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Preparing your garden

 

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Creating a garden is an exciting challenge with the potential to reward you with a lifetime of enjoyment. But the garden of your dreams doesn’t happen overnight; it requires careful planning, hard work and knowledge, and before you can even think about planting you must thoroughly assess and prepare the site.

Whether you have a new garden or are modifying an existing garden, the basic groundwork is all important and the more time spent in preparation the better the results. The benefits all the hard work may not be immediately apparent but your initial efforts will eventually be amply repaid.

Assessing a new section

You can avoid many of the worst garden problems by being careful when choosing a section. However, if you are building from scratch, it is important to remember that many of the construction processes can dramatically affect the section. The topsoil may be removed to make a more stable foundation and the soil is very likely to be compacted by machinery and builders. Levels, too, may be altered to suit building requirements.

Considerations, such as view and exposure to the prevailing winds, can be quickly assessed and are usually reflected in the price of the land; an exposed site overlooking a land-fill will be cheap. If you are completely new to an area it may be worthwhile viewing the site under various weather conditions, but your first impressions will generally be fairly accurate. Nevertheless, you should fully investigate the site, with special attention being given to the following points.

Levels and natural features

The natural contours of the site have the greatest influence on house and garden design. Severely sloping land can be difficult to work with and may complicate garden maintenance; it also demands a higher level of fitness from the gardener. However, if the slopes can be modified, either by excavation or terracing, you will find that variations in level are one of the best ways of adding interest to a garden.

Small streams and natural rock features are easily incorporated into a garden design and can be a very important reason for purchasing a particular section. Sometimes it is better to buy a garden with the natural features that you want and build a house to fit, rather than having to build the features into the garden.

Light

The site orientation will have a great influence on how much sun various parts of the garden receive. However, this can be difficult to assess with a bare section unless you know what the design of the house will be like and how it will be located. Sections that run north–south will experience greater variations in sun and shade between the front and back of the garden than those that run east–west. But until you have a clear idea of the house design it is difficult to determine exactly where light and shade will fall. The design of the surrounding houses will also have some influence.

How much sun the site receives will affect the range of plants you can grow and may also determine how wet the soil is in winter, how quickly it warms up in spring and dries out in summer. If you are building in an established area, the shading created by existing trees and buildings should be fairly apparent, but most new sub-divisions are reasonably open, at least initially, and it can be hard to determine how further development will affect your section. If possible find out the plans of those who will be building near you. Also remember that the passage of the sun through the seasons will affect various aspects of the site. Areas that are open to the high summer sun may be completely shaded in winter. Also, the sun rises and sets further to the north in the winter so even areas that receive low level summer sun may be shaded in the winter.

Existing trees

Most new subdivisions are built on completely cleared ground, but sections that become available when a house has been demolished or that are the last to be developed in an otherwise established area often have several existing trees and large shrubs. Left unchecked, builders will almost invariably remove such vegetation to make their job easier. It you want to preserve an existing shrub or tree, make sure that the builders are aware of your wishes or that the plant is lifted carefully so that it can be replanted later.

Weeds

Most new sections in city areas are thoroughly sprayed before construction begins, and are relatively weed-free. However, in country districts new houses are often surrounded by open fields that are full of potential nightmares like thistles, docks and oxalis. You will be able to control these weeds with hard work and selective weedkillers, but with a reservoir of seeds on your doorstep you could have a continuing problem. Some rural sections, while looking idyllic, may be subject to severe weed problems.

Utilities

Where the house will be sited is often determined by the access to utilities, such as sewers and electrical supplies, and this can have a considerable bearing on your garden design. You don’t want to be in the position of having to destroy the appealing features of your site, such as large established trees, in order to lay a sewer line. Consider how the existing contours and features may have to be altered to accommodate such utilities.

Soil

Good soil is the most basic ingredient of a good garden. The ideal garden soil is well drained yet moisture retentive, rich in humus, well aerated and teeming with beneficial micro-organisms and the all-important earthworms. Few new sections have soil anything like this and you can expect to have to put in a considerable amount of work to bring the soil up to standard.

Garden soil is composed of two parts: the topsoil and the subsoil. The topsoil is the layer of well aerated, well drained soil that will support plant growth. Its depth depends on the structure of the soil. A loose textured, humus filled topsoil will be deeper than a hard clay-based topsoil because it is better aerated. The subsoil starts at the limit of soil aeration. This anaerobic subsoil is often a different colour from the topsoil and sometimes smells unpleasant. It cannot support much plant life and is generally poorly drained.

