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How to Plant Tomatoes in a Grow Bag

Updated: October 2026

You have the plants, the bag is in position and you are about to cut the plastic. Before you do, read the label: how to plant tomatoes in a grow bag depends on that product's volume, drainage design and included-feed period. One manufacturer tells you to pierce the base while another explicitly says not to. The [tomato soil comparison](/hub/fertiliser-soil-and-bark-mulch/tomato-soil) records those listing differences. The safe sequence is to open the compost carefully, set plant count from its size, follow its own drainage instructions, keep moisture even and add liquid feed only at the stated growth stage. A qualifying purchase can earn us a commission at no extra cost to you.

You have the plants, the bag is in position and you are about to cut the plastic.

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UK GROW UK GROW tomato compost 10 litres

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Munnie Munnie multi-purpose compost 60 litres

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2you 2you vegetable grow bag 30 litres

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2you 2you vegetable grow bag 30 litres
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On this page
  1. 1.Before you cut the bag
  2. 2.Planting and drainage
  3. 3.Water and feed
  4. 4.When to stop and ask for help

Before you cut the bag

Move the unopened bag to its final sunny position first. Harden tomatoes off and plant outside only when conditions are reliably warm. RHS guidance uses temperatures above 16°C. Fit supports before you risk pushing canes through a reservoir. Open compost with your face away from the seal. The RHS advises avoiding dust, dampening dry mix and not storing bags in a hot greenhouse. Wear gloves and moisten dusty contents gently. Read volume, plant count and feed duration before you cut. Which? advises at least 10 litres per plant. The RHS recommends two plants per standard growing bag, while Gardeners' World uses two in 60 litres. Loose compost needs a pot, and the RHS recommends one tomato in a 30–45 cm pot. The grow bag versus pot guide separates those jobs.

Planting and drainage

Loosen the compost while the bag is closed. Lay it flat and cut only the marked planting openings. Make a hole deep enough for the root ball and bury part of a leggy tomato stem, because tomatoes can form roots along buried stem tissue. Firm the compost gently. Plant two tomatoes in a standard bag unless its volume supports another arrangement. Fit supports immediately. For a pot, plant one tomato centrally in the 30–45 cm container and leave a watering gap below the rim. Drainage is where generic instructions fail. Westland's Big Tom guide tells you to make holes in the underside. Levington Tomorite says not to pierce its base because the bag is designed to retain water. The 2you listing does not capture a drainage direction. Follow the wording on the delivered product.
UK GROW UK GROW tomato compost 10 litres

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UK GROW UK GROW tomato compost 10 litres
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Water and feed

Check moisture below the surface. Peat-free compost can look dry on top while remaining damp underneath, so use a finger test around 3 cm deep or lift a manageable pot. In hot weather, check daily rather than water automatically. Which? found that daily watering of coir-grown tomatoes in a six-week trial leached nutrients and gave the worst result among the tested routines. Water slowly around the root zone. The RHS links moisture fluctuations with split fruit and blossom end rot. Do not feed seedlings just because the bottle is available. The 2you bag claims feed for up to six weeks, Miracle-Gro up to three months, and UK GROW states no feed period. The RHS advises a high-potassium liquid feed every 10–14 days once the first fruits swell. Westland Big Tom uses the first flower truss. Levington times feed after its included nutrients, with different intervals outdoors and under glass. Those schedules are not interchangeable. The tomato compost buying guide shows why a printed feed window belongs in the purchase.

When to stop and ask for help

Levington says some white mould on its compost can be harmless, but that does not make every contaminated bag safe. If compost is soaking, smells rotten or contains glass or plastic, stop using it for food crops, photograph the batch and contact the seller. If you develop breathlessness, fever or flu-like symptoms after inhaling compost dust, stop work and seek medical advice, explaining the exposure. Urgent breathing difficulty needs urgent medical help. Do not reuse greenhouse compost from a blight-affected crop for tomatoes the following year. That is RHS advice. For a replacement, use the peat-free compost guide.

Common questions

How many plants should go in a grow bag?
The RHS recommends two tomato plants in a standard growing bag. Check the stated volume as well: Which? gives at least 10 litres per plant, while Gardeners' World uses 60 litres for two. Do not fill every printed square when the volume is small.
Should I pierce the bottom?
Only when your bag says so. Westland Big Tom instructs you to make underside holes, but Levington Tomorite says not to pierce its base. Their water-management designs differ, so the delivered label takes priority over generic grow-bag advice.
When should I start tomato feed?
Follow the compost and feed labels. The RHS advises high-potassium liquid feed every 10–14 days once fruit begins to swell, while some brand instructions use the first or second flower truss. Do not feed young seedlings simply because you have planted them.
How often should I water?
Check moisture daily in hot weather, but water only when the compost needs it. Test about 3 cm below the surface or lift the pot. Peat-free compost can look dry on top while remaining wet, and unnecessary daily watering can wash nutrients out.
Is white mould on compost dangerous?
Not always. Levington says some white mould on its compost is harmless, but that claim applies to its product guidance. Sour odour, waterlogging, rubbish or sharp material are different warning signs. Stop using a contaminated bag for food crops and contact the seller.
Can I reuse compost after tomato blight?
Do not reuse greenhouse compost from a blight-affected crop for tomatoes the following year, according to the RHS. Removing the old plant does not guarantee that the growing medium is suitable. Use fresh compost for the next tomato crop.

Related pages

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Best Tomato Grow Bag UK: A Top Pick With Conditions

Tomato Compost Buying Guide: Litres, Then the Label

Tomato Grow Bag vs Pot: Volume, Watering and Depth

Peat-Free Tomato Compost: What the UK Label Must Say

Sources

Public UK sources used for this comparison; listings checked on amazon.co.uk. Last accessed 15 October 2026.

  1. RHS: How to grow tomatoes (retrieved 15 October 2026)
  2. RHS: Peat-free compost (retrieved 15 October 2026)
  3. RHS: Tomato blight (retrieved 15 October 2026)
  4. RHS: John Innes compost (retrieved 15 October 2026)
  5. RHS: Minimising health risks in the garden (retrieved 15 October 2026)
  6. Which?: How to grow tomatoes (retrieved 15 October 2026)
  7. Gardeners' World: tomatoes in a growing bag (retrieved 15 October 2026)
  8. Peat-Free Partnership: legislation timeline (retrieved 15 October 2026)