> Does anybody know what frequencies are used by food plants? I
> understand that some frequency of UV is used and I have been told
> that some other frequencies may be used.
A very wide range of frequencies is used, IR thru UV, without much
distinction between food plants and others. The hydrogen UV emission you
mention is not a particularly significant direct contributor to the
overall solar spectrum as received on earth.. it is very broad band.
Since I am in your local area I will try to give you a ring tomorrow with
a reference or two.