Michael Bell wrote in
. uk:

Baz

The Birches are closely related to the Alders - both Betulacae. It is
a native species, Alnus glutinosa. My need is to get them up to 3 mm
diameter stem size by the end of May, that's what I need the light
for, and therefore the drying out risk.

Michael Bell


Michael,
If you need to you can have a "feel" to decide if they have enough water
and act accordingly. All native UK plants need water. I have a gut feeling
that I have not understood you properly, and am still unsure why you need
to have them out by the end of may, and be hardened off! They grow
naturally in extreme conditions in the UK. No extra lighting is required by
them.

Baz