Two plants per grow bag, not three. Water daily from midsummer and twice a day in a heatwave. Start feeding when the first truss sets, and expect the bag itself to run out of nutrition by late July, which is the point at which most crops quietly stall. Get those four things right and a grow bag will out-produce a border. Here is how to do it properly.
By Kel Dickson, Senior Brand Marketing Manager at Gro Worm, organic, peat-free grower. Reviewed by Brian McCuskey. Updated August 2026.
Why grow bags work, and where they let you down
A grow bag gives tomatoes exactly what they want at the start of the season, which is warm, free-draining compost that heats up fast in spring and drains after every watering. That is why they still outsell every other method in the UK, and why a bag on a sunny patio will usually beat the same variety planted in cold garden soil.
The limitation is volume. A standard bag holds somewhere around 35 to 40 litres, and a tomato plant in full production is drinking and feeding hard for four months. Split that between three plants, which is what the printed cutting guides on most bags suggest, and each one is working with barely a dozen litres of compost. That is the root of nearly every grow bag problem people report: drying out between waterings, blossom end rot, pale leaves in August, and trusses that set but never fill.
None of that is a fault in the method. It is a fault in the maths.
How many tomato plants per grow bag?
Two. The bags are marked for three because three sells more plants, but two gives each plant roughly half as much again to work with, and in practice two well-fed plants will out-yield three struggling ones every time.
If you are growing a large indeterminate variety like Gardener's Delight or Sungold up a string in a greenhouse, two is the maximum. For compact bush varieties grown outdoors, you can get away with three, because they finish earlier and never reach the same size.
Preparing the bag
Give the bag a good shake and a squeeze before you cut it, because compost settles hard in storage and comes out of the pallet compacted. You want it loose and open before anything goes in.
Lay it flat where it is going to stay, since a full wet bag is awkward to move without tearing it. Cut two planting squares rather than the three printed on the plastic, and pierce the base along the underside with a few drainage slits. Bags do not drain out of the top, and waterlogging at the bottom of an uncut bag causes as much trouble as drying out at the surface.
Then plant deep. Tomatoes root along their buried stems, so burying the first couple of centimetres of stem gives you extra root mass in a container that badly needs it.
Ring culture, if you want to do it properly
Ring culture is the old greenhouse trick and it solves the volume problem better than anything else. Sit a bottomless pot, or a ring cut from a plastic pot, into the planting hole and plant into that. The plant then develops two root systems: feeding roots in the compost of the ring, and water-seeking roots that push down into the bag below.
You water into the bag and feed into the ring. It sounds fiddly, and it takes ten extra minutes at planting, but it roughly doubles the useful rooting depth and makes the whole thing far more forgiving in a hot spell.
Watering, which is where most crops are won or lost
From the moment the first truss sets, a grow bag needs water every day. In a greenhouse in July, twice a day is normal. This is not optional in the way it is for a plant in the ground, because there is no reservoir of soil beneath it to draw on.
More important than the frequency is the consistency. Tomatoes take up calcium in solution, and when a bag swings between bone dry and saturated, that uptake is interrupted. The result is blossom end rot, the sunken dark patch on the base of the fruit that gardeners usually blame on a calcium deficiency in the compost. It almost never is. It is a watering deficiency.
Water at the base rather than over the foliage, in the morning if you can, and if you are away regularly it is worth sitting the bag in a shallow tray so it has something to draw on.
Feeding, and when the bag runs out
The compost in a new grow bag carries enough nutrition for around six weeks. That takes a plant potted in late May to somewhere around the middle of July, and it is no coincidence that mid-July is when people start posting photographs of pale, stalled tomato plants.
Start feeding at first truss, which usually falls a few weeks after planting out, and keep going every three to four weeks through to the end of the season. Our Tomato Boost granules go on at one handful, roughly 80ml, worked in around the base of each plant and watered in. If you would rather feed with every watering, the liquid version does the same job in solution.
If you want a bit more from the same product, brew a worm tea. A handful of casts in four and a half litres of water, left to steep for twenty four hours, gives you a microbe-rich drench that goes straight to the roots. It is the closest thing to a free upgrade in tomato growing.
The advantage of feeding with worm casts in a container specifically is that you cannot get the dose wrong. A concentrated synthetic feed measured by eye in a 12 litre root zone will scorch roots, and in a grow bag there is nowhere for the excess to go. Worm casts release slowly enough that overdoing it is close to impossible.
Rescuing a bag that has stalled
If the leaves have gone pale and the top growth has slowed, the plant has run out of food rather than caught something. Feed it, and then mulch the surface of the bag with two or three centimetres of fresh peat-free compost. That gives the surface roots something new to grow into and slows evaporation at the same time.
If the lower leaves are yellowing from the bottom up, that is usually nitrogen, and a feed will fix it within a fortnight. If the leaves are curling inwards but staying green, that is heat and water stress rather than nutrition, and the answer is shade and a more consistent watering routine.
Can you reuse a grow bag?
Not for tomatoes, and this is the one shortcut worth avoiding. Tomatoes are prone to soil-borne problems that build up over a season, and last year's bag has neither the structure nor the nutrition left to support a new plant.
It is not waste, though. Spent grow bag compost is genuinely useful as a soil improver on beds and borders, as a mulch around established shrubs, or mixed into a raised bed. It has simply done its job as a growing medium.
Frequently asked questions
How many tomato plants per grow bag?
Two for most varieties, despite the three planting squares printed on the bag. Two plants share around twenty litres of compost each instead of a dozen, which makes watering and feeding far easier to keep on top of, and usually produces more fruit overall.
How often should I water tomato grow bags?
Daily once the first truss has set, and twice daily in hot weather or under glass. Consistency matters more than volume, because irregular watering is the main cause of blossom end rot.
Do you need to feed tomatoes in grow bags?
Yes. A new grow bag holds roughly six weeks of nutrition, so feeding needs to start at first truss and continue every three to four weeks until the end of the season, otherwise growth stalls in midsummer.
Can you reuse tomato grow bags?
Not for tomatoes again, because of the build-up of soil-borne disease and the loss of structure. Empty the bag onto a border or raised bed as a soil improver instead.
What is ring culture?
A method where the plant sits in a bottomless pot pushed into the grow bag, giving it two root zones. You feed into the ring and water into the bag, which increases the effective rooting depth and makes the plant far more resilient in dry spells.
If you would rather start with better compost
Most of the problems above come down to volume and nutrition, which is why we make our Black Gold Peat Free Organic Tomato Planter larger than the usual grow bag. It is peat-free and enriched with worm casts. Pair it with Tomato Boost granules from first truss onwards and you have removed the two things that usually stop a grow bag crop in July.
