Welcome to Bad at Plants, a new column in which plant expert Maryah Greene, of Greene Piece consulting, answers your questions about plants, so that we might all become at least slightly better at keeping them alive.
“My aspidistra (cast-iron plant) has been perfectly happy in the same spot indoors for years — until recently. It has flourished and put out new leaves year after year, and though I haven’t repotted it, I have top-dressed it annually. Suddenly, however, some of the leaves have been turning yellow despite no changes in the care I’ve given it or in the house environment. When I first got it, I put pine cones on top of the soil to keep my cat from using the pot as a toilet. Is it possible that over time, the pine cones have affected the chemistry of the soil and caused the plant not to thrive anymore?”
Cast-iron plants enjoy continuously moist soil, so keeping the pine cones on top of the soil is super-smart, because it slows down the evaporation process. Some people put moss or rocks on top of soil, mainly for aesthetic reasons, but it also works to trap moisture in for longer. For some plants that don’t like moist soil — like my Marginata palm — that wouldn’t be a good idea, but for yours, it’s great.
You say you’ve been top-dressing it annually, which, for people who don’t know, is refreshing the top one-to-three inches of soil once a year. You scoop out the soil that’s on top and replace it with brand-new soil to give a boost of nourishment to the plant. A lot of people do that. I personally don’t, because to go through all that effort (replacing the soil and getting messy), I’d just rather repot, but some people do it and it works really well for their plants. It mainly comes in handy for really large plants that you don’t want to repot as often, so I’m assuming yours is fairly large.
Unfortunately, adding nutrients to the soil via top-dressing can’t replace re-potting the plant. It sounds like it’s been over two years that your plant has been in this pot, so it definitely wants to be repotted for two main reasons. First is the space issue: After a few years, a plant wants a new home. I hear this all the time: “Why do I need to repot my plant? The soil’s fresh!” I say: Imagine being at this point of your life and still living in your college dorm. Over time, it doesn’t make sense. You need more space. It’s the same exact thing for a plant. Inside, what starts happening with the roots is survival of the fittest. There’s only so much room, so those older roots get crushed by the newer roots coming in. It’s a plant — you can’t stop it from growing, so something’s gotta go. Those yellowing leaves you mention are likely the older ones that are getting crushed by the new leaves’ roots that are coming in. I know you have new roots because you have new leaves.
The other reason you need to repot is to give the plant fresh soil, which you’ve been doing to an extent with the top dressing, but the bottom roots are still in that old soil. Even though the nutrients are trickling down, they still want some new soil.
When you need to repot depends on the plant. Some plants are fast growing, and others, like cacti, are slow. Those only need to be repotted every five or so years. Then there are pothos, which are sold in just about every grocery store in New York — those grow super-fast, and can be repotted every year. The other thing it depends on is the size. If something’s in a 14-inch massive planter, it obviously has more surface area, so you don’t need to repot it as soon, whereas you’ll want to repot something growing at the same rate in a four-inch planter sooner rather than later. To be safe, I’d say you want to repot every two years on average. You always want to go one-to-two inches larger in diameter than what you’re currently in. So the plant outgrowing a four-inch planter should go into a six-inch planter. You can’t skip ahead to a huge pot, because when you’re watering your plant, you’re creating mud soup and space for mold to grow. You can’t take shortcuts with root care.
Right now, I’m creating a project plan for a client who has every size of pot imaginable, and every plant needs to be repotted. We’re essentially playing musical chairs with every plant, moving each one up a step. I just have to buy one massive planter for the largest one, and all the other ones are hopping into the next. I do that with all my plants. I have three of each size pot, and every time they need to be repotted, I shift one to the next.
Repotting can be messy, but that’s my favorite part. It’s one of the few things we do as adults knowing it’s going to be messy going into it, and that’s okay. I don’t think people get a lot out of it when they’re too worried about the dirt. Enjoy the process, because the plant is getting a lot out of it. I see people out on their stoops repotting, so wherever you can make the best mess is where it’ll be the most relaxing. We’re so used to perfection and performing that it’s nice to get our hands dirty.
Do you have questions for Maryah? Send them to firstname.lastname@example.org, and we’ll try to get you an answer.