Old-School gardeners are notoriously suspicious of trendy new ideas in horticulture, and with good reason. Planty social media is often filled with claims so colourfully fact-free they would make the average well-being influencer blush. But before older growers like me get too smug, it is worth remembering that much of the best-established gardening advice, when actually put to the test, also turns out to be based on pretty shaky evidence. It is hard to find a better example of this than the widely held fear of “over-potting”. Let me explain.
When potted plants outgrow their containers, the seemingly unanimous advice from the gardening establishment is that they can only be moved to a pot one size larger. In practice, this means giving the plants only a few extra centimetres around their root ball. Break this rule, and even august institutions like the Royal Horticultural Society that plant roots will start to rot – due to the excess moisture trapped by the large volume of potting soil – after which they will start to lose their leaves and then collapse.
The basic idea seems to be that bigger pots have a smaller surface-area-to-volume ratio, meaning they lose moisture much more slowly. Without a great mass of thirsty roots to wick it away, this excess moisture leads to decay. The problem is, when you actually look for evidence to support this idea in the scientific literature, it just doesn’t seem to be there.
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You might think this is due to a lack of trial data, which shouldn’t negate centuries of accumulated knowledge. But the effect of pot size on plant growth has been tested more than once or twice before – unsurprisingly, given that the horticulture industry is worth billions – and the findings of these studies are the opposite of what the theory of over-potting claims. In fact, a of 65 trials in a range of plant species found that “on average, a doubling of the pot size increased biomass production by 43%”.
So, according to a pretty hefty stack of studies, moving straight to a much larger pot size, rather than slowly transitioning up a series of containers, is better for plant growth. It also means putting in significantly less effort, and it is cheaper and potentially more sustainable, too, as you don’t have to buy all the in-between pot sizes. For plants that are sensitive to root disturbance, whose growth can suffer after repotting (and there are loads of those), skipping these interim steps is likely to be particularly beneficial.
Even if we give tradition the benefit of the doubt and imagine that over-potting has the claimed effect – of an over-accumulation of moisture – there is a simple way to get around the problem: just water less. In other words, reduce your workload even further, while delivering better results for your prized plants, like this tomato plant, pictured. I’m in.
James Wong is a botanist and science writer, with a particular interest in food crops, conservation and the environment. Trained at the Royal Botanic Gardens, Kew, in London, he shares his tiny flat with more than 500 houseplants.
You can follow him on X and Instagram @botanygeek
For other projects visit newscientist.com/maker
