Kenneth Setzer Fairchild Tropical Botanic Garden
As published in the Miami Herald, January 27, 2017
A plant’s life in a pot is different. It would seem pretty straightforward: A potted plant should have the same requirements of any other plant of the same species. However, life in a pot is not simply a microcosm of an in-ground planting. Conditions are intensified.
Succulents like this Stapelia appreciate a shallow pot, which holds on to less moisture. (Kenneth Setzer/FTBG)
A crucial time to repot often comes when you buy a new plant.
Plants are potted with shipping, storage and marketing concerns foremost, not horticulture. Quite often, plants come in soil that stays way too damp for the roots to survive long-term. Soil conditions are probably meant to help the plant survive time in a hot, dry truck, or while sitting on delivery bays.
Plants like succulents have evolved thick, fleshy leaves designed to hold on to moisture. They need soil that drains quickly and thoroughly or they rot. Repot them with haste into rocky, open, quick-draining soil, one with not too much humus or sand, which tends to clump.
Many holiday plants, for example, such as colorful cacti, are sold with pebbles glued to the surface. Clearly, the glue makes shipping neater, but I can’t stand the idea of entombing the plant like this. Break apart the pebbly crust and replace it with a loose dressing of pebbles or rocks. (I think I’ll avoid discussing the spray-painted cacti I’ve seen the past couple of years. As if cacti aren’t wondrous enough on their own. It’s plant torture, plain and simple.)
Some plants are sold in pots with absolutely no drainage — not a single hole! The pots are often glazed and very attractive, but they’re pretty deadly for most any plant. Soon, the roots would be sitting in water, a situation that would kill almost any plant other than true aquatics; even bog plants need some drainage. Crested euphorbias are often sold this way, a certain death sentence if not soon remedied.
A large pot holds more moisture, which translates to watering less often. (Kenneth Setzer/FTBG)
Remember being told to place a rock over the drain hole of a pot? Why was that? To stop soil washing out maybe? Well, don’t do it. We need good flow. A few small pebbles throughout the soil are also helpful for encouraging drainage. Think of water as a house guest: Please, visit regularly, but don’t linger!
Conditions are exaggerated in a pot. Plants heat up excessively and quickly without the insulation of the surrounding earth, especially in a dark pot. And like a bridge freezing in winter, potted plants succumb to frost long before their Earth-bound brethren.
While good drainage is essential, plants that like consistently moist soil will need baby-sitting, as potted soil can also dry out quickly. For a thirsty plant, a larger pot may help. Rather than having to water every day, the extra volume of soil should hold on to moisture longer. After repotting a Plectranthus (aka Coleus) into a large pot, I went from watering every other day to once every week or less, depending on weather.
The opposite can also be used to your advantage — plants that require aridity might benefit from a small planter. For example, cacti and succulents may do well in tiny pots, with the little quantity of soil drying quickly. Many plants of the American Southwest and similar areas do not easily survive our humidity and summer rain, but it may help to keep them individually in smaller pots, with some rain protection in summer.
Now, as to the issue of plastic or clay, again it depends. Clay is porous and allows moisture to evaporate quicker. However, plants that prefer moist soil may fare better in plastic.
Finally, if you fertilize, remember that most everything will be concentrated. Freshly introduced fertilizer won’t dissipate as it does in the ground, so be careful to follow directions for potted plants. Conversely, watering over time will flush out what fertilizer and nutrients were in the soil.
On the topic of flushing out, you might notice a white crust on the soil surface. This may be a buildup of salts from minerals present in water. If it’s excessive, scrape the top layer off. In addition, flush the pot by watering with about twice its capacity, so a gallon pot gets watered with about 2 gallons to flush out buildup.
If you see roots straining to escape through the drain hole or out the top or sides, your plant is crying out for more space. If you can pull the plant out, and there are more roots than soil, repot!