Do I need a male shrub to get berries?
Image by: Kim Fisher
Ilex aquifolium (English Holly) Monoecious and Dioecious
Many shrubs produce decorative berries. The majority of them are monoecious, meaning that there are male and female flowers on the same plant, or hermaphroditic, meaning that the flowers have both male and female parts. This means that berries are produced by every plant even if it is planted in isolation. However, some shrubs are dioecious, meaning that there are separate female and male plants. The female plants produce berries, but only if they are fertilised by pollen from a male plant. The best-known examples are many types of Holly. An even smaller number of shrubs produce fruit (seedless) without fertilisation. This is called parthenocarpy and an example is Nellie Stevens Holly. A few shrubs (eg. some Viburnum species) are monoecious, but require pollination by a non-cloned shrub of the same species to produce berries.
Planting for Berries
If your shrub is monoecious or hermaphroditic then all you have to do is plant it and you will get berries. It is possible to prune off the flowers or unripe berries and damage the display, or the berries may be eaten very quickly by birds, but it is basically very simple to get berries on your shrub.
Things are altogether more tricky for dioecious shrubs. In the wild, with hundreds of mixed male and female shrubs in the same area, pollination is almost guaranteed and berries show up. In a garden we often plant exotic (non-native) species or hybrids in isolation, but still want those pretty berries. The first trick is obviously to get a female plant that is capable of producing some berries. Then a male is needed, possibly nearby in the wild, but more commonly you will have to plant it yourself. The male has to be a sufficiently similar shrub to be able to fertilise the female and to flower at the same time. It also has to be planted close enough, although anywhere in most gardens is close enough. One male can pollinate several females within a few hundred yards.
It is not always easy to pick female or male plants. For dioecious plants, named cultivars are either female or male, but with some species it isn't possible to tell which plants are female until they flower. 'Blue Princess' holly is a female variety, but don't just rely on the name because 'Golden Queen' holly is male. Ilex aquifolium species is often used for hedging, but male and female plants look identical and are rarely marked at a nursery. You just have to buy a bunch and generally about half will be female. You could buy them in spring and sex them yourself since male and female flowers are noticeably different, but young or transplanted shrubs often don't flower. Obviously any shrub with berries on is female, but the ones without are not always male because the shrub could be too young to flower, might not have been pollinated, may be out of...