I'm sure you've read that phrase time and again: start small plants or seedlings off in a small pot, “... then pot them on as they grow.”
Ever wondered what it means, and why we do it?
“Potting on” means starting the seedling or small plant in a small pot, one which is only just big enough for the roots that it has. Then, when the roots reach the edge of the pot, we take the plant out and put it in the next size up, adding some fresh compost to make it fit. This can be repeated four or five times as the plant grows, and I've often been asked why we have to bother with it, and wouldn't it be easier to just pop the seedling in a big pot to start with?
The main reason we do this “potting on” procedure is to prevent the soil from becoming sour - but what do we mean by sour soil?
In a pot, the soil is completely isolated from the natural world: no worms or beetles move around below the surface, churning up the soil to aerate it, and often it gets less natural rainwater and more “watering can” water than plants which grow out in the garden.
This can easily lead to a potful of compacted soil which is lacking in nutrients, particularly if the water comes from the tap, rather than the water-butt: and even water-butt water can be quite nasty, if it has run off a house roof. Also, most commercial composts only contain nutrients for about six weeks (I'm not kidding! Read the back of the pack!) so after that time, the plant can struggle to put on new growth.
Potting on prevents this from occurring: disturbing the roots and adding fresh compost allows air into the soil, and every time we pot on, the plant gets a fresh supply of nutrients. I should say at this point that “air in the soil” does not mean having great big gaping cavities, which are very bad for roots: it means that the soil is not a solid clump, but has some degree of “crumbliness” about it. I seem to remember from my RHS Level 2 certificate, all those years ago, that the perfect soil was made up of 25% air, 25% water, 45% organic matter (i.e. dead bugs, decaying foliage etc) and just 5% actual solids, i.e. rock, stone and sand particles. Yes! Perfect soil is only 5% actual solid matter!
Once you get to a certain size of pot - I'd say about a 2 litre pot, but I'd be interested to hear your views - then things are not so critical: more rain water can reach the surface, there is a larger volume of soil available, and by the time you have potted on to that size, there may well be a few harmless creepy-crawlies working their way around inside the rootball, aerating as they go.
But small, new plants need all the help we can give them, hence this strange “potting on” regime, which seems like a lot of make-work but is actually very sensible.
And how do we know when to do it? Answer, when the roots have reached the edge of the pot, or are starting to make a break for it through the bottom. Many people will tip the plant out, to check on the roots, but this can cause damage to the plant, so I prefer to just lift the pot above my head and look for any visible roots starting to come through the bottom of the pot. If I can't see any, I put it back!
The photos show the two extremes of this regime: the Fascicularia bicolour (great name!) is not yet ready for potting on: there are a couple of good strong new roots, but as you can see, there is plenty of undisturbed soil for it to go into.
On the other hand, the other one is positively screaming out to be potted on, and might even have to go up two “sizes” in one go. I include this one to show you that even Professional Gardeners occasionally get caught out!