I want to slow a ship down, and thought it would be fun to cause it do be disabled by barnacles, tubeworms, or seaweed. I didn't find anything that would help encourage unnaturally rapid growth in my googling so I was hoping you could help.

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    – Someone_Evil
    Commented Mar 11, 2021 at 20:43

4 Answers 4


Plant Growth

There aren't spells or other tools for barnacles and tubeworms. They aren't really the kind of creature which the rules concern themselves with. For plants however, we do have a tool. Plant growth (PHB p. 266) includes the option to make

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

Which would include any presumed present seaweed think and overgrown. The spell's effect including a movement penalty for creatures moving through the plants, though how that translates to the ship they're attached to moving through water is up to your DM.

  • \$\begingroup\$ On a small scale, you could also use Druidcraft in the same way. While it wouldn't slow the ship, it is still a valid way to make plant life bloom. \$\endgroup\$
    – MivaScott
    Commented Mar 11, 2021 at 21:53
  • 4
    \$\begingroup\$ I've tried to sail a boat through thick seaweed before. Imposing a movement penalty for a ship sailing through overgrown seaweed is definitely a reasonable ruling from the standpoint of verisimilitude. \$\endgroup\$ Commented Mar 12, 2021 at 2:44
  • \$\begingroup\$ Could you do something similar with entangle? \$\endgroup\$
    – Mark Wells
    Commented Mar 12, 2021 at 6:23
  • \$\begingroup\$ Entangle wouldn't last long enough... DM accepted seaweed, disallowed barnacles an tubeworms. Thanks! \$\endgroup\$ Commented Mar 13, 2021 at 5:38

5e does not follow real-word definitions of taxonomy

The 5e Monster Manual does not distinguish between "Kingdom Plantae" and "Kingdom Animalia" as they would have been recognized by Linnaeus, or as they are currently recognized in modern taxonomy. Rather, it states a creature's Type as Beast or Plant, among others. The Plant Type explicitly includes creatures also called "Fungi", such as Gas Spores, Shriekers, and Violet Fungi.

The 5e spell Giant Insect says that it may be cast on centipedes, spiders, wasps, or scorpions. Of these, only wasps are considered insects (Class Insecta) under current real world definitions.

"Seaweed" is a catch-all term that includes many macroscopic algae, including things that are certainly not plants (Brown Algae, Red Algae). Whether or not Green Algae are in the Plant Kingdom depends on the real-world classification system one uses.

Barnacles, tube worms, and seaweed don't have their type(s) officially defined. There is no reason that a DM couldn't decide that they should be considered to have the type Plants rather than Beasts. As such, the spell Plant Growth is perfectly appropriate for your purpose.

  • \$\begingroup\$ On the flip side, there is no reason that a DM has to make that call, either \$\endgroup\$
    – NotArch
    Commented Mar 12, 2021 at 18:32
  • \$\begingroup\$ @NautArch Certainly not. Such a call, while perfectly valid, doesn't help answer the OP's question, though. \$\endgroup\$
    – Kirt
    Commented Mar 12, 2021 at 18:34
  • \$\begingroup\$ I'm not sure saying the DM can rule anything is also a good answer. Neither of these positions are very helpful. I think the answer would be improved without the last paragraph. \$\endgroup\$
    – NotArch
    Commented Mar 12, 2021 at 18:35
  • 3
    \$\begingroup\$ @NautArch A DM should not say that dinosaurs are affected by a plant growth spell. This is not about 'a DM can rule anything'. Given that the Plant and Beast types are not well-defined, and that something like a Needle Blight is officially a Plant, I don't think it is unreasonable to say that a barnacle would be as well, in the absence of an official ruling. If I cast Plant Growth on seaweed, I would not want my DM to respond with, "actually, Sargassum is a brown algae, not a Plant". \$\endgroup\$
    – Kirt
    Commented Mar 12, 2021 at 18:45

That’s what GM’s are for.

One of the main reasons we play tabletop RPG’s with game masters is to resolve situations like this, where the rules or other references are vague or nonexistent.

This is a fun idea for a good story, and any of the ideas here may apply, pending approval from your GM. My advice would be for them instead.

A tenet of improvisation is that you never say “no”. Only “yes”, “yes and…” and “yes, but…”. If you want barnacles to slow a ship down, either it is possible with one of the established mechanisms (spells, powers, skills etc. ) or if that somehow breaks the balance , then there are additional costs and/or consequences. If there are, make sure they are part of the fun.

P: Hey, can I use PlantGrowth to grow the barnacles on that ship’s hull?

GM: Hmm, good idea but you need seeds as a material component. You are not sure if ordinary land plant seeds would work but maybe some pearls would. You remember the duchess had pearl earrings. Care to persuade her to give you one? She’s in the captain’s quarters.


You could always collect the various animals and products but if you don’t want to go through the effort a summon animal spell could work or a home brewed spell could also work (if you play with the home brew rule)


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