The same tomato plant in four different containers produces completely different yields. The container is not a secondary detail.
The fabric grow bag performs best for a specific reason: when roots reach the permeable wall, the air at the surface desiccates the root tip and the plant responds by producing dense lateral branching further back — a process called air pruning. In a solid plastic pot the root circles continuously until it becomes pot-bound, compressing its own vascular system. In a fabric bag the root system fans outward in branched layers, maximising the volume of compost it can access.
Fabric bags also run significantly cooler than dark plastic containers. A black plastic pot in full summer sun can reach soil temperatures that slow root activity and reduce fruit set. A fabric bag in the same position will typically stay 6 to 10°C cooler.
What happens in each container:
Dark plastic pot — traps heat. Root temperatures above 28°C in summer conditions are possible. Roots circle and become constrained. Inexpensive to buy, costly in yield.
Fabric grow bag — roots branch rather than circle. Better oxygen at the root zone, better water distribution, better production. The best choice for tomatoes on a balcony or terrace.
Terracotta pot — transpires moisture through the walls, which cools the root zone naturally and prevents waterlogging. Excellent for herbs and drought-tolerant crops. For tomatoes it increases watering frequency, and in hot dry summers it can stress the plant. Works well with consistent attention.
20-litre bucket — conducts temperature change rapidly. Roots experience cold nights and warm days as sharp fluctuations rather than buffered changes. Works for tomatoes if the bucket is partially buried or insulated.