Series: Agnote 4-77 Edition: First edition Last updated: Jan 2007
Crab apples are usually regarded as ornamentals that are mainly used as garden showpieces. However, in recent years interest has focused more on their value as being pollinators for apple varieties, with worldwide research directed at testing their usefulness and effectiveness for this job. So how can crab apples be used to increase management flexibility in allowing a greater choice of pollinators for apples?
Choosing the pollinator is not often a simple decision, and the following factors need consideration:
Some orchardists prefer to use a pollinating variety whose fruit is completely different from the main crop. Either this pollinator matures much earlier and is picked separately, or its fruit has a different colour. The use of Granny Smith as the pollinator for a Red Delicious block will allow for easier picker discrimination between the red and green coloured fruit, and this would avoid the two varieties being mistakenly picked and mixed in the bin. This problem could occur with Red Delicious being pollinated by Bonza. However, with a typical pollination ratio of 1:9, 10 per cent of the fruit will be of the pollinator variety, which means finding suitable market outlets for small lines of this fruit.
High-density orchard blocks have minimal surplus space and there can be problems associated with using solid rows. Usually bees work down the closer planted row, which reduces the likelihood of cross-pollination across rows, resulting in decreased yields and or misshapen fruit. Scattering pollinators through the rows overcomes this problem, but other management problems may arise, such as different thinning requirements or having mixed fruit at harvest.
For varieties which flower much earlier, such as Braeburn, using crab apples could help overcome the problem of providing a good source of pollen when no other varieties are available.
Although a crab apple flowers at the correct time and the pollen is shown to be compatible from hand pollination tests, this does not necessarily mean that it is the best one. Research conducted in the United States and at Orange in NSW has shown that bees, when out foraging apple blossoms, tend to stay with the same colour flower and fly past a different colour, especially if the blossom colours are very different, such as deep red or white with a tinge of pink. This could negate cross-pollination. Although there are reports of using a red crab apple variety in New Zealand, pollination there is carried out by both honey bees and bumble bees. We have no bumble bees in Australia, and we rely on honey bees for most pollination.
Most crab apples have quite a long flowering period due to spur flowers opening first, followed by one-year-wood flowers. Delicious varieties tend to flower only on spurs, but many other varieties flower on spurs and one-year wood. Spur flowers when set tend to produce larger fruits of more even good quality, rather than a mixture of fruit on spur and one-year-old wood, for storage as well as for grading.
White flowers with varying amounts of pink are treated as the same colour by bees. The following three are the most suitable because of flower colour and honeybee activity.
Pink flowers are more intermediate for crossing with white. Some bees will bypass the colour but others will also cross from colour to colour.
Bees tend to forage flowers of a similar colour, so different colours are less likely to be cross-pollinated.
A comparison of the spread of bloom for the major apple varieties and these crab apples is found in Table 1. These times are based on observations made at Orange in NSW, which would be similar for Batlow (NSW), but the times would be earlier by 1–2 weeks for other NSW districts. The order of flowering would not change significantly from district to district.
There are many advantages in using crab apples. The most significant is that because crabs produce such small fruit they are not easily mistaken, and there is usually no need to worry about picking or thinning the wrong variety. Also they bear on one-year wood, which means they can be pruned each year after flowering and still produce an abundance of flowers for next season. Being slightly smaller than most other varieties, crab apple trees do not need a full tree space and so can be planted at higher pollination densities as single trees within the row.
The ideal management of crab apples is as follows:
Author: Delia Dray, Jill Campbell