Helping your lawn take care of itself
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Autumn is the traditional season for top-dressing and renovating lawns; the time for tidying up those bare patches and getting rid of invading weeds.
A top class lawn is the perfect complement to a formal garden, a tougher lawn can do multiple duties as pathway, barbecue and entertainment area, while a lawn of more rugged grass is the ideal children’s play area. But keen as I am to see the New Zealand cricket team improved by upcoming generations of backyard cricketers, there’s no denying that lawn maintenance is a lot of hard work and a lawn is among the thirstiest parts of a garden.
Saving water and labour are important, especially for those
of us in areas where water restrictions and drought are likely or whose ability
to undertake heavy work is limited by injury, disability or age. Yet when you
see all those garden sprinklers working overtime and listen to the
near-continuous summer drone of lawnmowers, it’s seems that lawns are not going
out of fashion even if they need plenty of work.
So what can you do to keep down the demands from the grass?
Well, obviously the first option is to eliminate the lawn entirely and replace
it with something that’s easier to manage. But that’s not always possible,
especially if your lawn doubles as a domestic sports field.
Another option is to plant a drought-resistant grass that
can be allowed to dry off over summer and which will
green up in autumn with the arrival of cool moist weather. That eliminates the
watering worry and, unless the summer is very wet, cuts out most of the mowing
too.
But even if you prefer a more formal lawn it’s still
possible to have the best of all worlds: a tough drought-resistant lawn that is
healthy and which requires only the minimum of labour. The key is giving the
grass the conditions it needs to look after itself.
Keeping your lawn healthy is really just like keeping
yourself healthy: if you can prevent the stresses and strains that lead to
damage and disease, most problems will be minor and able to take care of
themselves.
The essential ingredient for a healthy lawn, indeed any part
of the garden, is healthy soil with the right balance of aeration, moisture and
nutrients. Once you have the soil at its best, worms and beneficial
micro-organisms thrive too, which in turn perpetuates the cycle by leading to
better decomposition of organic matter and continued good aeration and moisture
penetration.
Keep the roots aerated
Grass roots need a good supply of air but are inclined to
suffocate themselves through the build up of a thatch of dead leaves and roots.
Although some thatch is quite normal (the grass clippings from mowing doesn’t
add much to it) once it gets over 6mm or so thick the grass begins to suffer,
often yellowing, dying out in patches and becoming progressively thinner. The
thatch is also an ideal home for grass grubs and porina caterpillars.
A build-up of thatch usually indicates a lack of the
micro-organisms that normally break it down. These micro-organisms also suffer
in poorly aerated conditions, so the cycle is self-perpetuating: poor aeration
leads to a build-up of thatch that in turn makes the soil even more poorly
aerated.
Excessive use of chemical fertilisers can kill the
micro-organisms, while organic fertilisers can help to increase the numbers of
both micro-organisms and earthworms. However, thatch problems often start with
soil that has become over-compacted through years of traffic and mowing. Once
the soil has reached that stage and a thick thatch is established, the lawn
will need mechanical aeration and dethatching using a coring machine and lawn
dethatcher. Sometimes these can be hired, though you may need the services of a
lawn care expert. If things are really bad the lawn may need to be dug over and
relaid.
Good aeration is vital and the best tools for doing it live
right there under your lawn — worms. Whatever else you do to your lawn, don’t
get paranoid about worms and worm casts. I just can’t understand gardeners who
insist on eliminating worms from their lawns because they find the worm casts
unsightly; they’re simply making far more work for themselves in the long run.
Eliminating earthworms, which can also
occur if the soil pH drops below around 5.2, will cause dead organic matter,
such as old grass roots, to accumulate in the soil. This soon compacts
down into thick thatch, leading to poor aeration, soil that is hard to wet and
providing conditions under which grass-destroying fungi thrive.
Encourage the worms, however, and your
lawn will have improved drainage and the worms will naturally feed your lawn by
pulling compost down into the soil and decomposing it while at the same time
removing any old dead grass roots. Charles Darwin made an extensive study of
earthworms and concluded that that in well-established pastures with a density
of some 600 worms per m² they can add as much as 5 mm of soil to the
surface each year. That’s effectively free annual topdressing.
Keeping the grass growing
Like any plant, grass needs feeding, and the better it grows
the more resistant it is to any potential problems.
Before you decide on exactly what fertiliser to use, it’s
important to realise that most lawn grasses grow best
on slightly acid soils with a pH of around 6–6.5. Old lawns can become very
acid, often approaching a pH of 4.5, so the first consideration should be to
make sure the pH is above 5.5 and preferably over 6. A light dressing of
dolomite lime, well watered in, rarely goes amiss. If your lawn is an old one
and really seems to be suffering, it may be advisable to have the soil tested.
