There are only two means by which hostas can be propagated: seed or division, which includes tissue culture (micro-propagation). Of the two, division is generally the most useful.
The simplest method to divide hostas is to remove a wedge-shaped piece from the clump, rather like a slice of cake, using a spade. The slice can then be planted elsewhere or potted up, and the space from which it was removed refilled with good soil or garden compost (decayed organic matter). The parent plant will grow back into the new soil and in a few weeks it will scarcely be possible to see where the slice was taken. An alternative method is to dig up the whole clump and split it into two or more, using two forks back-to-back. In either case, the operation should be carried out in the spring, just as the noses begin to push through the soil, or in autumn as the leaves are dying down.
If larger numbers of young plants are wanted the best method is either to dig up proportionately larger wedges of the original clump -perhaps as much as three-quarters -and split that into two or three big slices, or to dig up the whole clump and then shake or wash off the earth. Once this has been done the white roots and the rhizomes can be clearly seen, and it is easy to cut them into smaller pieces, each new plant having three or five buds each. The smallest units into which it is normally sensible to divide hostas is single terminal buds, that is to say the buds which occur at the ends of each short length bf rhizome. However, if a still greater quantity is required, it is often possible to coax the latent buds into growth. These are tiny, usually mauve buds that arise on the rhizome just below the terminal bud. The first thing to do is to make sure that there are in fact latent buds on the shoots: having located them, cut the terminal bud off with a very sharp knife and then split the rhizome longitudinally, as if slicing a carrot, each long thin slice having a single latent bud and, if possible, some of the original roots. Dust these plantlets with fungicidal powder and pot them up into a sterilized potting medium. They should then be stood out in a shaded but open position; if they are kept in close conditions the likelihood of fungal attack is much higher. It should be said that dividing hostas into pieces this small is not without its risks, and if the plant-lets are attacked by fungal infections all of them may be lost.
There are methods of preparing hostas for propagation which encourages them to make more buds than they would otherwise do, and for the patient this is the more prudent method of producing numerous divisions. The first is a practice known simply as ‘mowing’. In about June the leaves of the hosta are cut off about 0.5-1cm (1/4 – 1/2 in) above the ground, depending on the vigor of the variety. This causes the latent buds to break into growth, thereby producing more growing tips. Hostas can be mowed, three times in a season, if so desired.
Another technique is known as the Ross method, after its American inventor, Henry Ross. The earth is scraped away from around the crown on the hosta, revealing the rhizomes. A sharp knife is inserted into the side of the stem just above the basal plate and pushed downwards through the basal plate to where the roots are located. The knife is then removed and inserted into the side of the stem again, but at right angles to the original cut. After this the knife is removed and the earth returned around the crown of the hosta, which should then be watered. Over the following few weeks some of the leaves will turn yellow and some blotchy, but the hosta soon returns to looking as good as usual. By autumn hosta will have produced many more potential buds than the norm. Some growers find this technique is also successful when carried out later in the season. However, grinding the heel of a shoe into the young shoots of hostas just as they are coming through in the spring can be just as effective at producing more buds, and a great deal less trouble.
As a rule of thumb hostas are best propagated in the spring just as the new shoots are coming through, but it is not an inflexible rule. They can be divided all through the growing season, provided their leaves are cut off once they have been divided, they are properly firmed in the ground or growing media, and they are watered properly.
Hostas grow much faster in sunny conditions, which is why some nurserymen grow them in field rows. However, the leaves of some are liable to become scorched.
An extension of normal division is micro-propagation or tissue culture. This is a laboratory technique whereby cells from the tip of a flowering shoot are taken and grown on a sterile jelly in a test tube. By manipulating feeding and lighting, these cells can be made to turn into tiny plant-lets that exactly resemble the plant from which the flower was taken. The cells can be divided again and again, so that it is possible to produce huge numbers of plants quite quickly as compared to the older methods. It is this technique that has been largely responsible for many new varieties coming on the market far faster than would once have been possible, and for far more plants to be put on the market at an early stage, often before they have been adequately trialed in gardens or before unwanted sports have been rouged out.
A further reason for wanting to divide a hosta might be if a sport arose on an established clump. The safest procedure is usually to wait until the sport has reappeared for two or three years in succession, and then to lift the part of the plant that has sported, together with some of the surrounding parts of the plant that have not, preferably using the slice of cake technique as described above. The whole slice can then be put on a potting bench to bring it nearer to eye level. The soil should be shaken or washed off. It is then possible to see quite clearly to which piece or pieces of root the sport belongs: the rest can be cut away with a sharp knife, and the sport potted up and grown on. It may take several growing seasons to stabilize the sport as its natural instinct will be to revert to its parent.
Many hostas produce seed in great quantities, and most of the seed will germinate readily, producing an abundance of seedlings. Indeed, if the seed heads are left on plants in the garden for their decorative winter effect and the seed is scattered, seedlings will usually come up like weeds unless one takes steps to prevent them. It may be best to remove the scapes once the pods have formed to avoid this. Hybrid miniatures usually carry fewer flowers and are more reluctant to set seed.
Hostas raised from seed that has occurred naturally in the garden are unlikely to be of particular merit. In the first place hostas do not usually come true from seed (that is, the seedlings will not normally bear a close resemblance to the parent), apart from H. ventricosa, which is apomictic. Secondly, hybridizers pursuing deliberate breeding schemes have so raised our expectations of what a good hosta should be like that few seedlings will measure up.