Hybridising is a simple process: so much so that if left to themselves insects will do the the job for you. Unfortunately they will not also identify the pollen parent. It is impractical to carry out controlled pollination with plants growing out of doors unless they are to be individually covered in some way. This means growing parent plants in a greenhouse, a situation that most saxifrages do not really enjoy.
To cross-pollinate first select the seed parent and remove the petals and all the anthers. This is done for two reasons: first, it makes the flower less attractive to would-be pollinaters; second, it prevents selfpollination. Various methods can be used to transfer pollen to the seed parent, a brush or feather is often recommended, collecting the pollen in the fibres and brushing it onto the seed parent stigma. The problem of this system is avoiding contamination with stray pollen, and a better method is to use tweezers. First test to see that the pollen is ripe by carefully just touching the pollen with the very tip of your finger. The pollen will readily transfer from the anther if it is ripe. Then grasp the filament of the anther with the tweezers and remove it from the flower; gently brush the anther against the seed parent stigma coating it with pollen. Deciding when the stigma is receptive is more dilticult, but if the pollen sticks to it readily, it probably is. Early in the season it is fairly safe to leave the pollinated flower exposed but as soon as flies appear it is necessary to isolate the flower for a few days. Each flower that is pollinated must be identified by hanging a label from the stem marked with the name of the pollen parent (Fig.1 opposite).
Saxifrage seed is ripe as soon as the capsule opens, and the seed should be plump, not shrivelled. My practise is to sow the seed as soon as it is ripe. The compost used is of little importance as the seed does not know what it is, but a good mix is 50:50 coarse sand and broken up peat. A two inch pot (F6) is large enough for seed from one capsule. Scatter the seed on the compost and cover with a light layer of washed clean grit. If the grit contains a fine fraction it will encourage the growth of mosses. Never let the pot dry out, the best control is in the greenhouse. Some leave the pots out of doors but this is something I would not recommend. To keep the pots damp spray with water vertically from above, if an angle is adopted there is a danger that seed will be dislodged and washed over the side of the pot and lost, or even worse, washed into an adjacent pot. Some seed will germinate in the autumn but most will wait until the next spring. Even when seed does germinate in the autumn it is advisable to remove the young plants and prick them off as soon as possible, then keep the pots until spring when a second germination may occur. It may be thought that pricking off when the seedlings are very small is courting disaster, but experience has shown that the survival rate is good at this stage, i.e. when the first four true leaves have appeared. After this it is up to you to grow the plants on. Keeping them growing quickly is the secret to success: once they stop it is difficult to get them going again.
But if the method is straightforward the question as to what to cross is less so. Several alternatives present themselves: different forms of the same species may be crossed, two different species can be used, or a combination of an existing hybrid and a species, or another hybrid, may be chosen. Predicting the outcome of the interaction of two species is possible but the time-word phrase 'intermediate between the parents' immediately springs to mind. Some variation will occur. Most will be somewhere near the middle of the range but a few will be nearer to one or other of the parents. If three species are involved the outcome of shaking up the chromosomes is more difficult to imagine, and in the case of four species the point is reached where prediction fails and it becomes a lucky dip. Some people will look at a hybrid and state with confidence the parentage. Having raised many hybrids it has come to my attention that this is not possible except for certain species that have very distinctive appea rances. When Boyd produced the first Porophyllum saxifrage hybrid, S. burseriana x S. aretioides, identifying the parents was not a real problem, even though they were open pollinated, as few possible species were available at the time.
What can be achieved by hybridisation? Hybrid vigour immediately springs to mind, but this is not very easily seen in practice. Orange is the 'in' colour, but any new colour may be an acceptable goal. My view is that only three colours are available: red, yellow and white. The red is the rather blue-red of S. lilacina and the yellow may be pure yellow or have a green cast. Brighter reds need yellow to cancel the blue and white is the lack of red or yellow. Unfortunately information concerning the chemistry of colours in saxifrages is not available but it is quite probable that the porophyllum saxifrages may only have two colours available with all the intermediates being the result of mixing the red and yellow at various concentrations. Form of foliage and flowers are areas that have room for further work: in recent years there has been a drift away from the delicate in favour of the robust. More obscure but desirable properties are available such as disease resistance. One of my particular interests is in extending the flowering season with, earlier or later flowering.
Without planning, whatever is achieved can be largely put down to luck. One system to aid planning is to use a grid with the saxifrages selected for hybridisation listed along two edges so that progress can be easily seen. My particular interest is bispecific hybrids. It may be thought that this offers limited prospects. Hasn't it all been done before? The grid below includes many of the commonly available porophyllum saxifrages. Thirty to a side, means that 900 - 30 = 870 crosses are available , but as A x B is the same as B x A this number is reduced to 435. On the grid this is represented by the section above the diagonal. If it was wanted the experimenter could extend this by recording in both halves: with pollen parent tracked at the top and seed parent along the side. The hatched squares are crosses already published and the black squares are crosses I have made. It will be seen that there are many more crosses to try, as well as other varieties of the species which have not yet been exploited. Even after all my own non-published crosses are included there are many more to go and quite a number of other species which could be included.
Making a bispecific cross may not be an end in itself. Obviously there is the process of selection, for there will be variation within any bispecific cross. Then there is the possibility of the second generation F2 crosses either between the plants produced by the original cross or the cross back between them and one of the original species. The bispecific cross is the simplest situation: when hybrids are substituted for a species the opportunities for variation increase.
A major problem is numbers. On average something like five to ten seedlings can be expected from each seed capsule, though occasionally up to about twenty-five may appear. To put the problem into perspective, last year I made about 30 crosses resulting in about 300 seedlings. These will need to be kept three years before any selection can be made, but by then two more batches of seedlings will be growing. It could easily reach 1000 plants.
Consider the number of combinations that can arise from only five saxifrages, A B C D and E. There will be:
10 bispecific crosses AxB, BxC, CxD etc.;
20 back crosses (AxB)xA, (AxB)xB, (BxC)xB etc.;
30 trispecific crosses (AxB)xC, (AxB)xD, (AxB)xE etc.
though it should be noted that this last would be reduced to 10 if (AxB)xC is taken as the same as (BxC)xA and (AxC)xB. Already the number is 60 (or 30 depending on what is counted in), and combinations of four species (crossing bispecifics or trispecifics with a species) have not been considered. Furthermore it is assumed that there is only one form of each species. Nobody would mix five species .... would they?
And then numbers present the problem of what should be done with all the new crosses. Should each be described and named? If so, how would members handle perhaps 50 new names per year? If the crosses are not described then others will continue to plough the same furrow.
I can not claim to know the answers. The problem is becoming serious as more names are published every year. One answer that has been proposed is that names should not be given until the plant has enjoyed some popularity, but I feel that this is only a recipe for disaster with many plants circulating with only obscure identities. My personal view is that all primary bispecific hybrids should be described as soon as they are available, even if they will never enter widespread cultivation. If possible sufficient should be kept in cultivation for the future to make them available to breeders. More complex hybrids should earn their position in the cultivar world, and only be given a "trivial' name or identifier until they are established.
This article is based on the talk given by Raymond Fairbairn to the Society's meeting in October 1995 at Adel near Leeds.