Breeding Hostas

The essence of plant breeding is to bring pollen from the chosen male parent into contact with the receptive organ of the chosen female parent, while at the same time preventing any other agent from introducing pollen to the female. The mechanics of this are quite simple, although a minimal understanding of the structure of the flower concerned is required. A hosta flower is quite simple from this point of view, typically having one female organ (the stigma) and several (often six) male organs, or anthers, which surround it. The stigma is longer and thicker than the stamens, which bear a remarkable resemblance, especially in diagrams, to long eyelashes curling up at the tips.

Preparing the pod parent

In order to prevent the pollination of the selected flowers by natural agents such as bees the plant has to be emasculated. Before attempting this it is essential to study hosta flowers in order to know exactly when they are going to open, because it is necessary to catch the flower in the late afternoon of the day before it opens - that is, while it is still in bud but almost ready to open. The procedure is first to slit open the bud carefully, exposing the sex organs without damaging them, and then to cut away the petals and sepals. The final step is to locate the anthers and to cut them away without damaging the stigma. The removal of the petals and sepals deprives insects of a landing platform and makes it most unlikely that they will try to effect pollination, while the removal of the anthers eliminates any likelihood of self-pollination. Wind pollination is unlikely anyway since hosta pollen is heavy.

Gathering and storing pollen

Pollen is the medium in which the male gamete is transferred to the female. It is by its very nature short-lived and sensitive to changes in temperature. When the pollen of hostas is ripe and ready to use it has a powder-like texture. What governs its viability is enzyme activity, which is mainly temperature-controlled: the pollen needs a reasonable amount of heat to ripen, but in too much heat its viability deteriorates rapidly. The workable range would seem to be between 18°C (65°F) and 29°C (85°F), with an optimum of about 24°C (75°F).

Because it is so sensitive to temperature, it is important that pollen is gathered at the right time: too early in the day and the ambient temperature will not be high enough, too late in the afternoon or evening and the temperature will have fallen too low again. Mid-morning seems to be the time of optimum enzyme activity, but by this time of day the bees may have already got there and taken the pollen. For this reason it is better to gather the anthers, bearing their pollen, reasonably early in the day. The anthers should then be taken indoors, placed in the dark and allowed to come up to the optimum temperature before being either harvested or used.

Pollen may be used as soon as it has reached a suitable temperature, or it may be stored in folds of low-grade white paper, on which the source of the pollen can easily be written. It helps to fold the paper first and then open it out, tap the pollen into the middle section and refold it. The paper containers can then be stored in a refrigerator (not a freezer, which is far too cold) and will remain viable for the rest of the season and often into the early part of the following one. This of course means that one can mate hostas that do not naturally flower at the same time.

Transferring the pollen

The actual moment of mating is achieved when the pollen from the male anthers is transferred to the female stigma, which becomes moist and slightly swollen when it is at its most receptive: once it forms a dew-drop it is too late. The simplest and most natural way to do this is to brush the pollen-laden anthers across the slightly sticky stigma, thereby leaving a deposit of pollen on it. This of course is only possible when the anthers and stigma are in season at the same time.

Where stored pollen has to be used, a fine camel-hair or sable brush will be needed. Pollen is delicately taken on to the tip of the brush, which is then wiped across the stigma. Each cross should be made using a different brush, and the brushes should be cleaned afterwards in methylated spirits. As the brush then has to dry before use, it is practical to have a whole batch of brushes to work through before having to clean them. Another technique is to use a wisp of cotton wool held by tweezers to transfer the pollen, using a fresh piece of cotton wool for each mating.

Successful crossing

Some hostas are much more difficult to cross than others. H. plantaginea and some of its offspring are notorious for rejecting the pollen of other hostas. One technique that has been found successful is to use H. plantaginea pollen round the rim of the stigma, and the pollen of the parent you want to cross in the centre of the stigma.

Many breeders believe that there is no need to do anything further to defend the stigma once the cross has been made as the pollen takes only an hour and a half to reach the ovary. However, it is not unknown for bees to come and steal the pollen off the stigma after it has been put there. The simplest defence against this is to slip a short length of drinking straw over the anther. This can be removed the next day.

