I was actually thinking of Kelgore's Grave Mist, but didn't think it was on the Druid list...
You don't need to change ANYTHING. Unless your DM allows some really awesome splat feats, all you can really do for your spells is up the save DC a little or increase CL against SR (neither an issue for Spike Growth, and barely an issue for Entangle). The only thing you need is more spell slots. Which means money. Scrolls. Maybe pearls of power, but those are still pricy at your level. Wisdom +2 item if it'll nab you another 2nd or 3rd level spell per day. The only thing limiting you right now is your spells/day (and actions/round, of course, but that's a lot more expensive and "DM wrath triggering" to increase...).
Typical Druid either gets summoning feats (Spell Focus Conj, Augment Summoning, splat book overpowered summoning feats like Greenbound Summoning...) or melee feats. Whichever you have, they're equally not useful here. Fighting an army isn't a "normal" D&D encounter. You can't realy be expected to build towards that, and I'm not sure there's many great feat choices to build towards it with. Only thing that immediately pops out ot me is Sculpt Spell, but that's only good on spells that don't already have massive areas of effect, and it increases the spell level by 1. Since your high level slots are so few and precious, the only reasonable way to get use out of sculpt spell right now would be a lesser metamagic rod (3000 gp to use it 3/day on a spell level 3 or lower) of it.