Rock Garden Plants: Propagation
Once you have the garden established and your expertise evolves, you may want to turn your attention to increasing stocks of your collection; either by renewing plantings that are fading with age, sharing with fellow rock gardeners or simply selling (so that you can invest in a few more plants). Propagation is one of the joys of any type of gardening and the techniques for alpine and rock plants are the conventional methods of seed sowing, cuttings and division.
Most alpines will increase from seed easily as long as it is sown while still fresh. In fact, some seeds will germinate even before the pod is ripe, so eager are they to increase. Many alpine seeds will require a period of vernalisation — exposure to freezing conditions — achieved by placing the seed in a refrigerator for a few days or a week.
It helps to take a look at the seed; if it is flat it will germinate more readily if you take the time to sow it on its side or edge. Some seeds have long tails; if you poke the seed into the compost with this tail protruding, it will spiral as it dries and in the process twist the seed firmly and deeply into the compost.
Sow the seed in pots of sandy compost, cover with a mulch of fine grit and then put each pot into a frame or else cover with cling film. As soon as the seed germinates, remove the cover or else begin to ventilate the frame. As soon as the seedlings have developed two true leaves, they can be pricked out individually and put into larger pots to be grown on before planting out.
Do remember that only seed from species will come true and look just like the parent plant, although there may be some variation in the depth of flower colour or leaf markings. Seed from non-sterile hybrids will be mixed and you may surprisingly find your own treasure. Similarly, cultivars and forms will be like the parent but different in colour, form and markings. The only way to be sure of producing a plant that is identical to the parent is vegetatively, by cuttings or division.
There are two types of cuttings, soft tip and heel or semi-ripe. The former is taken from a healthy new shoot, preferably one which is not flower-bearing. Carefully strip away the leaves, leaving only a few healthy leaves at the tip. Then make a clean cut across the base of the stem directly below a leaf node (the place where the leaves are attached to the main stem). Dip . the cutting into hormone-rooting powder and insert the cutting into a loose, sandy cutting compost; a 1:4 peat/sand mix is good (but you will need to pot-up the cuttings quite soon after they have rooted into a more fertile potting mix). You must then cover the pots to retain moisture around the leaves, so that there is no chance of them wilting. Some gardeners use plastic bags, but I find the easiest way to cover pots is to use a 2 litre plastic drink bottle cut in half; they fit snugly into the top of a 7.5-10 cm/3-4 in flowerpot, turning them into miniature. Stand the pots in dappled shade.
Semi-ripe (heel) cuttings are made from new season’s growth that has had a chance to begin to mature, so that the base is becoming woody while the tip remains soft. These are taken by pulling the selected shoot sharply down so that it comes away with a small tongue or heel of old wood. Trim the end of this small flap and, if necessary, remove the leaves as before. Dip the cut into hormone-rooting powder and potup and cover as before.
This is the easiest method of propagation, but is only possible with plants which make small plantlets around their base or whose crown gradually increases in size by developing new shoots around its edges. These can be separated into individual plants and grown on in pots or else planted where they are meant to grow. Some plants will also make new growths by sending out, which root wherever they touch the ground. Once these have made an adequate root system, they can be detached from the mother plant to grown on their own.
Some bulbs and corms can be increased by a kind of division; if you lift a mature bulb or conn, you may find that it has developed tiny bulblets or mini-corms around its base. Remove these carefully and grow them on in a nursery bed for two years or so until they are large enough to plant out.
A few plants produce suckers, which are rooted shoots that emanate from the plant’s root system. These can be cut off and planted elsewhere.