Can you cast a sphere of plants in the air with Plant Growth?

Question

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.

Since the spell doesn’t specify that the plants grow along the ground or surfaces, I've been reading on some sites that I could allegedly create a "sphere of plants" with this spell. How would this work in practice if I ever cast Plant Growth in the air then? I have some difficulties visualizing it. Would it really form a sphere suspended permanently in the sky or does it simply mean that my plants could reach 100 feet of height once transmuted?

Answer 1

Nothing in the Plant Growth spell mentions shaping

You've quoted the most important part:

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.

The caster chooses a point, as defined in the Area of effect. Any existing plant within 100 ft grow thicker and "overgrown". But they do not become topiary. Nor do they just pop into existence.

Plants that are already in that radius follow their natural course of growing; trees would get taller and wider, bushes would get bushier, and vines will creep along the ground or climb an object.

If there was a preexisting sphere, you might convince a DM that vines would follow the form of the sphere but they would not create a sphere based on this spell alone.

For instance, you might use Mold Earth to create blocks that plants would grow around, or fill, as the case may be.

But on it's own, it's just magical fertilizer. Things grow big, but not formed.

Answer 2

Plant growth causes existing plants to be overgrown. It does not create plants.

Plant growth states:

All normal plants in a 100-foot radius centered on that point become thick and overgrown.

Plant growth only modifies existing plants, causing them to become overgrown and difficult to traverse. It does not create plants where there are no plants, nor does it cause a small area of existing plants to fill the entire area of the spell. If you cast it in the air and there are no plants within 100 feet of the point you chose, the spell does nothing.

Answer 3

It does not work like that

All normal plants in a 100-foot radius centered on that point become thick and overgrown.

This spell works with the plants that are already there, it makes them thicker and overgrown. It doesn't create new plants from scratch. If you cast it in the air and there are some plants on the ground within 100 feet of the point you chose then those plants will be affected but they will still be growing on the ground, just thicker and harder to get through than before. They might grow higher but still will not become a "sphere suspended in the sky". If you cast it somewhere where there's just air within 100 feet, it will not do anything since it has nothing to work with.

Answer 4

Most air is filled with pollen

As other answers have pointed out, the spell does not create plants, it merely changes plants that are already present.

Even if there are no macroscopic plants in the air, however, under Earthlike conditions the air is full of plant pollen. The lighter kinds of pollen are carried hundreds of miles from their parent plant. Only if it is currently raining or has rained a short while ago would pollen be lacking from the air. In conditions of more than 80% humidity there isn't much pollen in the air, but the air would then be filled with mold and other fungal spores, which the game also treats as plants.

Interestingly, living but normal plants are not creatures, they are objects. It would be the DM's decision then how much pollen is in the air, and whether, as a plant, it would be affected by the plant growth spell. When deliberately cast in the air where there are no macroscopic plants, the plant growth spell might (at the DM's option) affect the floating pollen, making it "thick and overgrown" until a hazy cloud appeared.

This could affect movement, as described in the spell description, but it might also break verisimilitude to have a cloud of floating particles, unattached to one another or the ground, require four feet of movement cost for every foot of travel. If running the spell as written would break immersion, a DM might consider treating the effect as dust of sneezing and choking within its area.

Answer 5

Only if there are already plants in the area of effect.

Plant growth doesn't create plants, it just amplifies the growth of existing, non-magical plants. As such, if cast on plants floating in the air, it would create the described "sphere of floating plants". This might be plants that are growing on an earthmote, plants growing in an air pocket in Wildspace, plants that generate lighter-than-air gasses somehow, etc. If you cast it on a point of space where these sorts of plants exist, you would get the effect you're asking for.

Alternately, you could cast it on a point in the air such that some ground-based plants were caught in the area of effect, so that they will grow and fill the 100-foot-radius sphere.

Once you have some suitable plants within the area of effect, they will grow to fill the entire area of effect:

A creature moving through the area [the 100-foot-radius sphere] must spend 4 feet of movement for every 1 foot it moves.

This movement penalty applies to the entire area, and it is caused by the overgrown plants.

As for the duration of the spell, it's listed as instantaneous - the plants will grow to fill the space, and then nature will take its course, depending on the nature of the plants. If you create a two-hundred-foot-diameter aerial sphere that grew from a five-foot patch of grass where it touched the ground, it's probably going to promptly collapse to the ground and die.