Yes you can siphon a planted tank. Try not to disturb the plant roots though. I siphon all the open areas thoroughly, stirring up the gravel as much as possible, then just lightly hold the siphon over the plants, gently pressing down as far as possible without damaging the leaves. How far this is depends on the type of plant. It is always better to only lightly siphon around the plants anyway as they feed on the muck that gathers around the base of each one.