The soil of a new section may alter considerably by the time the house is built. In many cases the topsoil is removed to ensure a solid foundation. This is often looked on as an unscrupulous practice, an extra revenue source for the builder, but it is vital to ensure that the house is on solid ground. Nevertheless, you paid for the topsoil and it should not be sold from under you. Make sure that it is replaced once the house is completed.

The opposite of this problem is the filled or reclaimed site. Such areas often have very poor, clay-based topsoil or a very thin layer of topsoil on a rubble foundation. This can lead to problems with stability, drainage and infertility that may be almost impossible to overcome. If you intend to build or develop a garden on a recently filled site, you must be sure of its stability and the nature and depth of the fill. Getting an independent geological assessment is an expensive option that may be necessary for your piece of mind.

Check the pH

Due to the presence of minerals and vegetable matter in the soil there will be variations in the soil’s acidity. This means that any moisture that is applied, and taken up by the plants, will become either acid or alkaline as it filtered though the soil. This has a considerable bearing on the range of plants you will be able to grow and how they will need to be cultivated.

Acidity is measured on the pH (potential of Hydrogen) scale, which is a 14 point scale based around the neutral point of 7. As the pH becomes lower (7-0) so the acidity increases, as it becomes higher (7-14) so the alkalinity increases. It is important to realise that this is a logarithmic scale, that is: 6 is ten times more acid than seven, five is ten times more acid than 6 and so on, therefore a pH of 4 is 100 times more acid than pH 6. The same applies on the alkaline side; pH 10 is 100 times more alkaline than pH 8.

The range of pH that will support plant growth is from about 4.5 to 8.5 with progressively less choice as you move in either direction away from neutral. Most ornamentals prefer slightly acid conditions (around 6–6.8) while vegetables generally do best in neutral to slightly alkaline soils (7–8). Woodland plants often prefer quite acid soils (5.5–6) as they have adapted to growing in areas with high levels of leaf mould, which tends to be acidic. Knowing your soil’s pH and planting accordingly can avoid a lot of problems later on.

Table of common naturally occurring soils

The following table describes the most common natural soils. Existing gardens tend to have modified soil that is a composite of two or more of these soil types but new sections will usually show a clear predominance of one soil type. The nature of the soil greatly affects the ease with which a garden can be developed and the rate at which larger plants, such as trees and shrubs, establish.

Heavy clays will be poorly drained while sand can dry out alarmingly quickly. However, a dry sandy site is easier to deal with than a poorly drained one. It is always possible to improve a dry sandy soil by adding compost, but correcting a heavy clay soil with extreme drainage problems is difficult and expensive. If at all possible choose a site with a deep, well-drained sandy-loam soil because it is naturally capable of supporting a wide range of plants and may be easily modified to grow almost anything.

