Horse Chestnut advice please?

[DiscoBeat]

We planted a horse chestnut in our garden around 3 years ago and it's very healthy and growing well. But it hasn't flowered yet. Anything I can do to help it?
If it's relevant, it was given to me by someone who had 'bonsaid' it by keeping it small in a pot and trimming the roots. I have been repotting it and letting it grow normally until planting it out.

It may just be a matter of time, although it does look to be of flowering size

Yes it is quite tall so I was rather hoping for flowers by now!

The word 'bonsai' jumped out at me there!

In the ground, it has got to start believing it is a mature tree, and it won't flower until it thinks so. You can help it along by giving it tomato fertiliser from mid-June onwards for a couple of months, which will ripen the branches ane enourage buds for next year. That would be better than an all-round fertiliser, which would just encourage large leafy green growth.

Now it has been in the ground several years, it will be established and will really start to take off.