The slow but sure method is to produce seedlings until you're close to the planet cap, then swarm the nearby planets to build seedling factories on them too. Try to always keep plenty of seedlings on each threatened planet, they are better at defense than the defensive trees, and even serve as a deterrent against enemy attacks. Your planets will be a bit vulnerable in early game, but this method gives you an enormous armada later on. I usually try to make the majority of my trees to be producing trees, not defense ones. Planets far from enemies can even go with zero defense trees.