Soil Type Description Advantages Disadvantages Best Plant Choice
Sand Very sandy soils contain over 80% beach or river sand. The soil is composed of large grains of finely crushed rock or shell. Depending on the material from which it was formed, sand can be quite fertile. Very free draining and easily cultivated. It is quite easily modified with added humus but this must be a continuing process and the humus will tend to break down so that sand again predominates. Beach sands are often salty and may have a high lime content. Sand dries out quickly and can be difficult to wet once dry. It tends to be very hot in summer and blows away in high winds. Pure sand is unsuitable for most plants other than those that are very drought tolerant. However, with some modification and regular attention it can be made to grow almost anything. Because it is so free-draining, sand is a good base from which to work, but the work involves means that it wouldn’t be an ideal first choice.
Volcanic Volcanic soils are a combination of small stone particles, sand and a silty clay. They vary in composition depending on the nature of the original stone. Some volcanic soils are very gritty and close to sand, others are more clay-like. Volcanic soils often have a high mineral content and can be very fertile. The soil may be too free-draining and porous, which will make it difficult to wet once it has dried. It will also dry out very quickly in summer. Finely ground volcanic soils will become very clay-like in wet weather. This depends entirely on the nature of the soil. Sandy volcanic soils are marvellous for root vegetables and can be easily modified with humus. Gritty soils tend to be too free-draining for most plants unless they are well mulched. Clay-like volcanic soils are best suited to shallow rooted plants and usually require additional humus to support trees and shrubs.
River silt River silt is usually about a 50/50 mix of sand and very fine clay or mineral mud. Many riverine areas have a shingle base not far below the silty topsoil. Silt is usually very fertile and capable of supporting most plants. It tends to be quite free-draining as well as being easily worked and modified. River silt dries out quickly and tends to form a crust once dry, although this is easily broken. If too much clay is present the soil can become compacted underneath and sandy on top, which leads to poor drainage and the sandy topsoil blowing away. Most plants will grow well in river silt, provided the soil is kept moist. Added humus is usually necessary to ensure good moisture levels and even aeration. Root vegetable do very well in river silt as do annuals and fast-growing perennials, such as most daisies.
Sandy loam A loam soil with a high proportion (up to 30%) of coarse river sand. It may also include some decomposed turf or other fine organic matter such as peat or sedge. This is usually the best garden soil as it is the ideal combination of fertility and good drainage with enough binding to ensure reasonably good moisture retention. It is also very easily modified to cater for plants with particular requirements. If the soil contains too much sand and the climate is very dry, excessive watering may be required. This is easily overcome by adding humus to the soil. An excellent soil for most garden plants. Annuals, rockery plants and root vegetables thrive in sandy loam. Roses and leaf vegetables may prefer something slightly heavier but the soil is easily modified to meet their requirements.
Loam Loam is a mix of sand, silt, clay and decomposed organic matter. It is described as sand, silt, or clay loam, depending on which component predominates. Loam is often confused with leaf mould or humus. Loam is more fertile than sandy soils and is not stiff and sticky like clay. It retains moisture while also allowing good drainage and aeration. If the soil contains too much clay and the climate is very wet, poor drainage may be a problem. Add humus rather than sand to correct this problem. Sand may bind with the wet clay to form an impervious concrete-like layer. Very good for leaf vegetables, fruit trees, roses and most large shrubs and trees. Acid-soil plants and root vegetables may require additional humus.
Peat Soil formed over many years by the gradual filling of ponds and wet areas. Grasses and sedges decompose in the swamp and form a blackish brown humus-rich soil. Peat retains moisture well and, provided the land is not low-lying, it drains well. Coarse peat does not compact and allows for easy root penetration. Peat retains warmth well in cool weather. Peat tends to be quite acid and of low natural fertility. It can also become very wet and boggy if present in large quantities on low-lying land. It can be difficult to work with in winter. Provided the drainage is good, peat is an ideal soil for acid-loving plants like rhododendrons, camellias and ericas. Many vegetables grow well on peat if given some lime to counteract the natural acidity. Roses generally do very poorly on peat soils.
Clay loam A loam soil with up to 35% clay. Many newly developed sections have a predominantly clay loam soil because the loamy topsoil has become mixed with a clay subsoil. This type of soil holds moisture well while still being reasonable well drained and offers a firm foundation for trees and shrubs. Clay loam can become poorly drained and badly aerated due to soil compaction. Careful attention to drainage will be required and the soil may require modification with added humus. Roses and most large trees and shrubs do well on clay loam as long as it is well drained. Root vegetables and plants with very fine roots, such as azaleas and ericas can find the soil hard to penetrate and may require additional humus to loosen the soil. Rockery plants may appreciate some additional fine grit to improve the drainage and aeration.
Clay Pure clay is composed of very finely ground mineral deposits. It is readily identifiable as it forms an easily moulded pasty mud when wet and dries to a hardened crust. Water runs off flat clay surfaces and ponds in any hollows. Good for building foundations and laying paving Pure clay is very unsuitable for most gardening due to its poor drainage ease of compaction. Excessive run-off in wet weather is also likely to lead to severe problems. The topsoil can be modified but its underlying clay base is likely to stunt the growth of large shrubs and trees. Unless you are prepared to work hard to modify the soil, don’t expect very much to thrive on clay. Most plants will make a start, but they soon run into problems with poor drainage and an inability to develop their root systems. Shallow-rooted, fast-growing annuals, such as portulaca, Californian poppy and many of the daisies will do well in summer.

 

Assessing an existing garden

When you are looking at buying an established house and garden or altering the one you already have, many of the considerations are the same as when starting from scratch. Things such as the natural contours, exposure to wind and sun and the basic climatic considerations are still important. However, in an established garden you are surrounded by a myriad of clues that can give you a good idea of the nature of the garden, how well the plants are growing and how easy it would be to make changes.

