That's a pretty hard level. Just keep trying till you get a lucky break in placement or enemies' choices. It also helps to steal your enemies' asteroids in the beginning. They start with fifty seedlings, spend thirty on trees, then send the other twenty to neighboring asteroids, and then you send yours to snag the undefended but fully built one. Go for the least-defended and try to get the good ones near the beginning, because they will take upwards of a hundred seeds in the late game after they get proper defense. This is a pretty densely packed level, so the standard land grab starting strategy won't work too well.
Aggressive use of laser mines helps a lot too. It never seems to go quite right for me, but if you can take out an enemy flower with your mine, you get a definitive advantage.