Importance of pollination
Most apple varieties are not self-fertile, and will not set a full crop of fruit without a compatible polliniser, particularly if flowering is sparse, or weather conditions are adverse. Even varieties that do have some self-compatibility will produce a much better crop with a different variety acting as a polliniser.
Successful pollination, and the formation of many healthy seeds within the fruit contribute to the eventual size and quality of that fruit. Poor pollination results in reduced yield and misshapen fruit. Consequently, when planning a new apple block it is important to include a suitable polliniser variety.
Commercial varieties can make suitable pollinisers if they have compatible pollen and good flowering overlap. Crab apples can also be considered as pollinisers, particularly where a second variety is not desired, or the use of another variety will make complicate orchard management.
Successful cross-pollination depends on several factors, all of which need to be considered when designing and managing an orchard.
Overlapping flowering
The selected polliniser must have an overlapping flowering period (see Figure 1). While two varieties may be compatible, if the flowering period for one is too early or too late for the other, successful cross-pollination will not occur.
Most apples require a certain number of hours of winter chill (below 7.2ºC) to break dormancy in spring and then flower, and requirements differ. Variation in winter chill from season to season can affect the degree of flowering overlap, particularly where the chill requirements of the two varieties are different. Select a polliniser variety with similar chill requirements to help ensure that flowering overlap is consistent from season to season.
The use of dormancy-breaking chemicals can affect flowering time, usually by shortening the flowering period and making it earlier. Consequently, this effect on flowering overlap should be considered when selecting a polliniser.
Pollen compatibility
Most apples will pollinate other varieties but there are some important exceptions to this rule.
In general, varieties that are closely related (that is, with similar parentage) are not the most effective pollinisers for each other. For example, Cripps Pink (which produces the Pink Lady™ apple) is only a fair to average polliniser of its parents, Lady Williams and Golden Delicious, and its sibling Cripps Red (which produces the Sundowner™ apple).
Varieties that are sports (derived from mutations) of the same parent variety are considered to have incompatible pollen and cannot be used to pollinate one another. For example, Royal Gala will not pollinate Gala or Galaxy; Naga Fu No 2 (Red Fuji) will not pollinate Fuji.
Triploid varieties such as Mutsu and Jonagold have sterile pollen and cannot be used as pollinisers. Where a triploid is grown as the main variety it is not uncommon to have two polliniser varieties. This ensures that an economic crop is obtained.
Environmental conditions
Apples, like many fruit trees, are reliant on insects for successful pollination. Favourable environmental conditions over the flowering period are essential to achieve good pollination. Where weather conditions are poor through prolonged rainfall and low temperatures, insect movement is limited and pollen viability also decreases. The use of well designed natural or artificial windbreaks can improve the orchard environment for beneficial pollinator insects.
Pollen-transferring insects
For consistent pollination across the orchard to be achieved there must be sufficient numbers of pollen-transferring insects, the most significant being honey bees. If pollination has been poor historically, or feral honey bee numbers seem low, it may be wise to bring in hives from a commercial beekeeper.
If you plan to bring in hives it is advisable to arrange this with the beekeeper well in advance, allowing suitable hives to be prepared. For more information about bees and availability of hives contact the Pollination Association of Western Australia, telephone +61 (0)8 9572 6116.
During the flowering period it is essential to avoid applying chemicals that will harm bees and other pollinating insects. Always choose bee–friendly chemicals where possible and ask your local chemical supplier about suitable bee–friendly chemicals at the time of purchase.
Distribution of pollinisers
A number of options are available when incorporating pollinisers into the design of a new orchard. They can be included in the following ways:
One in nine method
This is where pollinisers are included as every third tree in every third row in both directions (Figure 2). This method insures that every tree is bordered by one polliniser tree.
Full row method
This is where full rows of polliniser varieties are planted next to the main variety. Some examples are: every second row, one row in three, one row in five or two rows in six (Figure 3). The number of rows between polliniser rows will largely depend on the orchard density, the number of bees working in the orchard and the crop-setting characteristics of the variety being pollinated.
If pollination is a problem in an existing block, consideration should be given to re-working pollinisers (to suitable apple varieties or crab apples) into the block using the one in nine method. Re-working is where a new variety is grafted on to existing trees. This can include grafting entire trees over to the polliniser variety, or simply adding limbs to existing trees, usually at the top.
Crab apples as pollinisers
Crab apples can be used as an alternative to apple varieties as pollinisers because they flower profusely and produce compatible pollen. They also have some advantages over commercial varieties as pollinisers because they:
- are not easily mistaken by pickers for the main variety, which helps prevent accidents and unnecessary work at hand-thinning and harvesting
- have an upright growth habit, enabling them to be inter-planted in rows in some situations
- generally have a long flowering period
- flower on spurs and one-year old wood. This means they can be pruned each year without fear of a severe reduction in the level of flowering.
Research in the United States and at Orange, New South Wales, suggests that bees tend to prefer flowers of the same colour when they are foraging and have a tendency to fly past flowers that have a very different colour. It is therefore advisable to select crab apples with a similar flower colour to the variety you are trying to pollinate.
Contact your local fruit tree supplier to determine what crab apple varieties are available, their flower colour and flowering times.
