How do I cover a vast area of land with dense vegetation?

Question

I need help to introduce a plot device for the next scenario I intend to GM. I need a way to cover a large area of grassland (a few miles in radius) with high, thick and near impenetrable vegetation. It must be done fairly quickly - in a matter of one season at most, not a few years. How can I cover a vast area of land with dense vegetation? The objective is that:

• This must be accomplished with magic (spells, items, gates, otherworldly entities, etc) that is available on Faerun
• The means should be as low-level as possible, and can involve multiple people but fewer is better. The idea is a low-to-mid-level adventuring party might actually be able to overcome the people responsible.

I can think of some outer plane influence, perhaps spilling over Prime Material or overlaying it, but I don't know enough D&D lore to go beyond vague ideas.

Note: this change in the terrain takes place during the war with Tiamat. The premise is that while forces of order are tied up in the North, in the South trouble stirs as the forces of chaos take hold in absence of civilization's protectors.

Answer 1

Doing this mechanically RAW, is probably impractical:

1. At first glance, the third level spell Plant Growth could be what you're looking for

Plant growth is a third level spell on both the Druid and Ranger spell list, so available at 5th level, at the earliest. A character casting this spell repeatedly, might be able to create the effect that you want to achieve.

Plant Growth
Casting Time: 1 action or 8 hours
Range: 150 feet
Duration: Instantaneous

This spell channels vitality into plants within a specific area. There are two possible uses for the spell, granting either immediate or long-term benefits.

If you cast this spell using 1 action, choose a point within range. All normal plants in a 100 foot radius centered on that point become thick and overgrown. A creature moving through the area must spend 4 feet of movement for every 1 foot it moves. You can exclude one or more areas of any size within the spell’s area from being affected.

If you cast this spell over 8 hours, you enrich the land. All plants in a half mile radius centered on a point within range become enriched for 1 year. The plants yield twice the normal amount of food when harvested. (SRD p. 169)

As you can see, there are two ways it can be cast, with effects that I have assumed are exclusive to each method of casting (i.e. the 'eight hour' casting doesn't additionally produce the effects of the 'one action' casting).

You asked for a means of creating 'near impenetrable vegetation'. Plants that slow movement by three-quarters, seem to meet that criteria.

This spell also occurs instantly, and has no stated conditions (unlike Entangle) by which the vegetation later withers. So the limiting factor is how many times you'd need to cast the spell to cover your desired area.

2. But how many times would (the one action version of) Plant Growth need to be cast, to achieve the desired effect?

You stated that you'd like this area to be 'a few miles in radius', so let's start by calculating how many castings of the spell would be needed per square mile.

• Every time the spell is cast it affects an area of radius 100 ft.
• A circle or radius 100 feet contains 31416 square feet (rounded up).
• A square mile contains 27878400 square feet.
• So, you'd need to cast the spell approximately 887 times, per square mile.
• It would take a level 20 druid at least 60 days to cover one square mile using Plant Growth. That's assuming they use all 15 spell slots they have, of third level or higher, to cast Plant Growth and take one long rest, every day.
• (Note: it would actually take much longer than this. Circles tessellate very poorly, so casting areas would need to overlap each time if you wanted to not miss bits out)

Multiple Druids and or Rangers working together, could obviously achieve this effect more quickly. However, any high level casters you add to your game add extra complication to it.

An easier and perhaps more reasonable solution would be to adjust your expectation of how large this area needs to be. What are the motivations of the characters that are spending all day, every day casting this same spell over again?

3. What about the other (eight hour casting time) way of casting Plant Growth?

If you go with the eight hour casting of Plant Growth rather than the one action casting you can affect a much wider area each time. A circle of half mile radius is equal to 0.79 square miles. Therefore it would be easy for one relatively low level caster to affect an area 'a few miles in radius' over a period of months.

