This article is specifically about Calathea, but I think most if not all Marantaceae can exhibit really curled leaves.
Most plants can develop curled leaves, but Ctenanthe, maranta, and Calathea go all out – their leaves literally roll up into little cylinders when they’re under stress (not to be confused with new growth, which are also little cylinders).
Stress is the cause of curled Calathea, but here is a breakdown of the most likely reasons your Calathea is stressed:
Underwatering can cause curled Calathea leaves
This is the most likely reason that your Calathea has curled leaves – Calathea like to have evenly moist soil at all times, and will not respond well to drying out too much.
They curl their leaves when they sense that they don’t have enough moisture around the roots in an effort to reduce the amount of water evaporating through their leaves.
They’re not only reducing the surface area of the leave, but they’re also creating a micro-climate in the middle of the leaf tube (as it were) so moisture can be reabsorbed.
However, there are a couple of reasons that your Calathea might be displaying signs of being underwatered even if you water it frequently.
Your Calathea might be rootbound
When Calathea become rootbound then the soil in their pot is displaced by roots. Over time the volume of soil gets lower – and so, therefore, does the volume of water that that soil can hold.
Once upon a time the soil may have had the capacity to hold a week’s worth of water for the plant, but as the root system grew, the volume of water that could be held in the roots was reduced.
Being rootbound isn’t necessarily an issue – if you bottom water, for example, then the plant can get the moisture it needs by absorbing the water directly through it roots without it needing to be held in soil.
When you top water, however, the water just runs through the roots and out of the drainage holes at the bottom of the pot – the roots don’t have long enough to absorb much water.
You can either bottom water regularly (don’t leave your Calathea sat in water – that can cause root rot), repot your plant in a bigger pot, or trim the roots.
The soil might have become hydrophobic
Over time soil can compact into one big, hard lump. Water won’t run into the soil – it’ll flow over the top and down the sides between the soil and the pot, so the roots never get a chance to absorb any.
You can rehydrate hydrophobic soil by soaking it from the bottom up. If it compacts easily, you can add soil amendments like perlite and orchid bark to stop it from happening so easily.
As I said at the beginning, underwatering is the most likely reason that your Calathea is curling it’s leaves, however, there are a couple of other things that can cause it.
Pests can cause curled Calathea leaves
This isn’t something I’ve seen too often, but some Calathea with smaller leaves, like Calathea musaica, curl their leaves when they have pests.
One of the most common pests to Calathea is spider mites, which suck the moisture out of their leaves. It’s likely that the plant can’t discern between low water content in the leaf and insects sucking out their juices so they respond in a similar way.
Spider mites can be very difficult to eradicate, but I’ve found that consistently (twice a week) washing the leaves down with a spray bottle full of water and a drop of castile soap works well.
Also, be sure to keep your plant leaves clean; this not only helps with general health – because it allows for more consistent photosynthesis – but I’ve also found from personal experience that dust and spider mites are often found together.
It’s often said that spider mites don’t like high humidity so shouldn’t be found around Calathea, but whilst they do PREFER dry air, they’re definitely able to live (and thrive!) in most home environments.
I’ve yet to find them in my terrarium (touch wood!), but you never know.
Thrips are also quite common to Calathea, but in my experience, they’re more likely to leave brown spots on the leaves and crispy tips than make the leaves curl up.
Your Calathea might need higher humidity
Calathea tend to droop when they’re not getting high enough humidity (and crisp up, especially around the edges and tips) rather than roll up, but it can happen in very low humidity rooms.
I do advise people to check the humidity levels in their home BEFORE buying plants, but am also very guilty of seeing a plant, falling in love with a plant, and buying a plant, without doing so much as a cursory google search to see if we’re compatible.
If you’re thinking of getting multiple houseplants, it’s definitely worth getting a hygrometer beofre buying too many just so you know whether you can provide decent humidity for plants like Calathea and ferns.
Check your temperature as well – Calathea are from tropical rainforests so they need high humidity AND nice warm temperatures. Bathrooms, in particular, tend to be quite cold and have inconsistent humidity so are better for hardier plants like ferns.
Does poor water quality cause Calathea to curl its leaves?
Calathea are infamous for being picky about the quality of the water they’re given, but giving them tap water is unlikely to cause curled up leaves.
It does vary from species to species and even different specimens of the same species – some are more than happy with tap water, whereas others require rainwater or filtered water.
If your Calathea does throw a hissy fit about the water it’s receiving, this will most likely manifest itself as brown spots on the leaves, or crispy leaf tips. This is also usually the case if your Calathea gets too cold or is being misted (they don’t like having wet leaves).
Keeping Calathea consistently moist without overwatering them can be a struggle, which is why I like to grow mine in my terrarium. They do grow very quickly though, so I have to cut them back regularly.
If your Calathea is at the point where you’re scared for it’s life, read this article.
