When Plants Go To Pot

Ed: My daughter-in-law subscribes to what I consider a large number of magazines dedicated to the maintenance and improvement of home and garden, and when I visit I look at the tables of content for articles dealing with gardens. When I do find one it seems over enthusiastic about the results you can expect, especially with potted plants.

Leon: I see the same types of magazines here at St. James retirement community, but don’t pay much attention to them because they don’t make sense for our flowers/vegetables-in-a-pot kind of gardening.

Both of us wonder if the authors of those articles have had at least some experience in growing the types of plants they write about. It’s hard to tell, but the great enthusiasm they show will probably leave their readers disappointed, especially those who want to grow vegetables indoors or on a patio.

Most articles dealing with outdoor vegetable gardens stress the need for at least six hours a day of direct sunlight for plants grown for their fruits, such as tomatoes, eggplants, peppers, squash, cucumbers, and beans or peas, for example. Some concede that plants grown for the leafy parts, such as lettuce, spinach, or the various “greens” can get by with fewer hours of direct sunlight in a pinch, but they will grow larger and faster with full sun.

Well, that is about right, but the articles often fail to mention indoor pot gardening, where sunlight is coming through windows and the air is cool enough for our comfort. Warm weather vegetables do better with a little more heat than they will get in the house. Air conditioners also dry the air, so the plants will need more frequent watering.

Most vegetable plants require more space than you expect—several cubic feet per plant is common. If space and light are problems, try the newer leafy lettuces, many of them can be grown in about a square foot of space, and are as pretty as most ornamental plants. You should avoid the head types or romaine; it is more convenient to use the leafy lettuces—like spinach—that can be harvested a couple of leaves at a time to add to salads. The plants will keep making more leaves over a long period of time. If you have enough light but little space, ask at the garden center for one of the tomatoes developed for pot culture, especially the cherry or grape types that keep making those tasty morsels for a long time.

November may also be a good time to look back and discuss again some of the things we briefly mentioned through the year. Back when we taught our “Lagniappe” classes most of the people had been told that adding a bottom layer of coarse, insoluble material, such as rocks, improves drainage in flower pots. Most thought that it did, but we set out to show them that it resulted in the potting mix above the layer staying wetter after drainage than it would be without it. The demonstration we used is simple and may be something some of you Master Gardeners can use when it is your turn to make a presentation.

Fill a three or four inch plastic pot with tapered sides and several drainage holes around the bottom (rather than only one central hole) with one of the newer soil-less potting mixes. The mix should be moist, but not wet and the pot should be tapped lightly to fill it with the mix—don’t press the mix down. Carefully apply water until it starts to drain from the bottom, and even a little more, then set the pot where it can finish drainage, and pick it up to show that no more water is coming out. Now, tip the pot to about a 45-degree angle, and more water will drain out, usually to the surprise of everyone but you.

Where did this water come from? It was still in the mix, but capillary action in the channels of the mix held it against the pull of gravity. When you tilted the pot, some of these channels were lengthened and the weight of the water in them succumbed to gravity. This might seem strange but consider that the channels of that loose mix hold the water and oxygen a plant will need—especially in your air-conditioned room. So, what is the effect of that layer of material in the bottom of a pot on the length of these channels, or so-called “gravity pores in the mix? It shortens them so that the mix doesn’t have the proper balance of oxygen and water that it would with longer channels.

There is still a bit of conundrum here because we started by saying that the plants in your room will need more water. Why wouldn’t the shorter pots hold more water? Well, those gravity pores are needed both to hold oxygen and water for the plant. This gets us back to the “capillary action” that we mentioned earlier. You learned this back in high school science, but that was long ago. Water tends to stick to a solid surface a bit more tightly than it does to other molecules of water. Back then, the teacher showed you that water in a pipette sticks to the glass a little more tightly than it does to the water beneath it. That formed a crescent-moon shaped surface at the top of the water. Your teacher called this a meniscus. (Scientists love using foreign words—meniscus is a Greek word that means a shape like the crescent of the moon.)

Now we can get back to the taller pot holding more water for a plant than the shorter one does. The tall pot has longer channels that hold more oxygen and water than do the shorter ones. This means that the tall pot will allow the plant to have more oxygen and water—so you won’t have to water so often. Of course all pots need to drain excess water so that the plant can get oxygen. You do need drainage holes, but if you put rocks in the bottom of a pot it only makes the pot shorter and doesn’t improve drainage at all.  

Ed O’Rourke, Jr. and Leon Standifer are the authors of Gardening in the Humid South (LSU Press, 2002), in pots or otherwise, now available in affordable paperback.