Crabapple Pollenizers for Apples
Table of Contents
The commercial apple, Malus domestica (Borkh), is considered to be self-unfruitful. As a result, all apple cultivars (varieties) require the pollen of a different cultivar for cross-pollination in order to set a commercial crop of fruit. Apple cultivars such as Creston, Crispin (Mutsu), Rhode Island Greening, Jonagold and Spigold have a third set of chromosomes (triploid) and produce sterile pollen. These cultivars will not pollenize each other or any other cultivar. A pollenizer must be provided for these cultivars. If a commercial apple cultivar is used for this purpose, a second pollenizer cultivar should be provided for the first cultivar pollinating a triploid one.
Apple pollen grains are relatively large and, as a result, wind does not play a significant role in pollination. Pollen transfer in apples is carried out by several species of insects. Native wild bees, solitary bees such as the bumble bee, flies and other species can play a significant part in pollination. Honeybees are especially useful in pollination because of their social nature. Because honeybees live in hives containing thousands of individuals, pollen can be transferred from bee to bee in the hive as well as directly from flower to flower in the field. Some estimates suggest that a large percentage of the pollen exchange that takes place from honeybee activity occurs in the hive rather than in the orchard. Pollenizer arrangements based on hive transfer of pollen make it imperative that high honeybee populations be available each year to accomplish the pollination task.
Introduce strong hives of bees into the orchard at bloom time to maximize the opportunity for good cross-pollination and fruit-set. A good colony will satisfactorily pollinate up to 3-4 ha of young trees. As trees grow and fill in their assigned space, increase the number of colonies to 3 hives per ha or more.
Since an economic crop of apples depends on successful cross-pollination, it is important that a sufficiently large source of compatible pollen be present in the orchard.
Crabapples are generally heavy bloomers with abundant pollen; several crabapple species can set fruit on commercial apple cultivars. Pollen-compatible crabapples (i.e. cultivars that can set apple fruit) can be used as an alternative or additional source of pollen for large apple blocks where inadequate pollination was provided. This approach can also be used for single-cultivar plantings or triploid cultivars to maximize production. If pollination is in doubt, crabapples can be planted as insurance to supplement the normal pollination provisions in an orchard.
A major advantage of using honeybees for apple pollination is that the key factor in successful pollination becomes the amount of compatible pollen available. The actual location of the pollenizer cultivar(s) in the orchard becomes less important.
Crabapples can be interplanted between regular trees (Figure 1) in the orchard. Planting a compatible pollenizer crabapple tree in the row between apple trees with no additional space for the crab tree initially appears to be satisfactory. However, matching the vigour of the crabapple to the orchard cultivar is difficult, and if no space is assigned to the crab tree, it may either overgrow its space or be grown over by the regular tree on either side. Further, if no space is assigned, then the pruning of the crab tree may be especially heavy to create an unnatural columnar shape. This approach has not decreased flowering on some crabapples, but may not work with all types. Scattering crab trees throughout the orchard by interplanting means that caring for those trees is more difficult. They must be sought after in order to be pruned. Since crabs are normally pruned just after bloom, this creates a nuisance job just when other demanding orchard operations, such as pest control, are reaching a peak.
Figure 1. Crabapple tree planted in the row between apple trees with no additional space alotted for the crabapple.
One alternative approach for planting crabapples is to place them in specific rows assigned to them in the orchard. Another attractive option is to use crabs in hedgerow plantings around the orchard perimeter. Either approach permits mixing several crab cultivars together in the row, keeps the crab pollenizers out of the rows of apple trees themselves, permits mechanization of their postbloom pruning, and allows their use as windbreaks. Perimeter plantings of crabapples would provide a large number of trees and should be designed so the trees can still be sprayed for pests and diseases. There are many creative options available to the innovative grower that can both increase productivity and beauty of the orchard with the use of crabapples.
With the new concept of pollen transfer in the beehive a major component of pollen distribution in the orchard, the main idea with crabs, or with any pollenizer for that matter, is to have a sufficient number of them so there is a large volume of compatible pollen available when the cultivars to be pollinated are in bloom. There is no fixed figure on the appropriate number of pollenizers to have for any cultivar. It has been estimated that there ought to be a minimum of 20%-25% pollenizer trees in any orchard. The smaller the percentage of crab trees relative to apple trees, the more dependent the grower becomes on the satisfactory annual performance of each pollenizer tree.
Most crabapples exhibit some degree of bienniality in flowering. This trait usually does not become obvious until the crab tree is 5-6 years old and has developed a fully-fruiting canopy. This characteristic is one reason for using several crab cultivars in the pollenizer planting rather than relying on only one crab cultivar to do the entire job. The whole purpose of having crab trees is to obtain annual, heavy bloom. Mixing several crabapple cultivars reduces the likelihood of insufficient bloom. It is extremely important for growers to understand as much as possible about which crabs are biennial and which are more nearly annual.