Soil type

The most important thing here is to assess the drainage. Before starting your garden, dig several holes at least 50 cm deep in various parts of the section. Examine the subsoil, if it is very compacted clay there could be problems with drainage. Fill the holes with water and see how quickly the drain. If they are still holding water after three hours you will need to consider additional drainage or deep cultivation to break up the clay layer. If the holes start to fill with water naturally, it indicates that the ground-water table is high. This is quite normal in winter or after heavy rain, but if these conditions don’t apply it could indicate serious drainage problems.

Determining the pH

Many plants will only grow well within a narrow pH range and pH is also important in determining how efficiently soil nutrients will be able to be used. In general, trace element deficiencies will be more apparent on acid soils but very few soils are so acid that the effect is noticeable unless the soil is regularly cropped. On the other hand, acid soil plants will have difficulty taking up iron and magnesium if the soil becomes too alkaline. They will show all the signs (chlorosis and discoloured new growth) of deficiencies in these elements when, in fact, there may be ample present. It’s just that a higher pH tends to cause these elements to form compounds that make them unavailable to the plants.

A proper soil test is the only way to be absolutely sure of your soil’s pH. However, you can get a fairly good indication by looking at the plants growing in it. Erica family plants such as rhododendrons, will have an all over yellow look in alkaline soil and may not grow at all in very limey soils. Camellias, too, rarely look healthy in alkaline soil. Cabbages, on the other hand will probably be thriving in such soils. The large-flowered hortensia hydrangeas are among the best indicators of soil pH. Their flowers tend towards blue with increasing soil acidity and pink in alkaline soil.

With cultivation, most garden soils become slightly acid unless lime is added. This is ideal because it is what most plants prefer and you can always add a little lime if necessary. The exceptions to this trend toward acidity are limestone based soil or areas where the water supply is alkaline. If such conditions prevail, you’re almost certain to find out very quickly. Limestone districts, such as Oamaru and Waikari, are generally well known and you will probably be informed of the nature of the soil early on. Alkaline water, which is usually described as hard, is less obvious. Common signs of hard water are soap that is difficult to lather and crusty white deposits on cooking utensils and in the pipes. This “furring” is due to high concentrations of dissolved calcium salts. Litmus paper will quickly indicate alkaline water by turning blue when dampened.

Fertility

Poor soil leads to poor plants, so general plant health is probably the best indicator of soil fertility. Look for such tell-tale signs as stunted growth, yellowing foliage, premature leaf drop and a generally tired appearance.

Most soil fertility problems are easy to correct, either by improving the structure of the soil through the addition of compost or by using chemical fertilisers. Do, however, make sure that you are dealing with poor soil and not distorted growth that is the result of the use of long-term weed killers. This is something that you need to be particularly wary of if bought in topsoil has been used. Be suspicious if you see signs such as twisted and distorted tip-growth.

Weeds

A garden with serious weed problems should be immediately apparent, but you need to be wary of potential problems from more insidious weeds. Most weeds are dormant at certain times of the year and may not be obvious when you first view the garden. There is little you can do about this unless it is possible to see the garden over several months.

When you move into a new garden it is often a pleasant surprise when unexpected spring bulbs appear. However, if they are accompanied by oxalis, cape tulip and couch grass your joy will not last; they rapidly spread through a garden and are very hard to eradicate once established. If you are aware of the possibility of this occurring, you can eliminate the weeds before they become well-established, otherwise you may well have a major task on your hands.

Pests and diseases

All established gardens have their share of pests and diseases, but very few of them are serious enough to be a reason not to buy. If the garden contains a good mix of plants and has a generally healthy appearance then it is unlikely that there is very much to worry about. However, if there are fruit trees on the site or extensive plantings of one type of plant, such as roses or rhododendrons, you need to make a more thorough examination.

Fruit trees, particularly apples and stone fruits, are prone to several serious fungal diseases, of which fireblight and silver-leaf are probably the most devastating. Large groups of similar plants in a small area speed the spread of pests and diseases and are likely to quickly deteriorate if affected. Look for signs of powdery mildew, black spot and rust on roses; and thrips, leaf-roller caterpillar, mildew and rust on rhododendrons.

These problems can be controlled if they are not too well-established, but it may be better to avoid them. See the pests and diseases chapter for more details on identification and control.

