Potting On Tomato Plants – Root Bound – Root Capping

Potting On
Tomato plants grow at different rates depending on light and temperatures etc. Combine this with varying pot sizes and “when to pot on” becomes a matter of judgement and experience as every grower’s circumstances will be different.

The common advice is to pot on when roots start showing through the bottom of a pot.

However, if you’ve added perlite or vermiculite to the mix, potting a plant into a bigger pot can be delayed for a couple of weeks.

Root Bound
When plants become “root bound” they eventually run out of nutrients and oxygen. If they are not given more root space …

  • their health will suffer
  • or their growth may become stunted
  • or they flower early with very few fruit

There is a technique called “forcing” which keeps plants in a small pot and forces early flowering because the plant thinks its “time is up” because its roots tell it that resources are limited.

This technique should be handled with care because timing is crucial in order not to slow growth.

Root Capping
One of the few disadvantages with growing tomatoes in large pots and containers is that the surface area of a pot is limited.

Plants roots take oxygen through the soil surface, so the less surface area there is, the less access to oxygen plants have.

This is especially relevant for a mature tomato plant that has been in a large pot for some time.

Fine roots circle at the soil surface creating a dense cap that prevents oxygen from entering into the root zone below. As we know, roots need plenty of oxygen for respiration, without it, plants run out of steam.

One way to avoid root capping is to use air pots or fabric pots which allow air in through the sides.

If you are using a conventional pot, an “air pipe” is useful.

An air pipe is a plastic or rubber pipe that runs vertically from the surface to the bottom of a large pot.

Air pipe increases oxygen in tomato pots
Overflow pipe from Wicks – just over a £1.00 for 2 metres. I cut at 33cm to make six air pipes – you have to drill your own holes though!

Holes are drilled in the side of the pipe to allow air into the root zone – from top to bottom. An old length of hose pipe or any plastic tubing will do – as long as there are plenty of small holes to allow oxygen into the roots and other gasses to escape!

Before rockwool and perlite became the most popular methods of commercial tomato growing, air pipes were frequently used by growers who grew tomatoes in containers of soil.

As we know, a good oxygen supply to the roots means quicker, healthier growth and a bigger yield.

Germination Tips
One of our newsletter members, Rhys, has come up with an excellent way of germinating seedlings. This method has some of the advantages of a propagator but without the expense.

Here are Rhys’s comments and suggestions….

Broadly, the issues which need to be managed where germination is concerned is the temperature, the pH of the water used and the maintenance of soil dampness during germination.

Using small trays with 18 slots, I was finding that either the soil dried out very fast without cling film on top or even trying to water led to saturation, neither of which seemed to be very good on germination rates. Putting significant water on the tray beforehand led to soil compaction which didn’t seem to aid germination.

So: I did the following things:

1. Took an old ice cream tub and made four holes in the bottom by gently hammering a screwdriver on the inverted base.
2. Filled the tub just over half full with seed compost and fairly saturated it using warmish mixture of rainwater and boiled tap water (the pH of that mix should be neutral or slightly acidic).
3. Then added fresh seed compost on top (no watering) and sowed the seeds in 4 rows in it.
4. Put cling film on top and keep in boiler outhouse at around 20C for 5 days.
5. Remove cling film as by this point the upper soil should have sufficient water in it through upward migration.
6. Transfer the tub indoors into the light after 12 days in boiler house.

With 16 seeds sown, I have: 4/4 Bush Beefsteak ready to transplant; 4/4 Glacier ready to transplant, 4/4 Black from Tula already visible; 4/4 Black Cherry just coming through.

New Seedlings

I have a second tub sown a week later which we’ll see if it does as well, but this does appear to be a pragmatic, practical way of managing soil moisture without need for complicated bits of kit.

I’m sure you could do likewise keeping the tub indoors, it’s just that our outhouse boiler seems to be a good place to create the right temperature without using the airing cupboard.


Thanks to Rhys for his advice this week.

By the way, It’s always easier to prick-out seedlings when there is a good depth of compost to work with!

When to leave plants out in the greenhouse at night
It looks like we are going to have a few days of mild weather here in the UK. The end of April and beginning of May is the traditional time for leaving plants out at night in an unheated greenhouse or pollytunnel.

Condensation and damp air is an issue so make sure that there is plenty of aeration (for transpiration) first thing in the mornings.