In a previous short post I showed this old hawthorn (Crateagus monogyna) bonsai. In it I recommended to frequently trim to get new growth which is absolutely correct. However, to get flowers one needs to minimize strong vegetative growth and stimulate more mature growth to form. Flowers on hawthorn only come from short spiky growth called spurs, rather than from young vigorous stalks. The spurs are short branches with a clear thorn on the end. These are the ones that, when left unpruned, can over time start producing flowers.
Inducing flowering on hawthorn bonsai
It is important to realize that hawthorns only start flowering when they are mature. This normally is after maybe 15 years. Fortunately, Yamadori and plants sold in trade are often already flowering be it because they are old (yamadori) or grafted (store). But if you find a young plant in the garden and pot it up, realize it normally takes over a decade before it is of age for flowering.
If you do have a mature specimen, it can still be difficult to get it to flower. The 3 main causes all relate to the plant growing too vigorous:
- Repotting too frequently
- Fertilizing too heavily
- Pruning too strongly
Each of these lead to strong vegetative growth, and reduces the development of flowering spurs. As such, in order to get flowers one should lay back on the (especially nitrogen) fertilizer. Repot only when really needed (leaving it without repotting for 5+ years is not strange at all) and instead of pruning branches, take the tip away from vigorous growth as soon as you see it developing.
Here next to each other the fall-pictures of this hawthorn in 2019, 2020 and 2021. In 2019 for the first year I stopped pushing for structural branches and began focusing on flowering spurs. In 2021 the first flowers appeared. And in the 2021 fall picture it is clear there are many flowering spurs on the tree, let’s hope they give me lots of flowers in spring!