A few days ago I got a question from Cynthia about “potting up.” For those of you for whom this is an unknown phrase (and no, it’s not a euphemism for a certain herbal activity), it refers to the practice of moving plants into ever larger containers. She was wondering if there was any “real science” behind the practice – in other words, why not just start out with a larger container?
Hah! I needed no further encouragement and spent several days collecting and reading decades’ worth of research. And there is a LOT of research on this topic. As you might guess, it’s geared towards production nurseries and greenhouses. But the good part is that it’s been done on just about any kind of plant material you could want. Vegetables. Annuals. Perennials. Grasses. Shrubs. Native plants. Ornamental, fruit and forestry trees. Seeds, seedlings, cuttings, big plants, little plants. Ahhhhh…data!
Almost without exception, you get better growth on plants grown in larger containers, whether you’re measuring height, number of leaves, leaf area, stem diameter, shoot and root dry and fresh weights, whole plant dry and fresh weight,…you get the idea. This isn’t surprising, because with a larger root zone you can support more roots, which in turn support more above-ground growth.
The only parameters which tended to diverge for some species were flower and fruit production. Restricted roots can stimulate sexual reproduction in plants, possibly because poor growing conditions spur the plant to reproduce before it dies. Other drawbacks include increased probability of circling root systems, and higher ambient soil temperature, compared to plants in larger containers.
Smaller containers might be considered desirable when one is trying to limit above-ground growth – the “bonsai” effect. And they require less water than larger containers – which brings us to the bottom line, as far as production nurseries are concerned.
Larger containers take more space. And water. In at least one study, water costs were shown to be “prohibitive for larger container sizes.” Furthermore, smaller containers are preferred by production nurseries to “optimize production space.” Another economics-based study found that “the smaller of these was the more economical.”
But most of you probably aren’t interested in the economics of plant production – you want to know what’s best for your own container plants, whether they are houseplants or pots of herbs or punches of annual color on your patio. The science is clear: it’s best to pot up plants in small containers quickly into their final destination, rather than making several (pointless) intermediate transplants.