Modifying the garden conditions

All aspects of a garden can be modified to a greater or lesser extent, but with regard to site preparation, most of your efforts will be directed towards getting the soil in the best condition to support plants, levelling overly severe contours and improving the drainage.

Soil

As mentioned earlier, very few gardens start ideal soil. Extensive modifications to the topsoil are nearly always necessary. How far you need to go will depend on the nature of the existing soil and the plants that you intend to grow. Sandy soil, for example, will need plenty of humus worked in if it is to support woodland plants, but will require little modification to grow marguerite daisies. Clearly, preparation and planning are interlinked topics and it pays to consider your planting and layout options before going too far and perhaps doing a lot of unnecessary work.

By the time you reach the stage of preparing the soil you will probably have some idea of where drives, pathways are likely to be. These areas will still need to be levelled and surface drained but the soil will not need modifying. Indeed it may need rolling and compacting.

Before you attempt to make alterations it is important to understand the nature of the soil in your area. Study the table of natural soil types to determine which type the soil in your garden most closely matches.

Initial cultivation

Most gardens, whether they are new or existing, have soil that has become compacted. This may be due to the heavy machinery used during building or it may be the natural compaction that occurs after a few years without cultivation. Whatever the reason for the compaction, it badly affects the drainage and aeration of the soil, so the first task is to loosen the soil.

There two common ways of doing this are digging or rotary hoeing. Unless the area is very large, it is preferable to dig it over by hand. Although it is much slower, working with a spade cultivates more deeply, gives you more control and enables closer attention to be given to specific areas. Rotary hoes tend to be shallow cultivators that operate too quickly to give you much feel for the soil. Also, a rotary hoe capable of cultivating to a spade’s depth is likely to be large and very unwieldy in a small garden, as well as being expensive to operate.

A garden spade is the normal tool for digging. However, a fork is better if the soil is wet or has a high clay content. Such soils are inclined to form large clods when turned and a fork is better at breaking up the soil.

Do not dig down below the topsoil and do not mix the topsoil with the subsoil. In most cases there will be a distinct colour and texture differences between the two. If you have a very shallow topsoil (50–100mm) it may be advisable to try and increase its depth with compost and mulch before cultivating.

The worst situation is to have a very shallow topsoil on a hard clay-like subsoil. This makes it very difficult to establish plants regardless of the quality of the topsoil; you simply must do something to break up the hard clay pan. The most labour-intensive method, and generally the most successful, is the remove the topsoil and dig over the sub soil while incorporating plenty of high-humus compost. The topsoil should also be mixed with compost before returning it to the garden.

Breaking down heavy soils is never easy. Many products are sold as soil conditioners and most of them have some benefit. However, improving the soil is a continuous process and it would be foolish to expect that any product is going to make an immediate miraculous difference.

Lime or gypsum will break down clay and make it easier to work with, but the process is complicated by rather vague instructions regarding how much to use for specific soil types. The different brands vary widely in their suggested application rates, although they all agree that heavier soils require more. The best advice is just to follow the instructions on the packet and see what happens.

Apply lime or gypsum to freshly dug soil in late autumn or early winter and leave it to weather over winter. The material will wash into the soil with the winter rains and a spring cultivation will complete the distribution.

Some plants react badly to lime, so don’t overdo it. Lime is very soluble and will leach into the subsoil quite quickly, but even so, if you plan to grow lime-sensitive plants it pays to err on the cautious side. Consider increasing the depth of topsoil by adding compost, or growing such plants in raised beds, as alternatives to heavy liming.

Further cultivation

After the initial breaking up, the soil will need regular light cultivation to prevent weeds becoming established and to keep the soil loose. Light forking over or regular hoeing will be sufficient.

Newly worked soils are nearly always full of weed seeds and this continued light cultivation is a vital part of weed control. Provided you keep on top of the job you are unlikely to have any serious problems, but let the weeds set seed and your newly prepared garden will provide the ideal seed bed for reinfestation on a grand scale.

Levelling

Establishing levels and providing good drainage are related tasks. Most gardens have some variations in level and even if they are only very slight they can lead to problems with puddling and soil being washed away.

If the slopes are gradual, a simple smoothing off may be all that is required and this is most easily done immediately after the initial cultivation. Vigorous raking can eliminate slight humps and hollows and these can be smoothed over by screeding the soil with a board attached to the head of the rake. Heavier boards and rollers can also be used on lawn areas, but be careful not to undo all your fine cultivating work by re-compacting the soil.