There are a number of factors which may inhibit pollen from taking. Timing may be critical. H. plantaginea, along with most of its fragrant offspring, is a notoriously difficult hosta to encourage to set seed. Part of the problem would seem to be that it is a nocturnal species, opening its flowers in the evening just as the dew is descending; dampness on the stigma may prevent the pollen taking. This may explain why crosses made at 4pm the day before the flower opens may be successful, while those made the following morning may not be.

Watering and soil fertility may also be contributing factors. Plants growing in poor soils seldom produce a good set of seed, and the same is true of plants starved of water. A further consideration may be how long a plant has been growing in the same place: hostas seldom set good seed when newly planted and are much more likely to do so when they have been growing in one place for several years. Finally, early- and late- season crosses maybe easier to carry out if the pod parent is removed to a greenhouse or windowsill as the vagaries of the weather may prevent successful mating.

Labeling and recording

It is important to label crosses and keep records of them in order to be able to make the same cross again -or just possibly, having seen the progeny, to avoid making it again. Alternatively, it may be desirable to make the reverse cross, or to back cross, or to use a different pollen parent, and so on.

The first step is to label the pod parent. The label should be tied to the pedicel (the short flower-bearing stalk) where it joins the scape -if it is tied to the pedicel alone the swelling pod may push it off. Plastic labels are generally too heavy, bending the scape sometimes to breaking point. Jewellery tags are better, but the ink must be indelible. An alternative is to use colored wire or wool. The name of the pod parent should always be written first, followed by an 'x' and then the name of the pollen parent. Some people prefer to use numbers rather than names, and while this may be useful if secrecy is important, it does necessitate keeping a separate record of what the numbers stand for. It is extremely important to record each cross immediately, especially when making several crosses, in order to avoid confusion.

Harvesting and storing

It takes some six to eight weeks from the time the cross is made for the seed to ripen, and most early varieties can be left to ripen their seed on their own, out in the open. It is however important to clean the spent flower off the ripening pod: if it is left there it might become a source of disease. Many mid-season and late varieties will not fully ripen their seed out of doors, and they must be brought indoors and ripened in sugar water. This can be made by adding 1/4 teaspoon sugar to 600ml water, stirring until thoroughly dissolved and then leaving to cool before use.

When the pods go brown and begin to split, they are ready for harvesting. All the pods of the same cross should be gathered and placed loosely in a bag, preferably a brown paper bag to permit the pods to ripen. On no account should the bag be a plastic one, since this will retain any moisture that may be left in the pods and is likely to cause the seeds to rot. The paper bag should be sealed with staples or paper clips and clearly labeled. A lot of pods do not split open properly and have to be assisted: coax the seeds out of the pod by gently stroking them with the tip of a pencil. The seeds should then be winnowed from the husks by blowing on them lightly. The importance of removing all husks and husk fragments (which may have to be picked away with tweezers) cannot be over-emphasized, as any unwanted debris may harbor disease. Unless you have special facilities, keep only fully ripened seed and discard the rest.

Unless the seed is going to be sown straight away, which is often very successful, it will need to be stored, but only when it is thoroughly dry and clean. It should always be stored in a cool, dry place, and in a refrigerator if it is going to be kept for more than about a month. It can also be stored in a freezer, though most breeders do not favor this.

Sowing the seed

Seed generally germinates very easily, whether it is sown fresh or taken out of storage. Many people like to raise seed under glass because they feel they have more control over it, but this is really not necessary. Hostas and their seeds are perfectly frost-hardy, and seed can be sown directly into prepared seedbeds in the open garden, preferably in a shaded position. A greater degree of control over pests can be achieved if the seed is sown in cold frames, either directly into the ground or in pots or seed trays (flats). If frost causes the ground to heave, it may need to be firmed down from time to time.

Once the seeds have germinated the seedlings need to be kept watered and shaded until they are ready to be pricked out into small pots, an operation which is best carried out when they have four or five true leaves. They can then be set out in nursery rows once they have about 15 leaves. If the rows are spaced 45cm (18in) apart, and the plants are set out 20cm (8in) apart, this will enable them to reach sufficient size for their merits to be assessed.