Potting up - science or myth?

Mate in the pub swears that the best way to grow fruit and veg (maybe pretty much anything else for all I know) in pots is to start as small as possible, only put them into larger pots as they start to get root-bound in the existing pot-size, and then only use a slightly larger pot, until it outgrows that in turn, and so on. He describes this as "potting up" (I'm a newbie to this as you may gather, so I have no idea if it is accepted wisdom), and he says that it is standard advice (which I take on trust).

The practice is to be contrasted with the supposedly inferior technique of starting small (to save on compost if it dies etc) and then once you are certain that the plant is established, put it into its final destination pot, which will initially (but hopefuly not ultimately) be substantially too large for the plant at the point of transfer.

Lacking advice, my gut feel would have been to go for the second technique rather than the first, purely on the grounds that the less that you disturb the plant the happier it will generally be.

So, if this practice of potting up by gradual degrees really works, does anyone have any idea why it does?

 

I go along with the first theory, although not to the great extreme that you describe.

Main reasons being that a small plant in a large pot is more likely to get root rot in cool weather, also as a plant grows, the roots of many, head straight for the bottom of the pot and then go round and round.

If its in a small pot you can move it up into a bigger one, and then a bigger one, that way you get roots right through the root ball and not just on the outside.

A fresh bit of compost also gives the plant a bit of a spurt growth wise.

 

What mate in pub calls 'potting up' I call 'potting on'. 'Potting up' as far as I've understood and used the term refers to transferring a seedling into a pot fot the first time. I may well be quite wrong though!

 

The idea is ok as long as the root of the plant being potted is able to be inserted completely vertically. Hard to describe without a picture, but when potting any plant, the root of the seedling should not be planted in such a way that it is NOT bent sideways. This causes a check to the growth while it reorients itself and any plant which suffers a check like that is not as good as it ought to be. So while the width of the pot may be narrow, the depth is critical.

 

Interesting, I'll remember that.

 

Only applies to the initial pricking out though - after that you are only adding an extra 1/2", or so, of compost all round the original rootball.

I tend to go up in pot sizes by 1.5" to 2" diameter - so from a 3" to a 4.5" or 5". But books that I read recommend less than this - usually 1" increase at a time. I find that far too fiddly to try to get any new compost around the original rootball - a 1" increase in diameter is only 1/2" all the way round ...

For vegetable plants that are going outside I normal grow in seed tray in propagator, then "prick-out" / "pot-up" to 3" (if they are going to be potted-on, e.g. Tomato / Cucumber) or 3.5" if they are going to be planted out (e..g lettuce). I then pot-on by 2" - so 3" goes to 5" then 7" and then probably straight into 11" (final growing pot size).

However, for Fuchsias that I am trying to grow to a nice shape I subscribe more to the "give a little new compost often" approach of potting-on quite frequently.

My book on Fuchsias describes it thus:

"unlike the soil outside in your borders, the soil in a pot never encounters real weather and doesn't have worms to aerate it. The soil around an overpotted plant (i.e. one planted into a much bigger pot size) confined and constantly watered, will soon lose its air spaces, becoming compacted and sour. Whereas when potted-on 1" at a time the plant is actively making roots which keep the soil open and productive."

 

Jack, what the others have said is absolutely correct and I wouldn't disagree.

But ...... there is an other side to this and I would appreciate any comments or personal experience.



There are two plants above. They happen to be Scuteralia zhongdianensis (I find these cheap Chinese imports such good value :D), but I have had all sorts of other plants in a similar situation. In my mind there is no question that plants always grow bigger in bigger pots. Normally you pot on when the plant becomes rootbound, ie you can see lots of roots at the edge of the pot. These two plants were grown from seed at the same time and one was put in a larger pot by chance and has grown a lot bigger. But to my mind the smaller one on the right is not obviously pot bound. There are some roots at the edge, but it is not excessive. It looks happy, but not clearly in need of potting on. But if it had been potted on it would have been the same size as the one on the left, as I have several others all showing the same increase of plant size with pot size.

As a consequence I like to pot plants on pretty quickly - usually earlier that the textbooks might say. But I wouldn't put them straight into huge pots.