I think the planting density for cacti in a growing zone is broken. All other trees need space between them, but cacti can be planted as densely as normal crops. (Hella OP, for desert playthroughs. ;) See attached screenshot.
[attachment deleted due to age]
Looks like you missed a <blockAdjacentSow>true</blockAdjacentSow>
tag in the cactus definition. ;)
Or it is intended, since cactus don't grow that high like a tree.
But this only a dev can tell us.