Experience is lacking in managing cropping in crabapples to overcome bienniality. It is not known for sure that defruiting crabs after bloom will produce copious return bloom the following year. Chemical thinning of crabs has not been thoroughly investigated and virtually nothing is known of the potential for successful stimulation of return bloom by this approach. Growers should plant several crab cultivars and have a high percentage of crab trees to assure annual bloom in sufficient quantity to do the pollination job each year.
Crabapple cultivars vary tremendously in vigour, just as do regular apples. Variability of tree size is seen even when crabs are on dwarfing rootstocks. This fact makes it more difficult to manage crabs interplanted between regular trees in the orchard. In separate hedgerows or rows, the variability in vigour is not as much a problem, and can be dealt with by means of mechanical pruning.
Crabapples vary enormously in growth habit, just as do regular cultivars. The preferred growth habit is upright, in which the crab tree naturally assumes a columnar canopy shape. This canopy shape requires less space and is easier to manage. The spreading and/or weeping types should not be excluded on this basis alone, but will be more effective in planting systems where they can be mechanically pruned. The bloom dates for crabapples (Table 1) must be selected to coincide with the bloom of the main cultivars.
To select crabs with potential for any given cultivar, pay careful attention to bloom dates and use a series of crabs that bloom over the period the main cultivar is in flower. Peak pollination should be aimed at the king-bloom stage.
Crabapple blossoms come in colours ranging from pure white through shades of pink to deep maroon. Studies of bee activity indicate that bees identify particular nectar or pollen sources by colour, and will continue to preferentially visit that source. It is logical to presume that the same preference behaviour will be active when bees work trees. Thus, it is in your interest to have pollenizers whose flowers appear as nearly like those of the apple cultivars to be pollinated as possible. The majority of crabs used for pollenizers should be of the white flower type. Some red-coloured cultivars can be included in a mixed planting when those cultivars provide beneficial characteristics for the block.
Table 1. Average Full Bloom Dates for selected crabapple cultivars compared to average full bloom dates and estimated duration of bloom (20% open to petal fall) for selected apple cultivars, Horticultural Research Institute of Ontario, HES, Simcoe (between 1981-91)
( ) years of data to support bloom date calculations
[ ] average full bloom date
----- estimated duration of bloom
Since the objective with crabapples is to have the maximum amount of bloom, delay pruning until after flowering. Prune as soon as sufficient pollination has occurred, which means you can start at petal-fall. Prune as soon after bloom as possible to permit maximum regrowth during the growing season. With crabs, the pruning precautions preached for regular cultivars do not apply. The pruning objectives with crabs are to control canopy size and shape and to stimulate new shoot growth. Hence mechanical shearing is a good pruning approach for crabs. This represents another reason for having crabs in separate rows, so their different management requirements can be accommodated.
Most of the crab cultivars suitable for apple pollination will flower on 1-year-old (last year's) wood as well as on spurs, and will do so even with fairly heavy pruning. The production of lateral bloom is dependent both on crop load and on environmental conditions. A heavy crop on the spur portion of the crabapple canopy can suppress, and sometimes virtually eliminate, the development of lateral bloom. Similarly, a heavy set on lateral bloom alone can significantly reduce return bloom. These responses are cultivar-dependent, which represents another reason for mixing crab cultivars in a pollenizer planting system.
Little is known about the susceptibility of crab cultivars to various pests and diseases. Crabs are notorious for being sensitive to fire blight, but there are considerable differences in fire blight resistance among pollenizer crabapple cultivars. Trial plantings have not shown any major devastation due to fire blight so far. Some crabs are susceptible to other diseases and there are some large-fruited crabs which are probably more likely to harbour insect pests. Because little is known about the specific susceptibilities of crab cultivars to pests, it is recommended that all crab planting systems permit them to be sprayed as necessary for insect and disease control. Because most crabs are quite sensitive to apple viruses, they should not be simply grafted to existing trees in the orchard because of the risk that they will be lost to viruses or inadvertently pruned out during the dormant pruning program.
Among the crab cultivars rated as suitable for pollination purposes, fruit size varies from very small to 2 cm or greater in diameter. Large-fruited cultivars are disadvantageous because of the need to protect the fruit from pests and because of the possibility of the fruit becoming a food source for voles (mice) during the winter. Many of the pollenizer crabs do not drop their fruit in the fall. This is a desirable trait, as it keeps the fruit off the orchard floor. Others drop all their fruit in the fall, thereby creating a food source for voles. These are additional factors that should be considered when choosing crab pollenizers.
There are numerous crabapple cultivars representing several Malus species that can set fruit on commercial apple cultivars. Determining the appropriate crabs to use is primarily a function of bloom date, but other characteristics may also influence the grower's choice.
Table 2 contains some observations and comments on the crabapple selections evaluated at H.E.S., Simcoe, between 1981 and 1991:
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