If it turns out that it’s very acidic, extra lime may be required and you
should avoid acidic sulphate-based fertilisers.
You can use chemical or organic fertilisers, and both are
available through garden centres and farm suppliers. As mentioned earlier, some
chemical fertilisers can harm helpful soil micro-organisms, though this is
unlikely to happen if you follow the suggested application rates on the packet.
Considering that with regular watering and mowing a lawn can
yield around 1kg of dry matter per square metre during one growing season, it’s
scarcely surprising that there are lost nutrients to replace. It’s generally
reckoned that a season’s mowing removes the equivalent of the nitrogen,
potassium and phosphate content found in 100 g of urea, 50 g of
superphosphate, and 50 g of potassium chloride per m². Consequently you
should consider that level of fertiliser, or its organic equivalent, to be the
minimum application over the growing season.
Although other elements are removed in the clippings, most
fertilisers contain enough extras to restore the levels of calcium, magnesium,
and sulphur. Trace elements, especially iron, are also important but are again
usually incorporated in general chemical fertilisers. There’s no need to get
too fancy with the fertiliser; an off-the-shelf 8.8.8 blend with trace elements
will do the job.
If you intend to use organic fertilisers, consider
incorporating some clover in your lawn mix. Used at a rate of 5g/m² it can
provide as much as 10g/m² nitrogen per year, which should be enough to keep the
grass lush and green. Of course, using clover means that the use of broad-leaf
weedkillers is off the agenda, but then if you’re going organic you wouldn’t be
using those anyway.
Watering
One of the bonuses of improved aeration is that water
penetrates the soil more easily, which is vital because lawns need plenty of
moisture to stay green through summer. Unless you’re happy to make use of
grasses that can be left to dry off in the heat it’s almost certain that you’ll
need to irrigate. Better aerated soil also allows the grass roots to penetrate
more deeply and that helps the grass to better withstand drought and to use
more of the soil’s nutrients.
When the time comes to water, and you’ll know that by a
subtle darkening in the colour of the grass, try to water in the morning or the
evening so that less moisture is lost to evaporation. A few deep good deep
waterings during the week are far better than a quick soak every day. Water too
frequently and those grass roots will once again start staying near the
surface, the soil aeration will suffer and fungal problems such as brown
patches and fusarium wilt may develop. You’ll also leach out nutrients, meaning
that the grass will need fertiliser more often.
Mowing
All plants need to be able to photosynthesise light in order
to turn basic elements into vital life-sustaining sugars and starches. Grass
leaves contain the chlorophyll necessary for photosynthesis, so when we mow we
lessen the grasses ability to sustain itself. Mowing,
however, is not all bad for the grass because trimming the old growth
encourages new shoots to develop — it’s all a matter of balance.
Grass is under most stress in hot dry summer conditions, yet
that’s often when we cut the grass to its shortest. Don’t! Instead, raise the
mower blades as summer progresses — up to 50mm. While that may seem excessively
long, it’s fine unless you want a putting or bowling green,
and the grass will certainly be healthier for it.
Weeds
A thick healthy sward is more resistant to invasion by
weeds. If the conditions are right you shouldn’t be too troubled by vigorous
perennial weeds, but it’s hard to prevent the odd dandelions, Onehunga weed and
rank-growing grasses from appearing. Regular oversowing, top-dressing and
feeding to keep the grass thick, combined with hand weeding, are the simplest
organic controls, though if things start to get out of hand most of us reach
for a herbicide of some sort. There are many sprays, some specific to just one
or a few weed species, so ask at your garden centre for details.
Incidentally, always buy the best top-quality certified seed. The main grass in cheap mixes may not necessarily be an inferior type but the mixes often contain coarse rye grasses and more than a few weed seeds. There’s no sense in deliberately sowing weeds.
Moss, which often appears in lawns, is popularly associated
with acidity. However, it can occur on any bare patch or among grass that is
not thriving. Iron sulphate is the common method of controlling moss. It forms
moss-killing sulphuric acid in the presence of water. This can of course also
damage the grass, but only temporarily and the extra iron will soon green up
the turf. Don’t use iron sulphate if your soil is acidic, as it will only make
it more so.
This year, rather than just throwing on some grass seed as a top-dressing, first think about how you can improve what underneath that grass — the soil it’s growing on.
Copyright Geoff Bryant
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