I start small and pot on. I think if I put a small plant in a large pot, the nutrients in the compost could leach out with the watering before the plant roots got big enough to use it. That, and limited windowsill/coldframe space, and the fact that it's easier to dig a hole for a plant in a smaller pot come planting out time (for things that are vigorous enough to go out relatively small).
Doncaster, South Yorkshire. Soil type: sandy, well-drained
I start small and pot on. I think if I put a small plant in a large pot,
the nutrients in the compost could leach out with the watering before
the plant roots got big enough to use it.
I would think that small, annual plants, for example, don't need much in the way of food (for the home grower). Regular MP compost has enough fertiliser added to it in the bag for a few months growth, which is usually enough to get annuals to adult size and then we add tom feed or granules or whatever. Again, I'm personally not growing to scale like many members of the
forum, so feeding a few plants isn't very much of a cost consideration
for me. A bottle of tom feed with get me through the season.
Most of what we "buy", when we buy MPC is just a growing matrix that roots can be supported by. The nutrients are added. People like peat because it holds water well. It offers little in the way of nutrient on its own. We add elements like perlite or vermiculite or grit because it can open up the medium and makes it easier for the tiny roots to explore and push out. Tiny roots of say, a salvia cuttting, would have a harder time trying to explore in a pot of pure clay soil taken from the garden. That holds more water and less air and it risks the stem and roots rotting. That's my understanding.
RHS advice on the dangers of overpotting here: https://www.rhs.org.uk/prevention-protection/overpotting Using a large pot for a small plant doesn't necessarily lead to problems but where it can do is when the plant is not growing strongly, perhaps due to the time of year or slow rate of growth of that particular plant, or when there is a tendency to water too much.
So it seems the advice we are familiar with (don’t over-pot) is more of a problem for perennials - particularly in autumn. Less of an issue with annuals (or plants that we grow as annuals) like petunias or tomatoes. It then just becomes a matter of convenience, space available and time taken repotting.
Yes, I would think that in some cases the medium you are potting into is more important (re root development and water logging etc) than the size of the pot.
But if you are growing a shrub in a pot of a dense John Innes mix, then yes, it wouldn't be free draining and new roots on small plants might struggle, as they might struggle through the winter if planted straight into the ground.
I start small and pot on. I think if I put a small plant in a large pot,
the nutrients in the compost could leach out with the watering before
the plant roots got big enough to use it.
....... Regular MP compost has enough fertiliser added to it in the bag for a few months growth, .......
That's what I think would leach out before it was used, if plants were over-potted. I don't think I can water only the bit in the middle where the roots are. Nothing I grow needs extra feeding until it's been in its final size pot for a while, and only if it's getting kept in the pot for more than a few months.
Annuals never get to more than about a 3 inch pot before they're planted out. If I have a batch of perennials, generally when they fill a 3 or 4 inch pot I'll plant some out and pot some on. Tomatoes, cucumbers etc and non-hardy stuff for hanging baskets etc can't go into their final containers until it's warm enough for them to go outside anyway.
Doncaster, South Yorkshire. Soil type: sandy, well-drained
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Using a large pot for a small plant doesn't necessarily lead to problems but where it can do is when the plant is not growing strongly, perhaps due to the time of year or slow rate of growth of that particular plant, or when there is a tendency to water too much.
So it seems the advice we are familiar with (don’t over-pot) is more of a problem for perennials - particularly in autumn. Less of an issue with annuals (or plants that we grow as annuals) like petunias or tomatoes. It then just becomes a matter of convenience, space available and time taken repotting.