This method of casting does not produce difficult terrain. So, if that's what you want when you say 'near impenetrable vegetation' then this solution will not work for you. As DM, however, you could rule that the 'enriched' growth of the plants still makes them a good place for someone nefarious to hide. If all you need is a good hiding place then this could be enough.

But why not forget about RAW, and homebrew a solution?

There are lots of ways that you could achieve the effect that you want that would be narratively satisfying, without being bound by a RAW approach.

• Perhaps there's a liminal space there between this plane and the Feywild or Aborea, that's causing massive and inexplicable growth in the vegetation?
• Maybe upon fighting their way to its centre, your players will find a cursed legendary item from one of these planes, that's causing the growth (through a hugely magnified and ongoing version of the Plant Growth spell), and that must be destroyed by the players before a nearby town is subsumed?
• Could there be a monster at the centre, whose 'lair actions' are causing this growth? An Adult Green Dragon is probably the best fit - though I'd still suggest homebrewing this slightly as RAW you'd need multiple dragons to cover an area as large as you desire, and the thorns their 'lairs' grow could kill most PCs before they even find the dragon.
• Or, maybe you simply want to homebrew the spell, and rule that in your game, casting Plant Growth at higher levels increases it's area of effect (as suggested by Erik in comments)? Or, perhaps you'll allow Plant Growth to be cast as a ritual spell (as commented by Dan B)? Or that, in your game, the effects of the one action casting of Plant Growth, are also included in the wider area affected by the eight hour casting of the spell?

Any of the things that you've mentioned in your question ('spells, items, gates, otherworldly entities') could be a potential avenue for a solution to your issue, and narratively exciting for your PCs at the same time.

How something in your world has happened is up to you as a DM, but to get there you don't always need to be rigidly bound by RAW.

It's perfectly acceptable to use the Plant Growth spell as a jumping off point for your narrative inspiration, without having to worry about the players wanting to be able to replicate the exact situation themselves. NPCs may be able to do things that players can't and your PCs will almost certainly have skills that your NPCs don't.

Answer 2

Strangely enough, there's no real reason to resort to Magic to make this scenario work. The southern United States has been dealing with this problem in the real world since the 1940s!

That problem is a plant called Kudzu. (https://en.wikipedia.org/wiki/Kudzu_in_the_United_States)

Kudzu was originally introduced to the US in the late 1880s as a cover plant to prevent soil erosion, and as a nice shade plant. But, it spreads quickly, has few natural predators, and in the 1940s a large number of farms were abandoned (due to boll weevil infestations and crop failures)... and things sort of got out of control.

Kudzu currently spreads in the southern US at a rate of about 150,000 acres (61,000 ha) annually. That's more than 234 square miles a year - even with modern technology actively working to stop it! Further, it grows over and consumes everything in its path - trees, fences, houses - creating a thick, near impenetrable barrier of vegetation.

It wouldn't be too much of a stretch of the imagination to introduce a similar situation into your campaign. Perhaps local farmers themselves had introduced this plant into their gardens due to some beneficial property, but a significant number of them were away for a season or two dealing with the northern war with Tiamat... and things sort of got out of control.

Then, just add a sinister backstory, like the plant was sold to them by some Big Evil Bad Guy, and has some nefarious magical property... and bam!

And, a group of low-level adventurers could probably solve the Kudzu issue...

...with the applied use of an army of goats, like the United States is currently doing. :-)

Answer 3

Green Dragons have gone to live there

I had a quick look through the Monster Manual, wondering if any creatures have regional rffects that are suitable. As it happens, one of the regional effects that Green Dragons may have is the following:

Thickets form labyrinthine passages within 1 mile of the dragon's lair. The thickets act as 10-foot-high, 10-foot-thick walls that block line of sight. Creatures can move through the thickets, with every 1 foot a creature moves costing it 4 feet of movement. A creature in the thickets must make a DC 15 Dexterity saving throw once each round it's in contact with the thickets or take 3 (1d6) piercing damage from thorns. Each 10-foot-cube of thickets has AC 5, 30 hit points, resistance to bludgeoning and piercing damage, vulnerability to fire damage, and immunity to psychic and thunder damage.

