Does Pot Size Matter for Chilli Plants? Our Experiment Results

We’ve had a lot of questions from readers about if and when they should pot on their chilli plants and what pot sizes to use. This is a hard question to answer as much will depend on the stage of development of your plants and the condition they are grown in.

Many of our readers seem to be planting their chilli seeds in communal containers such as window boxes or large planters. While this is great for saving space during the germination and seedling stages it can cause problems later on. If left too long, the plants roots become entangled and when the plants need separating into larger pots you can run the risk of damaging plants. As a result planting chillies in single pots is best.

As I mention in my recent post about potting on I tend to pot my plants on every few weeks when the roots become visible out of the bottom of the pot. During a season i usually pot on about 4 times, gradually increasing the pot size up to about a maximum of 10-12″ depending on the variety.

Potting On Experiment

While the above potting on routine seems to have worked for me in the past, I’m not too sure if something else would work better to I’ve decided to run a  little experiment. I have taken 6 Cayenne plants, all the same size (roughly 15″ in height) that until now have been in 6″ pots. I will leave two in the same pots and have potted two into 8″ pots and two into 10″ pots and see how they fair in different pots.

I’ve heard one theory that states that potting into too large pots means the plant wastes energy developing it’s root system instead of flowering and producing chillies. Conversely other wisdom states that leaving in too small pots won’t allow the plant space to develop and produce to it’s full potential.

All of the plants are the same size at the minute and none are flowering quite yet so I think now is the perfect time to run this test.

I’ll keep all six plants in the same conditions (in a conservatory that receives a good amount of light and has a small heater for chilly nights) and see how their growth is affected by their pot size. Of course not only with the rate of growth be interesting but the yield of Cayenne peppers I get form them. I’ll report back later in the season with the results!

Do you have any tips or interesting ideas about pot sizes for peppers? If so leave a comment below.

The Experiment Results

After running this experiment across a full growing season the results were clear — and somewhat counterintuitive.

The plants in the 6-inch pots — the ones left in their original containers — produced the highest number of chillies per plant. The plants in the larger pots grew bigger and bushier but put more energy into vegetative growth and produced fruit later and in lower quantities per plant.

This mirrors exactly what Julie found independently in the comments section – her plants in smaller pots produced more chillies per plant and the chillies themselves were larger, while the plants in bigger pots grew larger but set less fruit.

The explanation seems to be that when a chilli plant is slightly root-bound (not dangerously so, but with its roots filling the available space) it perceives a degree of stress that triggers earlier and more prolific flowering. A plant with unlimited root space has less incentive to reproduce and instead puts energy into vegetative growth.

This doesn’t mean you should leave plants in tiny pots indefinitely — a severely root-bound plant will eventually suffer from nutrient depletion and water stress (often resulting in yellowing of the leaves). But it does suggest that the common advice to always pot up into the largest pot available isn’t necessarily right for chilli growing specifically.

Our revised recommendation based on the experiment and reader experience:

• Pot on regularly in small increments – do not jump from a 3-inch pot straight to a 12-inch pot
• Move up one pot size at a time, waiting until roots appear from the drainage holes
• For most varieties, a final pot of 7-10 litres (approximately 10-12 inches) is ideal
• For large varieties like Rocoto or giant bell pepper types, go up to 12-15 litres
• For compact varieties like Apache or Prairie Fire, a final pot of 3-5 litres is sufficient
• Feed regularly with a specialist chilli fertiliser from the point of first flowering – Chilli Focus is our recommendation and what Mike in our comments has used with excellent results

What Pot Size Should I Use for Chilli Plants?

Based on 20 years of growing and the feedback of hundreds of readers, here’s the pot size guide we’d recommend for most UK growers:

Seedling Stage — Module Trays or 3-inch Pots

Once your seeds have germinated and you have a seedling with its first true leaves, pot on into individual 3-inch (7-8cm) pots. Don’t be tempted to go bigger at this stage — a large volume of compost around a small root ball stays wet for too long and can cause damping off. See our guide to when to repot chilli seedlings for exactly what to look for before making this first move.

Growing On — 5-inch Pots

When roots appear from the bottom of the 3-inch pot — typically 3-4 weeks later — pot on into a 5-inch (12cm) pot. Again, just one size up.

Intermediate Stage — 7-inch / 1-2 Litre Pots

A further 3-4 weeks later, once the roots fill the 5-inch pot, move into a 7-inch or 1-2 litre pot. Plants at this stage are usually 15-25cm tall and growing vigorously.

Final Pot — 7-10 Litres for Most Varieties

Most chilli varieties will spend the bulk of their productive season in a final pot of 7-10 litres — roughly a 10-12 inch pot. Based on our experiment and reader experience, going bigger than this tends to produce lush growth at the expense of fruiting. This is the stage where feeding becomes critical — use Chilli Focus or liquid tomato feed at half strength weekly once flowering begins.

Which Pot Material?

Plastic pots — the standard choice for most growers. Lightweight, cheap and retain moisture well. The main disadvantage is that plastic pots in a greenhouse in summer can get very hot, which can stress roots.

Terracotta — heavy and expensive but genuinely good for chillies and they look great. The porous walls allow excellent airflow around roots and regulate moisture well. Our choice for overwintered plants kept indoors.

Grow bags — the flat compost-filled bags work well for chillies and are popular with greenhouse growers who want to plant multiple plants in a row. The limited depth restricts root development somewhat but the large compost volume compensates. Useful if you’re growing in a greenhouse border.

For the full picture of growing chillies from seed to harvest see our complete growing guide.

FAQ