Hi! I know horse chestnuts are not ideal for bonsai but is there any way to decrease the size of it's leaves? Also is it possible to defoliate?

Defoliating will only temporarily reduce leaf size, it will take many years to reduce the leaf size. A friend of mine had one and it was about 20 years to get the leaves down in size, and even then keeping them small was difficult.

Thanks for the info I guess I will be in for a bit of a wait

Yes. Using proper bonsai techniques leaved reduce well (But still remain larger than for most).

Once you have the main trunk and branches, bud-removal in winter is a common route towards refinement in chestnuts.

Leaf pruning involves removing some of the leaves from the tree in order to stimulate the growth of smaller, more compact leaves. You can do this by simply pinching or cutting off some of the larger leaves with a pair of sharp scissors.

As for defoliation, it is possible to defoliate horse chestnuts but it can be a bit risky. Defoliation involves removing all of the leaves from the tree in order to encourage the growth of new leaves. This technique can help to reduce the size of the leaves but it can also put a lot of stress on the tree, especially if it is done too frequently or at the wrong time of year.

Yes, you can reduce leaf size. In order to do so you have to ratify branch structure. So, here is step by step:

- In the late winter/early spring you remove all main single buds. They will be swollen and sticky with sap at that time.

- two opposite buds that were below the main bud will start producing opposite branches. Let those opposite branches grow. Each one will produce two, or more sets of leaves. Allow those leaves to mature somehow. By now it will be probably end of May/ early June.

- prune newly created each branch to just past the first set of leaves.

- defoliate that pair of leaves.

- keep your tree in light shade.

Repeat this process year after year. After a few years your tree will develop many branches. The nodes will be reduced, and so will leaf sizes. At this stage you will be very satisfied with your tree leaves, However, some of the leaves will have the tendency to grow large. Defoliate them as the spring and summer progresses.
Of course, you will have to prune and shape your chestnut tree to keep it in satisfactory shape.
This method works very well for me. I’m located on Vancouver Island.

I just want to add that mid/end of spring defoliation is a good idea in more way than one:- after defoliation you can see well the branch structure. That helps to decide which buds and branches to prune,- defoliation encourages new branch growth.Of course, I would not recommend defoliation process after repotting.

My elms and maples will develop a branch with shirt internodes and small leaves and at some point stretches the internodes and increases leaf size. I cut right there.