Tomato plant roots have a great sense of touch and it is this sense of touch that tells them to “get on with it!”
When their roots reach the sides of a small pot, it triggers the plant into quicker growth and earlier maturity. This is because the plant knows it has a limited area in which to grow, and therefore, a limited time in which to produce tomatoes and seeds for further generations of its kind.
This is one of the reasons we start with small pots and then pot on into bigger pots. To encourage plants to maturity sooner rather than later.
It also saves space – if you had a hundred seedlings, each in a 10 inch pot, you’d need a lot of room!
The later in the season that you start, the less pot sizes you’ll need. This is because the period between seedling and final position is shorter owing to plants growing more quickly due to better weather – hopefully!
Here’s a photo of my pot sizes – I started in January. If you started in March or the beginning of April, you can omit a pot size.
After the seeds had germinated, I began with a 3 inch pot, then 4 inch, then 5 inch and the final large pot is about 16 inches at the top – any large pot 9 inches or bigger is fine.
It’s best if the plant in the 5 inch pot is in flower before planting in the largest final position pot. This will prevent it from becoming too leafy.
I bought the large pot for £1.00 in Poundland – it’s perfect for a Red Alert bush or trailing variety like Tumbling Tom. Tall pots will keep the tomatoes off the ground as they cascade over the sides.
Tall varieties are best grown in grow bags or tomato pots that aren’t so high off the ground. It makes it easier to pick the higher trusses.
Why not keep a plant in a 3 inch pot until its final position (big pot)?
When plants become root-bound (kept in a small pot for too long) they run out of steam and become unhealthy.
Some growers use this technique to force plants on to maturity sooner – it’s called “forcing”. However, you need to know exactly when to pot on otherwise it will have the negative affects already mentioned.