Self-Sterile
Needs a pollinator to produce a crop.

Partially Self-Fertile
Produces some crop without a pollinator, but will benefit greatly from their presence.

Self-Fertile
May produce a good crop without a pollinator, but will often benefit from their presence.

Pollination Group

For successful cross pollination to take place, the flowering periods of the different cultivars must overlap. This means that when choosing groups of pollinating trees, you will need to take into account their 'pollination group'. This is an indication of their flowering time, with group A flowering the earliest, followed by group B and so on.

Varieties within each group flower at approximately the same time and would in most cases pollinate each other well. Trees in adjacent groups may also be compatible, so a tree in group B may be used to pollinate a tree in groups A, B or C.

Pollination Date

The flowering time of a fruit tree may be further specified by an optimum pollination date. Rather than being an absolute date (as flowering times vary with location), this is an indication of its relative flowering date. Thus, trees with the same pollination date will usually flower at the same time in the same location, even though the actual date on which they flower will vary depending on the location and weather.

For example, a pollinator in group A might have a relative flowering date of 1, whilst another might have a date of 3. Group B pollinators might, for example, vary in date from 4 to 6. This date can help you to select your trees more precisely, as successful cross pollination between two varieties must occur within a particular timeframe. Flowering of both varieties should take place within 3 days for apples, 6 days for pears and 10 days from plums. This means that although many trees in adjacent pollination groups may be compatible, two cultivars with pollination dates at the extremes of their group might not. For instance, an apple cultivar in group A with a relative date of 1 would not be suitable for a group B tree with a date of 6, as they flower 5 days apart.

Incompatibility

There are some species (pears and cherries) that will not cross-pollinate other trees of the same species, even if they flower at the same time. These cultivars are divided into a number of incompatiblity groups; fruit in these groups cannot pollinate any other cultivar in the same group. For example, a cultivar in group 2 cannot successfully pollinate any other tree in group 2, but can serve as a pollenizer for cultivars in any other incompatibility group. However, there are some sweet cherry cultivars (group 0) that can pollinate each other as well as cultivars from other groups; these are known as universal pollinators.

Triploids

Triploid cultivars have little pollen and so are not suitable to be used as pollinators. A non-triploid plant must be provided to pollinate them; if this second tree is to produce fruit, then a third pollinator will need to be planted. Apples triploid varieties include 'Blenheim Orange', 'Bramley's Original' and 'Bramley's Seedling', whilst pear cultivars include 'Catillac', 'Merton Pride' and 'Beurre Alexandre Lucas'.