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What are other good power sets to go w/plant control?
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Plant/Storm and Plant/Trick Arrow both have a lot of synergy. They leverage Seeds of Confusion well, add more damage, and add control which helps with confusion resistant enemies like Nemesis. And they even fit thematically.
Arc #40529 : The Furies of the Earth
Rad goes well with ALL Primaries, including Plant. If you have a tank teammate who can gather the foes to concentrate them, letting you cast Radiation Infection before Seeds of Confusion, the Defense Debuff lets you get just about everyone. Enervating Field and Ling Rad keeps them from doing too much damage to each other, letting you actually get more XP from the confused foes. AM helps Seeds and Carrion Creepers recharge faster, along with all the other buffs it provides. You get a self- and team- heal, one of the best Rez powers in the game, and one of the best Hold powers in the game as a great "panic button" power. Plus, you can blow up the dead bodies of your teammates if you want to. What more could you want that a little necro-explosive?
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Guide Links: Earth/Rad Guide, Illusion/Rad Guide, Electric Control
I've said it before, I'll say it again...
PLANT/KIN is awesome! My 50 Plant/Kin is just a beast, is incredibly fun to play, and with Syphon Speed and Hasten, not only is seeds of confusion up for every mob (often twice in one mob) but so is carrion creepers. Carrion Creepers gets buffed by fulcrum shift as well, so an awesome attack chain is to seeds a mob, watch them gather on each other, carrion creepers the mob, then fulcrum shift and roots them until they die.
I have played plant/thermal to 40 and plant/rad to 50. Based on that, I would say the following:
Secondaries that are primarily focused on single target or long-recharge buffs (as much of empathy and thermal are) do not present any particularly good synergy with plant, since the pet is considered quite weak and doesn't contribute nearly as much to your offensive output as many other sets' pets do. This is of course a non-issue when teaming, as you'd generally use those buffs on teammates anyway, but you won't find it nearly as useful as forting three imps, for example.
In terms of synergy with plant, you will generally be looking for sets that use (de)buffs to enhance the strong damage of this primary, that provide additional recharge, and that diversify your control options. Not surprisingly, this makes storm, rad and kin all very good choices (which go well with most or all primaries).
I can't speak too much to trick arrow as I don't have much experience with the set, but I would expect the variety of debuffs and controls to also pair fairly well.
Force field, sonic, empathy and thermal, while all worthwhile sets in their own right, are not particularly synergistic with plant. While they all provide powers that you may find useful, such as thermal's debuffs at the top tiers or the mez protection of bubbles/rings, you are likely to find many of the powers useless when soloing. If you team exclusively you probably will not find it affects you that much.
Another consideration is the amount of attention you can spend on the powers in combat, which will depend on your style. Roots can be a very strong damage option, and I certainly found it worthwhile to develop a ST chain for my plant/rad, so I found a secondary that doesn't necessarily take up too much of your time to be ideal. You may also go through endurance quite quickly this way, so anything that provides recovery is also nice (cough cough rad or kin, though empathy does fit the bill here too).
In terms of slotting the primary, I would look to the following.
The most important powers are Strangler, Seeds of Confusion and Carrion Creepers. Strangler can be slotted purely as a hold, purely as a ranged damage power, or a mixture. Get 6 slots into it when you can. Seeds of confusion needs accuracy and recharge. I would get at least 4 slots into it quickly. Confuse duration is occasionally useful but is not a priority. Carrion Creepers should get 6 slots asap. It needs a little accuracy and then as much recharge and damage as you can get. The recent fixes to the power allow sets to properly enhance all aspects of the creepers, so frankenslotting is a good choice.
Following that I would look to roots and entangle (if you take it). I treat these both as damage powers and slot them as such. Note that roots does substantially more damage than similar powers in other sets. Don't bother slotting for immobilize duration.
The remaining powers are a little more optional and depend on your taste. If you take vines it benefits from heavy slotting (usually an even mix of acc, recharge and hold, with accuracy first thanks to the lower hit chance), but do not slot this much before the higher priority powers. Spore burst is, well, spore burst, and is usually skipped. A little accuracy, and maybe some recharge or end reduction if you really love this power. Spirit tree never interested me, but if I were to take it I'd prioritize heal slotting and recharge only if it was convenient and I found myself using the power a lot. Fly Trap is a take it or leave it kinda thing; I personally found it worthwhile but I wouldn't complain about anyone skipping it. Some accuracy with plenty of damage, like most pets. I thought about trying achilles heel in it, but I didn't find the use of fling thorns frequent enough to bother.
Dark Armor/Stone Melee Tank Guide [I12]
Empathy's a great secondary for duo/trioing with damage dealers. Personally I think that's its best use (aside from all-empath defender teams, which rock HARD. You laugh.) To that end, I like it a lot with Mind. It's the first controller secondary I played to 50, and maybe I'm jaded, but I'm pretty sure it's not my favorite.