This sounds extremely difficult to get through. The thickets form a maze spanning miles, making it almost impossible to traverse. If they try to go through the thickets themselves, they must move slowly and take damage, or spend time destroying them (likely by burning them down, which might well attract attention to themselves...)

As far as I'm aware, there are no rules for how long it takes regional effects to appear after a dragon sets up its lair. It could easily be less than a season- it's up to the DM.

The 1 mile radius means that you'll need several dragons in order to cover the whole area. Why are all these dragons there? Maybe they're working together for some purpose, or perhaps someone else convinced them to set up shop there. That's up to you.

Now, the dragons need to be legendary in order to get regional effects around their lairs, so they need to be at least Adults. Even one Adult Green Dragon will be a serious challenge for a mid-level party to take on directly. RAW, a legendary creature's regional effects only fade when the creature dies, but as a DM you may want to change this. As a suggestion, perhaps the effects also fade if the dragons permanently leave. This could open up a lot of alternate solutions to the problem...

Answer 4

This sounds like the perfect place to implement Regional Effects to a creature's Lair

In the Monster Manual, there are many monsters that have Lairs with Regional Effects. The best related example I can find is Silver Dragon which has these effects (emphasis mine):

• Once per day the dragon can alter the weather in a 6-mile radius centered on its lair... the effect is identical to the control weather spell
• Within one mile of the lair, winds buoy non-evil creatures that fall... Such creatures descend at a rate of 60 feet per round and take no falling
• Given days or longer to work, the dragon can make clouds and fog within its lair as solid as stone...

The first Regional Effect can be similar to the Silver Dragon's Lair, but with plant growth (Tiggerous's answer covers this topic well). You can change the radius of the effect to whichever you want, as the control weather area of effect is also changed.

The third benefit here is really what gives you a highly similar effect to what you are trying to accomplish. This can be modified to something along the lines of "any vegetation within X miles of the lair form a wall as hard as stone as if create by the wall of stone spell until the creature is killed." You could make it take as long as you deem necessary considering the line from the Silver Dragon's Lair: "Given days or longer to work."

Whatever creature (possibly enslaved or allied to the villain hiding out in the region) you decide to use to create this dense vegetation could even have Lair Actions that use the vegetation during combat to make things interesting (such as a free usage of the entangle spell).

Answer 5

Mirage Arcana

This 7th level spell can change the look, feel, sound, and smell of an area.

Open fields or a road could be made to resemble a swamp, hill, crevasse, or some other difficult or impassable terrain. A pond can be made to seem like a grassy meadow, a precipice like a gentle slope, or a rock-strewn gully like a wide and smooth road.

It can also be made thick enough to impede movement:

The illusion includes audible, visual, tactile, and olfactory elements, so it can turn clear ground into difficult terrain (or vice versa) or otherwise impede movement through the area.

The spell lasts for 10 days, and thus must either be refreshed by the caster every so often. It also covers only 1 mile square, so to cover the large area, you'll need repeated castings or multiple casters.

Answer 6

Experimental magic

As pointed out there's probably nothing RAW that covers this, so I'd say get creative

Maybe, a druid in the south is trying to develop a better version of plant growth, the experiment went wrong and, well, it was certainly a bigger effect...

Or, time in the area has become fractured. The rest of Faerun has experienced a few months; here it's as if aeons have passed.

Perhaps the mere presence of some extraplanar entity causes insane plant growth - and the party must slay/banish it before this spreads too far.

It could be the result of a wish spell - as intended or not

---

As the DM, you are pretty much the god in a world of magic

The rules merely exist to support your storytelling - if you can't find a rule to explain/justify/support something cool that you'd like to do, go off grid.

Always remember: The rules are just guidelines.