Propagating erica family plants
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What do Erica, Calluna, Rhododendron, Pieris, Andromeda, Agapetes, Arbutus, Gaultheria, Arctostaphylos, Kalmia, Leucothoe and Vaccinium have in common? Well apart from the fact that there can’t be many of us that don’t have at least one of these genera in our gardens, they’re all members of the erica family.
Although a few erica family plants have a reputation for being difficult to propagate, many are remarkably easy, especially considering the relatively high prices asked in nurseries and garden centres.
Cuttings are the most reliable and commonly used method. Layering is another easy and reliable method. Indeed, some, such as the low-growing evergreen azaleas, will self-layer where their stems are in prolonged contact with the ground. Seed is used less often as the popular garden specimens are often hybrids. However, if you are looking to experiment with hybridisation you’ll need to germinate seeds.
Cuttings
Spring and early summer are the best times for cuttings, though the larger rhododendrons strike well right through summer and into autumn. Use only small tip cuttings of non-flowering new growth. Of course, such soft, tender cuttings require very humid conditions to prevent them from wilting and drying off. Fine if you have a mist propagation unit, otherwise it may be better to delay taking cuttings until the growth is a little firmer.
The exact size of the cuttings varies from around
1.5–2 cm long in the case of ericas and calluna through to 10–15 cm with the larger rhododendrons, but in each case the cutting is small compared to the stem length available. So small in fact, that it’s quite possible to fit 450–600 heath or heather cuttings in a standard nursery propagation tray.
With bottom heat and mist 75–90% of the cuttings should strike in 6–12 weeks. Some will strike just as well with less ideal conditions, but most will take longer and the striking percentage will be lower. In cold winter areas the freshly struck cuttings may need to be overwintered under cover.
Layering
Layering is a very simple process that is just a matter of bending a branch so that it touches the soil and then keeping it there long enough to form roots. It helps to wound the lower surface of the branch to expose some of the cambium layer, and it’s also advisable to dust the wound with rooting hormone. Peg the layer down with some wire hoops and mound some soil over it, leaving the stem tip exposed. Layers pegged down in spring will be well rooted by the following spring, possibly even by autumn. All that’s required is to lift the rooted stem, cut it off the main plant and grow it on, either in a pot or the garden.
Aerial layering
Aerial layering in effect takes the soil to the stem rather than the stem to the soil. It’s a good method to use with shrubs and trees that have no stems suitable for bending down to the ground for layering.
Select a firm stem that still has green bark; older wood will strike but it takes longer. Aerial layering can be done any time such material is available. Remove the foliage from around the immediate area and make a shallow upward cut into the stem. Lift the flap of bark and wedge it open with a matchstick or a small pebble. Lightly dust the wound with root forming hormone powder and wrap the stem with wet sphagnum moss. Then wrap the ball of sphagnum in black polythene and secure it with wire ties or tape.
The moss stops the wound drying out and healing, and with time roots will form at the wound. Check it after about 10–12 months. When it is apparent that a good root system has formed, remove the struck layer and pot it up for growing on.
Seed
Spring is the best time to germinate the very fine, often dust-like seed. Sow it on a finely sieved 50/50 mixture of sphagnum moss and peat- or bark-based potting mix. Don’t cover the seed, just sow it on the surface and gently moisten it with a fine mist. If kept lightly shaded and moist it should germinate in about 10 days to six weeks depending on the type.
The young seedlings are minute and slow growing. To achieve the best results they should be grown in a controlled environment. They are generally unsuitable for sowing outdoors either in trays or the open ground as they require high humidity, steady even warmth (around 18°C) and occasional dilute liquid feeding.
The seedlings can be potted on once they have formed their first true leaves. As they may still be too small to handle conveniently at this stage, you may prefer to delay the pricking out. No harm will come to the seedlings provided their nutrient requirements are met.
There may be a considerable waiting period before you see any flowers, up to 10 years for some of the tree-sized rhododendrons.
Grafting rhododendrons
A few of the larger rhododendrons perform poorly when grown from cuttings and are usually grafted. Rhododendron ‘Cunningham’s White’ is the most common rootstock and is preferable to the formerly used R. ponticum as it produces fewer basal suckers.
The best time to graft is in early spring before the new growth begins to develop. A saddle graft is used and is easy because it is just a wedge cut made to fit a matching V-shaped notch, but you will still need a sharp knife and a steady hand. Practise with a few old twigs before cutting any valuable plants.
The stock and scion should be about the same diameter to ensure a good fit. Start with 75–100 mm pieces of non-flowering tip growth from your scion. Remove the lower leaves and trim back the remaining foliage by about half.
Next, trim the stock back so it’s just high enough to work with comfortably — about 75–100 mm is adequate. The top of the stock should be cut to a wedge-shaped point by make shallow cuts on opposite sides of the stem.
Make a matching V-shaped cut in the base of the scion stem. The cut base of the scion should slip neatly over the top of the stock. A little trimming may be necessary to get a good fit. Bind the stock and scion together with grafting tape. It’s all a little fiddly at first, but you’ll soon get the hang of it.
Keep the freshly grafted plants in a warm humid environment to ensure good growth and to prevent the graft drying. Once the scion is growing well and the graft has callused over, the tape can be removed. Don’t leave this too long or you may find the tape starts to cut into the stem.
Copyright Geoff Bryant
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