Lots of pruning...

[JusticeZero]

In a campaign where the characters will be fighting plant type creatures a lot... Like, a lot, much like players in a "zombie horde/vampire scourge" type game loaded with clerics and paladins fight undead...

What sort of things can I do to help certain classes to not be made ineffective? Who is going to be hit the worst?

The plan was a low magic game (deeply restricted access to spellcasters and spellcasting, but unrestricted magic item access).

[Doctor Despair]

What sort of things can I do to help certain classes to not be made ineffective? Who is going to be hit the worst?

The plan was a low magic game (deeply restricted access to spellcasters and spellcasting, but unrestricted magic item access).

So I'm hearing rogues are going to be very sad. No sneak attack AND no magical items to UMD? Going to be a hard time.

[JusticeZero]

They can get items to UMD, but there's a lack of list casters. Are plants immune to critical strike? I didn't see it on the template.

[ShurikVch]

They can get items to UMD, but there's a lack of list casters. Are plants immune to critical strike? I didn't see it on the template.
Yes, they are immune - Plant Type (https://www.d20srd.org/srd/typesSubtypes.htm#plantType):
Not subject to critical hits.

Vine Strike spell (Spell Compendium) - 1st-level spell for Druids and Rangers - allow to Sneak Attack (but not critically hit) Plants

[daremetoidareyo]

Yeah, homerule up a knowledge nature check to identify a plant ctitter as a gateway to full sneak attack/precision damage with a slashing weapon and maybe 1/2 with piercing or bludgeoning.

[tyckspoon]

Plants are.. actually a lot like fighting Undead; they share many of the same immunities including the common ones that make undead annoying for a lot of classes to fight, so your least-happy classes/character builds are those that rely on critical hits, sneak attacks, and social manipulation/mind-affecting stuff.

There are a lot of magic items that can help, at least in regard to the sneak/crit aspect, but most of them are pretty strictly use/day limited - you're not really expected to be fighting like ten of the same creature type thing per day. Take a look at the Magic Item Compendium for:

Deathstrike Bracers: 3/day spend a Swift action; for one round you can ignore type-based immunity to crits and sneak attack (so doesn't ignore, for example, Fortification Armor, but lets you crit/sneak on plants, undead, constructs, etc.)
Enemy Spirit Pouch: Improves your Favored Enemy bonus against one Type. If you don't have Favored Enemy, you pretend to have it against the item's keyed type for a +2 bonus.
Blighter's Hex-Eye: 3/day Swift activation deal 2d6 extra damage against plants (and also elves, for some reason. The default fluff/lore on this is it's an Orcish item intended for helping them attack woodlands.)
Weapon Crystals: ..ok, there technically isn't one of these for anti-Plant. There is however one for undead and one for constructs that could be used as a sample to homebrew a Plant one. The Least effect is +1d6 damage against the target Type, the Lesser effect is some type-specific bonus (Construct crystal makes your weapon count as adamantine, Undead one gives you ghost touch/lets you attack incorporeals. Figure out some common defense or special mode of attack Plant type creatures often have and give some benefit to countering it - bonus to saves against Poison maybe) and then the Greater effect is allowing you to sneak/crit the Type.

..and of course putting a Wand Chamber (Dungeonscape, if my memory is right) with a wand of Vine Strike in your weapon.

[Biffoniacus_Furiou]

Rogues can get Penetrating Strike in Dungeonscape at level 3, which give them half their sneak attack dice against sneak-attack-immune opponents they're flanking. It forces Rogues into a melee role, but it's better than nothing.

Honestly, in a low-magic game the classes that get no magical class features are often hit the hardest due to their heavy reliance on magic items.

[Fizban]

In a campaign where the characters will be fighting plant type creatures a lot... Like, a lot, much like players in a "zombie horde/vampire scourge" type game loaded with clerics and paladins fight undead...

What sort of things can I do to help certain classes to not be made ineffective? Who is going to be hit the worst?
Clerics and Paladins are not effective because they're good against undead- that's a bonus/backup safety measure. Classes that "rely" on sneak attack will of course have a problem, but depending on the op-level you're running at, classes with sneak attack do not in fact have to rely on it.

But for most people's games (even mine) it's expected that sneak attack will work on at least like 50% of enemies. It's probably been mentioned by now (yup), but the Vine Strike spell can be made easily available with magic items and just ignore that problem.

