I'm mainly a team player, so I would say, be aware of everything that is going on around you as you are the one controlling the battle field
And I think you mean Plant/thorns? :P
@Under Control
Ok quick note I have played 2 different Dom's to lvl 10, and I have a Dom at lvl 5 and one at lvl 3. On all of these I have noticed that several things are important for low lvls.
1) Get the hold asap
2) AoE controls can be iffy. The Immobs at low lvls are morelikely to cause more aggro than they nulify where as things like the AoE sleep and and confusion are good for negating nasty aggro.
3) I prefer attacks at the lower end over controls as the quick you build domination the better.
4) Soft controls like knockdown are only useful depending on the situation. My two lvl 10 Dom's (fire/fire and mind/energy) both have air supp as there pool power at lvl 6 as I planned on flight for both due to concept. This power is very useful as for bosses it is great but for groups of minions it can get annoying.
5) Address all the targets. This is probably the most important piece of advice I can give. You must plan your attacks by considering what mobs you take out with attacks first which you hold and which you tie up. An example is one against mooks:
3 mooks 2 blckjack and one normal now the plan I decided on was that the blackjacks hit harder than the normal mook in combat so the further they were from combat the better and the normal mook I could deal with by killing first so what happend was the first blackjack was held then the second got a single immob and I then started attacking the mook. As the immob and hold recharged they were applied straight away to the blackjacks as Although it may mean that you have to wait for them to recharge before the next group that takes less time than going back to the mission from the hosp because someone you should have locked down got free and smakced you around.
Just my points based on some limited low level experience you can probably get better advice.
Have only reached level 17 with any of the dominators I've tried (Ice/Psi and focused on soft control over damage output) I'd say the 'best' way early on is a bit like this.
Open with Aoe hold/sleep/confuse
Single target hold the biggest threat in the mob
single target immob the next dangerous one (or most dangerous melee critter)
use the attacks on the minions (alot of Aoe/cone moves) building up the domination bar and once ready spam that Aoe immob for some reasonable damage
Apply Aoe sleep/confuse as needed untill all are a pile of bones.
At least thats the idea I have with mine (level 9) 
Yup have to say, I focused on holds more than attacks. After lvl 10 your damage really does start to become pitiful, and teaming becomes the way forward. If your gonna team and hope to be invited back you want to be useful. That whole crowd control and debuff thing is the way forward.
I only took 3 powers from my thirns secondary set, but I slotted them all up with def debuffs and accs. I figure, compared to the team I do very little damage, however def debuffs make them do more, and that makes me useful even if not overtly.
Thorntrops, well just like caltrops these are really useful in choke points like doorways. Recharge slot them and you can put 2/3 down at same time. If you slot them with slows and put them in same spot you can almost immobilise mobs. Again makes you useful, great if team is retreating and quite obvious.
My primary set, well I skipped the sleeps. Why? Well if your gonna team (which you will need as you go up levels) odds are someone will be using some form of aoe attack, have pets etc. So your nice mass sleep effect is now gonna get woken up by that fireball/blazing aura/death shroud/caltrop etc only thing is they are probably all gonna be mad at you.....uhoh your a squishy that cant be good.
Holds, repels, debuffs are all great. Personally I also took hover and stealth to give me some defence. I also have phase shift as an emergency panic. I also took tp foe, so I could pull mobs out for a team.
But this is just what worked for me.
My Plant/Thorns dom has single hold, area immob, cone confusion, and area sleep as controls and all the ranged thorns (so far); I slotted for accuracy, as missing a hold or immob can lead to very quick faceplanting...
I'm currently playing on the fourth level of difficulty, without too much trouble (occasionally a boss causes difficulty, such as the one who I couldn't keep held or immobilised despite hasten and domination and one of the thorn attacks causing immob...), which is two levels higher than even my mastermind plays at, so it must be doing something right
The only issue with my build is that the immob can't be used on a partially slept spawn due to waking the sleepers.
My Plant/Thorns is at L32. I find that I used Seeds of Confusion a lot (slot with acc and confusion duration) and Strangler (2 Acc, 3 Hold, 1 Recharge) at the low levels. Vines is okay, but appears to have a significantly shorter hold time than Strangler. Roots just aggroes lots of mobs onto you. Spore Burst...well, I respec'ed out of it because it just didn't work for me. Carrion Creepers is the big primary power, really (level 26).
The thorns side of things are a little weak - just play around with them to see what works best for you.
Hope this helps
I use a dominator called devil seed and my fave powers are:
Vines - awaasome some people dont like it
Seeds of confusion - use alot
Strangler- Awaasome
Creepers- obviously good
Fly trap- PLANTY!!
Impale- Awasome
Throw thorns- first attack, but fast
Aim- gotta love aim
Thorn barrage -
Also thorntrops - good for pvp and okay for pve
didn't like sleep and the others but thats my opinion.
Oh and ill say some of my battle secrets, yes i know you all wanna know em.
JUST RUN LET OUT A SCREAM AND AGGRO EM ALL. And your team will call you nuts but its okay cause they just jealous they might also call you a coward when you run away and let them take the damage but THEY JUST DONT understand at all the pure SKILL it takes.
PS, i aint drunk :P
Well as the others in this thread have said,
1- Single target hold, fill it with slots, 2 acc, 3 hold, 1 recharge is probably the best combination.
2- Use AoE Immoblise with extream care, this power attracts argo like nothing I have ever used before, the reason seems to be this:
Enemies tend to attack what ever has hit them last/for the most damage/most offten. Now the damage on your AoE immoblise is low so shouldn't cause too much? wrong, because the immoblise seems to count as a continius attack while they are immoblised (I've watched a fire/ corruptor lay into my immoblised mobs with a fireball and they still targeted me, then he used rain of fire, after that finished they still went for me 
)
3- I found the hardest part was the teens to eairly tweenties, you need to team a lot to get through these levels.
4- I the secondry set remember that your melee ranged attacks do Far more damage than your ranged. This said don't over look your ranged attacks ether.
5- Sleep is good for solo but not always so usefull in teams.
6- The AoE hold is a bit wimpy, but it is a great emergancy power when you are getting swamped or when a confuse has missed a big chunck of a group. Another good use is stack it with our single target hold to hold a boss fast.
One use I have for the AoE immoblise is to drop it on a group right after a cone confuse, this will stop they running all over the place, nicely bunched for a AoE attack and it will do some damage so as to make sure you do not get 0xp if the mob's kill each other before you can hit them again.
You can add to this trick by dropping Throntrops on them as well (an immoblised mob cannot run off the trops and the damage should start to stack up 
)
Carrion creepers is one of the best powers in the set, the damage is low, the knockdown chance seems poor BUT the vines are counted as pets so you drop this on a group and the alpha strike goes on to the vines instead of you, aloughing you to get up close to confuse them without being attacked 
On teams, still great, why send in the MM's pets when you can send in the viens they are far more expendable I've seen a MM lose half his pets in an alpha strike because he charged them in before I could drop my carrion creepers.
OK, I've made a new alt, a Dominator, she is my first dominator, and I'm a thorns/thorns dominator. What skills do I need to make a good PvE dominator?
At the moment, she's level 3 with the first two holds, the single mob immob, and the single mob hold. With one slot in each (from level 2)...
P.S. Domination rocks! ^_^