Skimmias and sex
This is one of a new series of posts based on real questions I have been asked. Feel free to add to the answers or include your ideas and experiences
A lady inherited a skimmia in her garden and, despite it being 60cm high and wide it has never had berries. She wanted to know if it was male or female and if she could prune it.
Most plants have ‘perfect’ flowers, with both male and female parts in each flower. But, from a survival aspect, it is an advantage for plants to be pollinated from another individual, to expand the gene pool as much as possible. Plants that have flowers with both male and female parts often go to great lengths to avoid self-pollination – I wrote about this here.
Of course, the most extreme way to avoid self-pollination is for a plant to be one sex or the other. Such plants are called dioecious. These are not of great importance to gardeners except where we need the berries as well as the flowers. The most important of these, in our gardens, are hollies (ilex), skimmia and gaultheria (gaulnettya). The flowers of these plants are not obviously different in structure but the male flowers usually have a vestigial stigma and ovary but fertile stamens and the female flowers are devoid of stamens. It should be possible to look at the flowers to see if they are male or female – the same is true of hollies.
There are always a few horticultural exceptions – freaks of nature that have been selected by gardeners. There is a self-fertile skimmia (Skimmia japonica subsp. reevesiana) and holly (‘J C Van Tol’), which muddy the water. The rest are either male or female.
Among skimmias, the most popular is Skimmia japonica ‘Rubella’ (photo at top of page). It is widely available right now. Plants of all sizes, covered in red buds are available in garden centres and shops. Most male forms have showy heads of buds, that are attractive all winter before they open, in March. Males have larger heads of flowers with many more buds than the females. The flowers of both are fragrant and rather small but only the females will produce berries and then, only if there is a male within flying distance for the bees to transfer the pollen.
So a skimmia without berries may be a male or a lonely female.
Skimmias are strange plants and although I frequently see fine specimens or healthy drifts I always struggle with them. In theory they prefer acid or neutral, organic rich soil with a little shade. In hot sun they can scorch and be chlorotic and in dense shade they can be straggly and not bloom well. And yet, when I was a student at Kew there were dense plantings of them in the dense dry shade of (evergreen) holm oaks (Quercus ilex) that were perfectly fine. I think one of the problems is that they are usually sold as clumps of cuttings in a pot, to make a saleable plant more quickly, and these compete once planted out and never really make good plants. At present I have one, that is struggling but holding its own, and a variegated one that always looks thoroughly depressed, and if either ever looks really happy I will plant more.
As for pruning, skimmias can be pruned, in spring, and even pruned quite hard. It is sometimes necessary when old plants spread and get bare at the base.
The Biking Gardener 
It is uncommon for Ruscus to be self-pollinating but I have one, a small shrub named ‘John Redmond’ which holds berries all year round.
That is a great plant. I like all ruscus but the only one I have at the moment is R. hypoglossum. I think it is a female but it is lonely so I never get berries. I need to find it some partners because I like ruscus, despite their strangeness, or more likely because of it!
Although hollies are uncommon here, I learned very early that in old landscapes, almost all hollies of a particular species were female, with a male pollinator nearby. For example, a formal hedge of English holly (which would be horrid to prune) would be a single female cultivar, so could be just as uniform as a boxwood hedge. A male cultivar might grow as a single small tree in the background of a bed enclosed by the hedge. Unfortunately, as horticultural expertise waned amongst so-called ‘gardeners’, the odd male tree was likely to get cut down. Because hollies are uncommon here, the closest male pollinator might be a significant distance away. Of course, skimmias, particularly old types, are even more uncommon here.
That is interesting that, in the past, people were so sensible and organised with their hedges! I agree that holly hedges are awful to prune.
Not only was horticulture respected more back then, but this is the Santa Clara Valley, where horticultural commodities formerly sustained local economy. People were aware of the importance of pollination for some of the stone fruits, particularly the cherries.