Is there a particular formula for calculating planting distance between trees? Are you meant to plant both of them so the limits of their spread are barely touching or can they go closer?
Given mature width of 14m for both trees it would be around 7m but the birch will reach 4 times the height off the crab apple when mature so you could go a few metres closer. Closer still if you aren't in your 'forever home'. It also depends on whether they will be in a line or one behind the other.
It depends. Do you want them just touching, happily mingling and overlapping or standing as separate trees whose forms you can easily see and admire.
You can check the eventual height and width or many trees on the RHS website. The malus will get to 8m wide in 10 to 20 years as will the betula tho this will also be much taller. Anyway, for just touching you need to plant them 8m apart, closer for mingling and further for keeping them distinct.
Vendée - 20kms from Atlantic coast.
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Closer still if you aren't in your 'forever home'.
It also depends on whether they will be in a line or one behind the other.
You can check the eventual height and width or many trees on the RHS website. The malus will get to 8m wide in 10 to 20 years as will the betula tho this will also be much taller. Anyway, for just touching you need to plant them 8m apart, closer for mingling and further for keeping them distinct.