It might surprise you that most trees aren’t grown from seed, but are instead produced from scions, which are grafted onto rootstocks. Trees are produced this way for two reasons: most varieties do not grow well on their own roots and most species do not produce seeds that are true to type, but an entirely new variety, so they have to be propagated via vegetative cloning. This is because many species, such as apples, have to exchange chromosomes to produce seeds, just like humans.
Scions are small cuttings of one year old wood (twigs), which are taken from a mother tree. Mother trees are among the most important assets in any nursery from which hundreds 3-4 inch scions are taken and then grafted onto rootstocks, producing hundreds more of the same variety. These trees have to be in tip top health, so are well looked after. Infection by a virus would spell disaster as it makes grafting ineffective.
Grafting refers to the act of attaching a scion to a rootstock. In order for a graft to be successful, a scion and stock’s cambium must be in alignment, which ensures quick establishment and effective union. The cambium is a continuous layer of cells, which in stems, produce wood towards the inside and bark towards the outside. Thus once aligned, the two stems can begin to merge, allowing nutrients and water to reach the scion from the rootstock.
Rootstocks are plants, usually of the same species, which are selected for their interactions with the soil. In most cases, rootstocks are selected for their reliability, which includes such factors as drought, pest and disease resistance, and cold hardiness, producing rootstocks suitable for climates as extreme as Siberia and the outback. In the UK, neither cold hardy, nor drought resistance rootstocks are necessary, so breeders look for pest and disease resistant rootstocks.
Otherwise, rootstocks are selected for their effects on a tree’s size, which is determined by the effectiveness of a rootstock’s roots in gathering and channeling nutrients and water. Rootstocks are thus categorised into such categories as vigorous, semi-vigorous, semi-dwarfing and dwarfing, with vigorous rootstocks producing large trees and dwarfing rootstocks producing small trees.
Dwarfing rootstocks’ roots are ineffective for a whole range of reasons. Their root systems spread out over a smaller area (1-2sqm as opposed to 10-12sqm on full sized trees) and possess a smaller feeding depth. Their roots are brittler, narrower in diameter and less efficient at channeling nutrients towards the top part of the tree. This is because their transport vessels feature fewer, narrower, more misaligned cells and are sometimes partially blocked by lignin. They also produce and transport less growth hormones, which play a profound role in promoting vegetative growth.
They produce and store less carbohydrates over winter, which is used for early spring-summer growth. A tree will always put more resources into fruiting (it wants to spread its genes) at the expense of leaves, shoots and roots. One study compared carbohydrate distribution in an apple tree on dwarfing M7 rootstock with an apple tree on a seedling rootstock (its own rootstock). The effects were profound, with the 3.1m dwarf tree putting 75% of its carbohydrates into fruit production, and the 8.4m seedling putting slightly less than 50% into fruit production. Thus, dwarfing rootstocks are significantly more efficient and should be first choice for anybody interested in fruit production.
In many cases, trees are simply too large for the average garden and thus dwarfing rootstocks are preferred. For orchards, dwarfing rootstocks are preferred as they produce fruit earlier on smaller trees, which are easier to manage and harvest.
So how does this process work? A nursery will have fields of clearly marked rootstocks. For apples this might be M9, M26, M27, M1106, which each have different interactions with the soil. They will have a mother tree of each variety to take scions such as Cox, Bramley and Egremont Russet, for example. Come winter, hundreds of scions will be harvested and come spring they will be grafted onto various rootstocks, producing very dwarfing Cox trees, semi dwarfing Cox trees and so on.
After this, trees will be fertilised and watered for optimal growth, and pruned to promote maximum flowering and fruiting. Some will be trained as to produce cordon trees and the like. They will be sent only once they reach a minimum height, dependent on what type of variety is growing. The fast-growing eucalyptus may have a lower minimum height than the slow growing oak, for example.
Interestingly, grafting is not a modern introduction, but goes back thousands of years. Indeed, we have an example from antiquity, whereby Alexander the Great sent back a dwarfing apple rootstock from Persia back to Macedonia. Grafting doesn’t have to be used to add a variety to a desired rootstock, but can be used to add multiple varieties to a tree. Today, you will see trees marketed as “family trees”, which produce Cox, Bramley and Egremont Russet apples on one tree.
Not all trees are produced through grafting as some species grow well on their own roots, are difficult to graph, or a suitable rootstock hasn’t been developed. In general, fruit trees, such as apples, have the best selection of rootstocks as they are so important to the economy.
When browsing for trees, you’ll see many newly introduced varieties, and in some cases new rootstocks. New varieties are produced by through traditional breeding techniques, mutation breeding and more recently genetic editing.
Traditional breeding might involve putting two varieties together in the hope they will produce offspring with the best traits of both. Mutation breeding involves bombarding trees with radiation to speed up evolution, while genetic editing is more precise, involving splicing genes responsible for a certain characteristic, such as colour in fruit, for example. It is with the introduction of the powerful gene-editing tool crispr that you are likely to see more and more unique varieties introduced in the near future.
Jorge works in the Primrose marketing team. He is an avid reader, although struggles to stick to one topic!
His ideal afternoon would involve a long walk, before settling down for scones.
Jorge is a journeyman gardener with experience in growing crops.