Severe slopes can be very difficult and tiring to work with. They can also lead to potentially dangerous run-off problems and slips. Terracing is usually the best option and in all but the most basic cases it is a job that requires professional contractors. They have the equipment and expertise to do the job quickly and easily. Also, building retaining walls above 50 cm high requires careful planning and is best left to those with experience. Poorly built retaining walls can turn a potentially dangerous slope into a disastrous slip just waiting to happen. Ambitious terracing jobs may require planning consent or building permits and this is another area where contractors can help.

Drainage

Drainage serves three main purposes: it directs run-off water away from the house; eliminates poorly drained garden areas; and lessens the risk of any earth movement in sloping parts of the garden. Good drainage is vital for ensuring that your house is pleasant to live in and that your garden thrives. Poor drainage can lead to rotting and mildew in the house and is tolerated by very few plants.

The top 60 cm or so of soil can easily be altered but the subsoil is largely permanent and unalterable, and it has a significant bearing on drainage. Anyone can see that clay soils tend to hold too much water while sandy soils often drain too freely. What is less apparent is the effect of a very heavy subsoil, often known as a clay-pan or hard-pan. Plants that don’t send down deep roots do well but deeper rooting plants often struggle when they hit the clay.

Conversely, peaty soils, which can be marvellous to work with, are often saturated with water at lower levels. This can lead to root rots and may cause trees to become very shallow rooted, which can lead to stability problems.

The best time to install drains is while the house is being built or just after it has been completed. The roof will need to be on and the guttering installed before you can establish where the roofing drains will run. The builders will usually take care of this type of drainage, but if you work in with them it should be possible to make these drains so that they help to drain the garden too.

Drives and pathways will also create run-off, especially if they are sloping, and their position will need to be considered when installing drains. Surface drainage, such as edging gravel or small edging gutters, will remove much of this run-off, but sub-surface water is a greater problem. The only way to deal with it is to remove it from the property by underground drains. These should always run to the lowest part of the property where the water can be channelled into a stormwater drain.

If you do not have access to main stormwater drains you will need to create a soak pit. A soak pit is simply a deep hole filed with coarse gravel or hardfill into which drainage water is piped and left to drain naturally. A soak pit is not practical in areas with a constantly high water table but otherwise it works well enough provided it is well below the level of the garden.

Installing drains

Most gardeners are capable of designing and creating a drainage system for gradual slopes, but severe grades and hillsides usually require an experienced drainlayer and possibly a surveyor.

Simple drainage systems are usually built around one or two main drains that are fed by smaller secondary drains if necessary. Generally, the main drains run with the slope of the land and the secondary drains run across and down with the slope to intersect with the main drains at about a 45–60° angle.

Fortunately, with modern plastic drainage pipe this is no longer the tedious job that it was when clay field tiles were the only option. Perforated plastic drainage pipe, such as ‘Agflow’ comes in a variety of diameters and is easily cut, which greatly simplifies the joining of main and secondary drains.

The drains must have a gentle but constant fall of at least 10–15 cm per 30 m of pipe. Sub-surface drains should rest level with subsoil if possible and seldom need to be more than 60 cm deep. Often an inexperienced drainlayer will overestimate the fall and depth required, only to find the ditch getting deeper and deeper at an alarming rate. The best way around this is to measure the length of the drain and establish depths for each end. That sets the depth for each point along the ditch and enables you to check the depth as you progress with the digging.

The ditch-digging process is straightforward. As with digging over the ground you have the choice of a quick mechanised method: a trenching machine, or a labour intensive but cheap option: digging the ditches by hand. Either method works equally well but digging ditches by hand soon loses its novelty. Once the ditch is dug and you are satisfied with its flow it should be lined to about a 15 cm depth with coarse gravel. The pipe is laid on the gravel bed and is then covered with a further 15-30 cm of gravel. The object of the gravel is to prevent the pipe quickly becoming clogged with silt. This will happen anyway, with time, but the gravel will considerably lengthen the silting period. If the main drainpipe runs to a stormwater drain, and both ends can be left open, blasting water through the pipe with a hose every few months will remove any silting.

Maintaining a constant fall over a long distance on undulating land is more complicated. Such drainage jobs are best left to the professionals who will have surveying instruments to aid them with their calculations.

Altering established drainage

Because drainage requires extensive digging, is messy and may necessitate the removal of established plants, it is better to take care of it before the garden is developed. Occasionally, however, nearby building developments and roading changes can alter the drainage patterns in established gardens.

