This article focuses on the subject of encouraging Hawthorn bonsai to flower, however, the basic principles can be applied to flowering bonsai of all species.

Maturity and Reproduction

All trees are genetically predisposed to be dominant over surrounding trees and plants in an effort to reproduce. The most successful specimen (of any one species) are those that are able to outgrow their neighbours in an effort to gather as much light, water and soil-space as possible.

Once they have grown to their fullest extent (as high and as wide as they are able, given their local environment and circumstances, (whether that be 100 metres or just 1 metre tall) they then begin to try and reproduce themselves by flowering and spreading their seed.

These two different phases, of gaining maximum height and then of seed-production, are known as immature and mature growth.
In the immature phase of growth, a tree will put out predominantly (or exclusively) vegetative growth in order that they can ‘outcompete’ its neighbours by growing as tall and wide as possible, commonly known in most tree species as apical dominance. This is a pre-disposition and it does not matter whether the tree in question is growing wild or in a bonsai pot.
Conversely, during the mature phase of growth, the tree reduces the amount of energy put into growing new vegetative shoots and begins to try and reproduce by flowering.
This mature or flowering stage of growth is triggered when the tree can no longer spread upwards or sideways and continues the process of dominance by trying to reproduce itself.

These same events occur with a bonsai; a bonsai will continue to grow vegetatively in an effort to grow taller and wider until such time that it is unable to grow any bigger and begins to enter a mature, flowering phase. Ergo, in order to encourage a bonsai to flower, first it must be encouraged into maturity.

Encouraging a Hawthorn to flower

Trees need to reach a certain age before they will ever begin to flower. The age varies according to the exact species; some species will flower after just a couple of years, while others, such as Hawthorn need to reach 15-20 years of age before they will begin to flower.

Feeding regimes high in phosphorous can help encourage more flowers on a tree that already produces flowers each year, (do not feed high nitrogen as it will encourage vegetative growth), but will not make a tree enter maturity and start flowering.

The procedure is first to gently slow the vegetative growth by allowing a Hawthorn bonsai to become on the rootbound side, if repotting and root pruning is absolutely necessary (for the health of the tree), only root prune lightly. Newly available space around the rootball encourages new root growth and therefore new vegetative top growth. A confined rootball dissuades the tree from trying or being able, to spread itself and remain immature.

Do not prune the tree hard, reduce the trunk or remove heavy primary branches. Doing so will result in vigorous vegetative growth, very much at the expense of flowering. New collected yamadori (wild trees) that may have flowered well for many years in the wild, will frequently stop flowering for many years after being chopped or pruned hard, until they re-enter a mature-growth phase. When a previously mature tree is in a vegetative/immature stage of growth, existing flowering spurs will simply open a rosette of leaves in the Spring but will neither flower or extend. Occasionally they will produce a vegetative extending shoot but this cannot be relied upon when trying to develop the branch structure of a bonsai. Bonsai that are still having their branch structures developed or are poorly ramified, should be ‘completed’ before encouraging flowering.

(On bonsai species that flower easily, the reverse is true, remove flower-buds to encourage more vegetative growth on trees in development)

Once the vegetative growth of a well-ramified and pot-bound Hawthorn bonsai slows down, pruning to contain the size of the tree naturally becomes more gentle. The tree begins to produce a new type of shoot that contains flowering ‘spurs’. This is mature growth and is subtly different than immature, vegetative growth.

Two new shoots on a Hawthorn bonsai during the Summer. On the left a mature flowering spur, on the right, a vegetative shoot carrying just leaves.

Flowering shoots on a Hawthorn will have a thorn at its tip, as shown in the image above. These shoots should not be pruned, if possible, otherwise they may become vegetative. Flowering shoots that are left unpruned will produce flowering spurs from which flowers will emerge sometime in the future. Unfortunately with Hawthorn bonsai, encouraging flowers can take a few years to achieve. The thorn itself should not be removed for the same reason.

Importantly, vegetative shoots (without a thorn at their tip) should be pruned by pinching out their tip as they extend to stop them becoming too long rather than allowing them to extend fully and then pruning them back (which encourages further vegetative growth).

A flowering spur on a Hawthorn beginning to open in Spring and revealing a cluster of flower buds…….

4-6 weeks later, the flower buds are about to open……..

The Hawthorn bonsai finally in flower

Some of the research was initially a bit confusing, as bonsai are usually trimmed all year to maintain the shape of the tree, and one source recommended only pruning flowering bonsai in the winter. The article I have posted today, at least explains it a little better.

It has been suggested that the best course of action for all flowering bonsai, is to find out whether they bloom on new growth or old, as this can differ, depending on the species. That way you can at least try to keep them in shape.

So far so good, now I need to research crab apples…