Alternatively, you can just use trapfinders that aren't sneak attack based, by adding a Trapfinding feat or using the Dungesonscape ACFs for Barbarian and Ranger (or a low SA multiclass build, or. . .). The Ranger which could have favored enemy plants, and end up making you wish they had sneak attack issues.

The plan was a low magic game (deeply restricted access to spellcasters and spellcasting, but unrestricted magic item access).
Shouldn't have any particular problems with plants, just the expected problems from reduced spellcasting (less status removal, problem solving, and mass/physical ignoring offense).

One thing you might want to do since there aren't many plant monsters, is take existing monsters and just reskin them as plants. Though a lot of plant monsters reproduce by killing people, so maybe they're the point of your focus. And many of those spawn abilities likely reference Remove Disease and/or Remove Curse, deal ability damage or drain, and so on, all of which are supposed to be guaranteed by the Cleric. So you'll need to account for that.

[Biggus]

If you're willing to let the players know it'll be a plant-heavy campaign before they start, a lot of classes can choose options which will help, or at least avoid options which will be useless (like specialising in stunning or critical hits).

Also I found this feat the other day, it's homebrew but doesn't seem too unbalanced: https://www.dandwiki.com/wiki/Enable_Criticals_%283.5e_Feat%29

[liquidformat]

I would give penetrating strike from Dungeonscape and Disruptive strike PHB2 to rogues free of charge that way they have some options against the plants that don't rely on using vine strike from a wand for every attack they make.

Scouts and Ninjas are SOL

Paladins are pretty much worthless except if the plants are evil though there might be a dragon mag alternative that makes them useful not sure.

Beguilers are pretty much useless due to plant's Immunity to all mind-affecting effects

Druids should worship Eldath, Mielikki, or Silvanus and take initiate of nature

Clerics should by all means have the plant domain and probably also take Initiate of Nature

Ranger's should take plants as favored enemies, taking Nemesis (Plant) sounds like a sure win)

Everyone should have plant bane weapons

[Psyren]

OP - are you playing 3.5 or Pathfinder? Because Pathfinder nixed their immunity to sneak attack and critical hits, so a rogue there would be fine. (Many plants do have weak points, like roots, bulbs, petioles, and vascular systems that a sneak attack could reasonably target.)

Enchantment focused characters (like enchanters, bards, mesmerists etc) will still have trouble with them, but the Verdant Spell metamagic feat is one way to assist those characters, or they could just charm/dominate a bruiser to take care of the plant on their behalf.

[TheCount]

Sandstorm has a weapon enchantment called dessicating iirc, which is good against both plants and water elementals

[JusticeZero]

Pathfinder 1, Epic-6. Low and restricted spellcasting access, automatic bonus progression, but plenty of magic items.

[Psyren]

Pathfinder 1, Epic-6. Low and restricted spellcasting access, automatic bonus progression, but plenty of magic items.

Those last two are an odd combination, but this does solve one of your problems - plants are subject to sneak attack in PF baseline, so you won't need vinestrike or penetrating strike etc.

As for which classes will be hit the worst - classes that depend on influencing or controlling minds will have a hard time, so I would consider a rod or two of Verdant Spell since magic items are expected to be plentiful.

[AvatarVecna]

A quick glance over several random plants in the PFSRD confirms a general suspicion I had going in: plants tend to have lower speed, Dexterity, Reflex, and initiative than I'd usually expect for a creature of their CR. They also tend to have DR, and they don't infrequently rely on poison abilities. Based on these, I'd still say rogue has a bit harder time than most - not because of SA immunity, cuz it's pathfinder, but because ranged rogues have more trouble than melee rogues, and rogues in general don't tend towards high Fort saves or poison resistance. Nothing that can't be overcome with a bit of resource expenditure though.

I didn't look over absolutely every plant, so this isn't a hard and fast rule. I'm willing to bet there's dumb stuff out there, like a plant that thinks its a fey and forces will saves, or a rosebush that shoots thorns, or a tumbleweed monster with high Dex/Ref/movement/init.

[Dimers]

You could put in some bane weapons, especially odd weapons that the robe-wearers don't have proficiency with.

[farothel]

I've recently seen in the Magic Items Compendium an item that allows sneak attacks and crits on plant creatures. Don't know the name anymore, but that would be something that rogues could use.

[hamishspence]

After a little googling - it appears to be Deathstrike Bracers.