If such problems are the result of the building activity of others, you may have some legal recourse with regard to correcting the problem. However, anytime that an existing garden has to have its drainage altered it is vital that great care is taken. Buried electricity and telephone cables, sewer pipes and existing drains are all potential hazards that you must be aware of when digging drainage ditches. Hiring a professional drainage contractor is usually safest and often turns out to be the cheapest option in the long run.

If your garden has areas that prove impossible or impractical to drain, you could consider turning these wet patches to your advantage by using them for bog gardens or ponds. However, if the poor drainage is caused by excessive run-off to a particular point, you may that your garden is frequently scoured by run-off water and that your pond silts up.

Trees pump up a huge amount of water from the soil and those that can tolerate very wet soil, such as swamp cypress (Taxodium distichum) and tupelo (Nyssa sylvatica), can be used to drain wet patches.

Manures and fertilisers

The next step is to incorporate plenty of humus containing material. Humus is rotted vegetable and animal matter. It provides nutrients for the plants, opens up the soil to make it more easily penetrated by roots and also helps to retain moisture. Regularly adding humus is one of the best ways to maintain nutrient levels and conserve soil moisture in light sandy soil or gritty volcanic soil.

A common way of adding humus is to grow a bulky green crop, such as potatoes, or nitrogen storing legumes, such as lupins or peas, to be dug in at the end of the season. If you have access to good compost, leaf mould or high humus material this can be dug in immediately thus saving the time required to grow the cover crop. Don’t be skimpy with the compost; it’s not possible to have too much humus.

Next you need to add some fertiliser to ensure good levels of soil nutrients. Composting and plant nutrients are dealt with more fully in later chapters, but to briefly summarise there are three main ways of adding nutrients.

Next you need to add some fertiliser to ensure good levels of soil nutrients. Composting and plant nutrients are dealt with more fully in later chapters, but to briefly summarise there are three main ways of adding nutrients.

Manures, which may be of animal origin, such as horse, cow and sheep manure; or green manures such as those already mentioned. These are a valuable source of nutrients, particularly nitrogen, and also add a considerable amount of humus.

Fertilisers, either chemical or organic, liquid or powdered. These are usually formulated as general all-purpose fertilisers, but they may also be nutrient specific. Many include trace elements and all should be provided with an accurate chemical analysis.

Mulching material and soil conditioners, such as pea straw, stable manure and seaweed work well as mulches or may be dug in as a source of nutrients and humus.

Altering the pH

Excessive acidity is easily corrected by adding lime but altering highly alkaline soil is more difficult. Incorporating plenty of compost material and using acid based fertilisers will help, but lime is very soluble and will eventually seep back to neutralise any added acidity. Planting in raised beds filled with specially prepared soil offers the best long term solution because it keeps the plants above the level of the native alkaline soil and lime is unlikely to seep up into the raised bed.

Raised beds should have at least a 60 cm soil depth and should be wide enough to allow for a good root spread; a large rhododendron requires a bed at least 1 m wide. Raised beds can be made from treated timber or more permanent materials, such as bricks or concrete. Remember, however, that fresh mortar and concrete will release free lime and should be left to weather for several months before planting.

You will often be told that you should never add lime to plants that prefer acid soils but there is an exception to this rule. In very acid soil, calcium can fall to dangerously low levels and all plants, even acid-soil plants, need calcium, so this problem requires treatment. Calcium is available as calcium sulphate, which will not raise the pH, but obviously this does not correct the underlying extreme acidity. The best solution is to use dolomite lime, which will slightly increase the pH, thereby freeing up calcium, and will also add some magnesium.

Dolomite lime (as opposed to hydrated lime) is largely composed of mineral dolomite, a natural form of limestone with quite a high proportion of magnesium. It is a more gentle form of lime, one that acid soil plants will tolerate, and the extra magnesium helps too. Very few garden soils are acid enough that you would need to add dolomite lime to grow acid-soil plants, such as Rhododendron and Kalmia but potting mixes and composts may be. That is why you will often find a little dolomite lime listed in the ingredients of potting mixes, even those for acid-soil plants.

Results

You should now have moisture retentive, humus-rich, well-drained, aerated, fertile soil with a pH of around 6.5, which is the best canvas that you can start with, now it's time to apply the colour.

 
Copyright Geoff Bryant