Underpot to Keep Indoor Trees Under Control

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Do you really want your indoor trees to reach the ceiling? Photo: apartmenttherapy.com

Indoor trees are marvelous things. They give you the feeling of being in the great outdoors even while you’re in your own living room. They provide shade, making reading easier, they help purify the air and they perk up your décor like nothing else. But they do tend to get big over time. Ficus, scheffleras, money trees, corn plants and other indoor trees just want to keep growing. In nature, some can reach 50 feet (15 m) or more in height. That’s just not possible indoors. So, what’s an indoor gardener to do?

Benign Neglect

Once your indoor tree (here, a Ficus benjamina) has reached the size you want, slow it down by no longer potting it into bigger containers. Photo: Julie Deken, pinterest.ca

One thing you can try that requires very little effort on your part is to simply stop potting them up. That is, start leaving them in the same pot year after year instead of moving them to a bigger one every two to three years, as you’ve been told you ought to do. With their roots severely constrained, most trees wisely start to grow more slowly, some putting on almost no new growth worth mentioning, but simply replacing older leaves with fresh ones. 

Severe underpotting has helped keep this nearly 400-year old bonsai small. Photo: Ragesoss, Wikimedia Commons

Underpotting in this manner has been used in producing bonsais, tiny, pot-bound replicas of full-sized trees, for 2,000 years. On indoor trees, you simply apply it at a later stage, when they’ve reached the size you want.

Now, depending on the species, some selective pruning may also be needed, cutting back taller branches to stimulate denser, shorter growth, but by the time a tree has been in the same pot about three years, you’ll find you won’t need much of it. Underpotting will have slowed it down considerably.

Saves Time, Effort and Cash!

Repoting a large plant is a lot of work. Do you really want to do it? Photo: roomfortuesday.com

Besides, repotting a big plant is a major hassle. They’re heavy and awkward to manipulate. Just getting the old pot off without a second pair of helping hands can be a struggle! If you keep the plant in the same pot year after year, you avoid that.

Plus, you save the price of fresh soil and the cost of a new, bigger pot. OK, admittedly, potting soil is not that expensive, but pots—certainly big pots suitable for an indoor tree—certainly can be.

What About Mineral Salt Buildup?

Mineral salt buildup inside a pot. Photo: whav.net

One of the reasons for repotting houseplants has always been to reduce the buildup of mineral salts, deposits caused by the use of hard water and fertilizer that have high concentrations of dissolved substances such as calcium carbonate, sodium, and iron. They’re often visible as a sort of white to yellow crust forming on the inside of the pots and are a sign the potting mix is becoming slowly toxic to the plant. By repotting into a larger pot, thus changing part of the old growing mix and adding fresh, uncontaminated mix around the root ball, you can help alleviate that. 

So, what happens if you’ve stopped moving your tree into bigger and bigger pots. Won’t the mineral salts build up until they kill them?

Not if you try one (or both) of two things: leaching and top dressing. 

In leaching, you pour fresh water over the soil until it drains out the bottom. And keep it up until you’ve applied the equivalent of two to three times the pot’s volume. That will dissolve and carry away much of the minerals. Then you simply throw away the drainage water. 

It’s easy enough to leach a small houseplant in the kitchen sink, but trees may need to be leached outdoors. Ill.: Claire Tourigny, from the book Les 1500 trucs du jardinier paresseux

Leaching a tree while it’s indoors is pretty much impossible, though. It’s not as if you can put it in a kitchen sink to leach it as you would for a smaller plant. And it may no longer fit into the shower. I leach my indoor trees simply by putting them outdoors for the summer without a saucer underneath. There, Mother Nature does the leaching by supplying regular downpours. In a drier climate, simply water more than strictly necessary and the leachate will drain off, carrying the excess minerals away. 

Top dress by scraping off the top layer of potting soil and replacing it with fresh mix. Ill.: Claire Tourigny, from the book Les 1500 trucs du jardinier paresseux

To top dress, just scrape off the top layer of soil (about an inch/3 cm or so) annually and drop it into the compost pile. That works because mineral salts tend to migrate upwards and mainly accumulate in the top part of growing mix. By removing the top layer of soil, you’ll severely reduce mineral salt buildup … and all that’s left to do is to replace the inch (3 cm) of soil removed with an equal amount of fresh potting mix.

How Long Can You Delay Repotting?

But how long can you keep this underpotting thing going? 

Actually, I don’t know the upper limit, but I have some fairly imposing corn plants and ficuses that haven’t been repotted in 20 years and they’re still doing fine just through leaching them outdoors in the summer and annual top dressing. And remember 400-year old bonsais in their tiny, root-cramped pots. 

I therefore figure there is probably no real upper limit. You can just keep not potting up pretty much forever.

So, slow your trees down by not repotting them, a laidback solution to a common houseplant problem!

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