Autumn blaze maple trees - too many?

Question

I am based in Melbourne, Australia.

I just moved in to a new property and there are 20 young autumn blaze maple trees in the small backyard, planted 1m apart from one another - some on the garden bed of timber retaining walls, some in front of the retaining walls. They would have been planted there the past 2 years.

I have read that these trees do grow big!

My questions are:

• If they are planted so close together, would they then restrict one another's growth, like how plants are grown in pots?
• Is it possible to restrict these maple trees' growth effectively over time or would it be wiser to look at rehoming some of these whilst they are young?
• If I continue to keep them, would I be risking damage to my timber retaining walls with its root system?

Thank you in advance for all your opinion and advice.

Answer 1

Depending on how much shade you want, one Autumn Blaze would be fine for a small yard (15m x 15m); two or three would probably be fine in a 30m x30m yard. Any more than that is a forest, and one that won't be healthy, either, due to intense competition.

My recommendation is to re-home as many of your trees as possible, as soon as possible. Given that Autumn Blazes tend to suffer from chronically bad crotches and rampant branch growth (especially when young), you'll also want to spend some time each year removing poorly growing and excess branches for at least the next 5 years. The bad crotches (anything < 40 degrees from vertical or so) mean that the tree is prone to split or drop branches in storms. These trees can also get multiple leaders, which also can cause splitting years down the road. Pruning will resolve these issues.

As for your timber retaining walls, any tree planted closer than 4-5m could indeed damage them, but not for many years yet.