I need some advice. My players have had a silly idea, and it's interesting enough that I've been trying to entertain it.

Basically, I am running a pirate campaign, and they have their own ship (a caravel), and they want to (bear with me here) plant a tree at the bottom of the ship.

So essentially, they want to install a planter box, plant a tree, and cultivate it. They were strategic about it, utilizing the grates on the deck of the ship that open up into the bottom portion of the ship. (FYI when trying to work this out I was using this reference image > https://i.pinimg.com/originals/7a/72/f1/7a72f1bb2be26d5dcd89dd58a5fb4fe7.jpg < so you can see the way theoretically the grate on the main deck could be positioned right over that wider hole with the ladder to the cargo hold.)

My questions are these:

1. Should I even allow this? They do have a shipwright who was blustered by a few good charisma rolls, so he was kind of trying to figure out if it would work or not. I kind of played this off as him needing to consider the design implications while I turned to you all for some feedback.
2. If I allow it and it ends up even a little successful, what are some of the negative impacts their ship experiences as a result of having a tree growing from the cargo hold?
3. Kind of a follow-up to the above: if the shipwright says they can make it work, what are some of the things they need to keep in mind to keep this tree alive? I assume this will be a very delicate thing and it will not be easy to sustain.

First problem is that salt water isnt good for trees. As waves and leaking timber let water in, it will eventually get into the root system sickening the tree. (exhaustion from lack of food possibly although trees can go a lot longer before reaching starvation than a human) The soil it will need will be less likely to gather its ecosystem of worms and insect bringing in any new material and it will soak up saltwater, becoming a problem. Watering the tree sufficiently for its needs will require a rather larger amount of freshwater than a crew would otherwise take on a journey, relegating it to a coastal or marina pleasure craft.

The weight of the tree could cause the ship to become unseaworthy due to incorrect weight distribution or its roots reaching and damaging the hull. a tree is also damn heavy and eventually unless its branches were pruned it will interfere with the sail.

It might be easier to turn an existing structure like the aft castle into a sealed green house and have the tree or other plants inside, but unless magically reinforced, the first ocean storm is likely to shatter that to pieces.

Do you know why they want to do it? 

Beyond the actual practical requirements,  if they have a purpose in mind, that may open up more possibilies, reasons potentially to allow it, create challenges out of it, or ways around the challengs inherent to it already.

Conversely, it is a fairly impractical thing to do, for reasons Moondruidsneversleep has pointed out, especially depending on the size and type of tree desired, so if there's an easier way to meet their objective,  that might be suggested. 

Part of the reason they want to do it is just because one of the party members is a little silly, but then they realized the druid could use it for "Transport Via Plants", so then they became more insistent on making it work. But yes, otherwise I agree that it's fairly impractical.

SO a few things they will need to worry about with planting a tree in the cargo hold:

1. Sunlight for the tree, it needs sun in order to grow.

2. Water for the tree, they need fresh water for the tree because salt water will kill it.

3. Something that you could use as an interesting mechanic is the danger of the tree growing too large for the area. Its branches and trunk start growing too large and may need to be removed.

These are some basic things that the players will need to consider and be aware of while trying to take care of the tree. I hope this helps and i wish you luck.

mangrove grow in saltwater, and many palms are very saltwater tolerant.

I can't think of any reason this would not work.

I would make it a coconut tree, and call the ship "the Bounty"... like the chocolate bar! mmmmm. what could go wrong?

i mean if its for "transport via plants" why not use a sea dwelling plant be much easier to cultivate a butt load of kelp on a ship that a whole tree

Here are my thoughts:

The tree only has to be wide enough for the largest person/animal to fit through.  Cruise ships have full sized trees on them.  I would:

1) Determine what type of tree they would need

2) Determine how heavy that tree would be

3) They would have to reinforce the ship and p;anter box somehow

4) They would need to do maintenance on the tree with Sunbeam, Create water and other druidic spells to make sure it grows correctly and stays healthy.

OR

You could look at them and say "Np, you've lost your mind."

Thanks all! They did think to look for a more tropical or sea-worthy plant so at least I have that going for me.

As much as I like the kelp idea, one of the players just genuinely wants to have a tree, not just any old plantlife, so I'm going to consider all these replies and try to come up with a solution that challenges them. I really like the idea of having to take care of it using druidic spells and also the risk of the tree growing too large.

So...if they somehow open a stable rift or portal to the elemental plane of earth for the tree's roots to grow into, and let ocean storm rains water the thing from time to time......sure, why not?

First up. It's a really good idea if they can think it through. Allow it? Well if it was up to me I would give you the simple and probably best answer. Nope.
But if you are running a more or less silly campaign. You know where the characters are never serious or they are all just really ****ed up freaks. Like using glitter dust to cast fireball and being a half unicorn celestial centaur with a 22 charisma. As in that's a serious moment for you guys then probably GO FOR IT! I mean I would just say noooo. But fun > rules/reality.
Or just do the "You can certainly try". It's mostly up to you. But if you're going for if it would work? Nope. Never. If there is a druid or ranger they would also know that it's not possible. (A not of worthiness: Trees need sunlight to grow. Water to grow. And from what I know salt water isn't good for trees but I'm not dru- well I'm not expert on trees or plants, though I do play a druid... ^^

Just keep us updated! I wanna know how it went! ^^

I like the idea of a tree on a ship. If you're ok with them having a portable teleportation station, then go for it. I have always felt that my main requirement for players having a base (and this really is kind of a base) is that it bring something unique and interesting to the story. This has the added benefit of being useful. As the party's reputation grows, so will stories of their unusual tree ship.

At my table, I would be looking for a reason to say yes to this.

Yes! I don't like saying no to players if I can help it because... c'mon... it's fantasy D&D world, so why not have some fun? I'm more of a "You can certainly try" kind of DM. I love them having the teleportation station; they're a high enough level where it's really useful for them, and I think they've earned it.

I was mostly looking for feedback on potential side effects or things that CAN go wrong if they don't care for the tree properly, and I've gotten some good feedback on that!

A short list of things that could go wrong in this scenario because this is D&D:

• Dire Gulls have taken up nesting in the tree.
• If an Elemental Earth rift was used, creatures from the Elemental Plane of Earth might occasionally pop out of the ground. 
• The tree itself and its branches create a problematic windbreak behind the sails slowing the ship.
• The nautical oddity attracts pirates, keen on staving off scurvy with fresh tree-fruit.
• In the fall, someone now has to rake leaves off the deck. In conjunction, aquatic monster unfamiliar with leaves attack the ship, investigating the strange botanical pollution.
• The tree's branches now catch a lot more wind during storms, making navigating the ship safely a major challenge.