Plant control is a great aggressive/big-team control set-- probably my favorite for that purpose. The area damage boggles the mind. I like it with Rad, and LOVE it with Storm. I'm sure I'd like it with Kin or Trick Arrow, too. It might be interesting and balanced with Sonic.
Choosing a Controller V2 | Splattrollers | Plant/Rad | Fire/Storm | Mind/Emp & Mind/Rad
Weird Controller Powers | Conf & XP/Time | Controller Damage
Being a Healer | The word Necessary | Natural Concept Characters
I am in the process of re-rolling my plant / rad. I'm at 23 and just not finding it working for my solo play style. I've leveled an Ice/Storm controller and found the secondary very helpful when both teaming and solo. I'm looking toward Trick Arrow as a new secondary for plant. I like the extra control it offers and with the damage provided in Plant, I'm thinking I can get away with it's debuffs and extra perks.
Demonfest - 50 - Demon / Thermal Mastermind
Covered Shadow - 50 - Dark Melee / Shield Scrapper
I've got a Plant/Storm, and she's tons of fun. Everything gets rooted down pretty well for Tornado and Lightning Storm to wail on without knocking back, and Steamy Mist helps keep the pets alive. With IO slotting and Hasten you can keep stuff like Creepers/Lightning Storm/Tornado up all the time and really just brutalize your opponents.
Try Plant/Sonic, great on teams, great solo.
You get a huge -res power as your starting. Which makes enemies tearing themselves apart so much better. Did I mention you get it at lvl 1?? Great for that posi tf!!!! Although it is not auto like other sets and does require accuracy.
You get good shields for your teammates & Audrey and a great -res toggle which goes well on Audrey's melee-centric style.
You get a great toggle shield that gives mez protection to all inside.
Sonic can also be cheap to slot giving you more slots to put into your primary without fear of gimping your secondary. If that matters.
A lot of people shun it, condemn it, and various other things but sonic has a lot to leverage and shouldn't be lightly dis-missed.
I second plant/storm. Mine is 39 and she's pretty awesome. I've played a rad/ defender and a /rad corr and can say that rad emission works great either solo or on teams.
Hmm... plant/TA sounds interesting. Might have to try that.
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/Storm /Rad /Kin are I'd say all top contenders for plant.
Seeds of confusion/roots/ then just crank out the damage through debuffs, buffs or whatever. Creepers, get them perma as soon as possible they are amazing.
For very clicky secondaries like empathy I usually like a more passive control set like Illusion.
To be a great team player plant/therm is a good way to go. You can shield your team, confuse every spawn at least once, maybe twice if the fight is long, and heal your team mates. It just depends on what you want. If you want damage though, plant storm would rock, with tornado and lightning storm and creepers and your pet, which is pretty decent with single target damage now in my opinion. It would also give you a lot of control.
Wait a bit. Roll a Plant/Cold. I'm going to reroll my own Plant/Therm as a /Cold. Nothing wrong w/Therm, but /Cold works better w/Plant for stuff like Snow Storm (for the -fly) and Sleet. Sure, you can do the same w/Storm, but Storm doesn't have Heat Loss. 
An Offensive Guide to Ice Melee
Actually I think Plant would be one of the better sets to play with Empathy. If I were making the character, then I would go with Plant/Storm. I also have a baby plant/Kin that's pretty fun but pre-stamina I am having serious endurance issues.
As for slotting your Plant powers for teams:
Seeds of Confusion: 6 slots immediately with accuracy, recharge, and confuse duration.
Strangle ( single target hold ) 5-6 slots for accuracy recharge, and hold.
Roots: 6 slots for accuarcy, recharge, damage and enduracne reduction.
Creepers: 6 slots 1 acc, 2 damage, 3 recharge.
IO frankenslotting and later slotting sets helps. If you play your plant part right, you shouldn't have a lot of need of your Empath heals, and can put your empathy slots into buffs instead.
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I've been taking my plant/kin to hazard zones until 20 when I plan to AE blast it to 50 (yaya I've played plenty the hard way to 50 already thx).
I swept Perez and am now sweeping Boomtown. Seeds+roots is way too easy. So far it has been faster than my fire/rad doing similar.
Plant (so far) doesn't even need a secondary. In that regard it is kind of like mind, but this is against team sized spawns and pumping out decent aoe damage.
I think offensively plant will pair very well with kin, rad, storm, cold and TA. So really I think you'd be safe picking w/e you enjoy the most.
The reason I personally picked /kin is because it will be really easy to pl 
and I've recently run a storm troller and two TA's. I also wasn't in the mood to run another /rad, which I've played to death over the years.
I recently created a plant toon with empathy. I havent got far enough to say "yay" or "nay" but I was wondering:
Is emp a good secondary with plant control?
What are other good power sets to go w/plant control?
How should I slot my troller powers (team player)?