What Are Grow Bags and Grow Bag Pots?
Grow bags are flat, rectangular bags of compost used to grow vegetables, fruit, and flowers — particularly in greenhouses and on patios where border soil is poor or unavailable. They're one of the most popular ways to grow tomatoes, cucumbers, peppers, and aubergines in the UK, offering a fresh, disease-free growing medium every season.
Grow bag pots (also called grow bag planters or toppers) are rigid plastic rings or frames that sit on top of a grow bag, creating a deeper, more manageable growing container. They solve many of the frustrations of growing directly in a flat grow bag — and if you've ever tried to water a standard grow bag without flooding it, you'll understand exactly why they're so popular.
The Problem with Standard Grow Bags
Grow bags are brilliant in theory but notoriously tricky to manage in practice:
- Watering is difficult — the flat surface makes it hard to water without runoff; water pools on top or runs straight out of the drainage holes
- Shallow depth — standard grow bags are only 15–20cm deep, which limits root development for larger plants like tomatoes
- Instability — tall plants like tomatoes and cucumbers can topple a flat grow bag, especially when canes are inserted
- Drying out — grow bags dry out very quickly in a warm greenhouse, sometimes needing watering twice a day in summer
Grow bag pots address all of these issues by adding depth, creating a reservoir for water, and providing a stable base for canes and supports.
Types of Grow Bag Pot
Standard Grow Bag Pots
A simple rigid ring, typically 25–30cm in diameter, that sits on top of the grow bag. You cut a hole in the grow bag beneath the pot, allowing roots to grow down into the bag while the pot holds additional compost above. Most come with a built-in water reservoir or can be used with a separate watering system.
Grow Bag Watering Pots with Reservoirs
These feature a built-in water reservoir at the base of the pot that slowly releases water into the compost, reducing the frequency of watering needed. Ideal for busy gardeners or anyone who struggles to water consistently during hot spells.
Large Grow Bag Planters
Deeper, larger versions that sit over the entire grow bag, effectively converting it into a deep container. These are particularly good for root vegetables and larger fruiting crops that benefit from extra depth.
Grow Bag Frames and Supports
Some grow bag systems include integrated cane supports or frames, making it easy to support tall plants like tomatoes and cucumbers without separate canes and string.
What to Grow in Grow Bags
Grow bags are best suited to crops that produce a heavy yield from a relatively small number of plants:
Tomatoes
The classic grow bag crop. Plant 2–3 tomato plants per standard grow bag (2 is better for larger varieties). Cordon (indeterminate) varieties like Gardener's Delight, Alicante, and Moneymaker are ideal. Bush varieties work well too and need less support.
Cucumbers
One or two plants per grow bag. Cucumbers are vigorous and need strong support — train up canes or a trellis. Keep well watered and fed once flowering begins.
Peppers and Chillies
Two to three plants per bag. Peppers prefer slightly drier conditions than tomatoes — don't overwater. They do well in a warm, sunny greenhouse or polytunnel.
Aubergines
Two plants per bag. Aubergines need warmth and consistent watering. Pinch out the growing tip when the plant reaches about 30cm to encourage bushy growth.
Strawberries
Grow bags work brilliantly for strawberries on a patio or greenhouse staging. Plant 3–4 plants per bag and keep well watered during fruiting.
Courgettes
One plant per bag — they're large and spreading. Grow bag pots are particularly useful for courgettes as they help retain water around the roots.
How to Set Up a Grow Bag Pot
- Position the grow bag — place on greenhouse staging or a patio in a sunny spot. Shake to loosen the compost if it's compacted
- Cut the holes — following the grow bag pot manufacturer's instructions, cut circular holes in the top of the grow bag where each pot will sit
- Place the pots — press the grow bag pots firmly into the holes so they make good contact with the compost below
- Fill with compost — fill each pot with fresh multipurpose or tomato compost to within 2–3cm of the top
- Plant up — plant your seedlings or young plants into the pot compost, firming gently
- Water in well — water thoroughly through the pot; the water will drain down into the grow bag below
- Insert supports — push canes through the pot and into the grow bag for stability
Watering and Feeding Grow Bags
Watering is the most critical aspect of grow bag success. In a warm greenhouse in summer, plants may need watering once or twice daily.
- Water through the pot — always water into the grow bag pot, not directly onto the grow bag surface
- Check daily — push your finger into the compost; water when the top 2–3cm feels dry
- Don't let them dry out completely — a severely dried-out grow bag is very difficult to re-wet evenly and can cause blossom end rot in tomatoes
- Start feeding at first flower — once the first flowers appear, begin feeding weekly with a high-potassium liquid tomato feed. Grow bag compost contains enough nutrients for the first 4–6 weeks, after which regular feeding is essential
- Consider a drip system — a simple drip irrigation system connected to a timer takes the stress out of grow bag watering, especially during holidays
Common Grow Bag Problems and Solutions
- Blossom end rot (tomatoes) — caused by calcium deficiency, usually due to irregular watering. Water more consistently and use a calcium-containing tomato feed
- Yellowing leaves — usually a sign of nutrient deficiency; start or increase feeding
- Wilting despite watering — check roots aren't waterlogged; ensure drainage holes in the grow bag base aren't blocked
- Poor fruit set — tap flowers gently or use a soft brush to transfer pollen; in a closed greenhouse, open vents to allow air movement and pollinating insects in
Reusing Grow Bags
Grow bag compost can be reused after the season, though it will be depleted of nutrients and may harbour disease. Options include:
- Add to the compost heap or dig into borders as a soil conditioner
- Use as a mulch around established shrubs and trees
- Mix with fresh compost for a second season of less demanding crops (avoid reusing for the same crop family)
Never reuse grow bag compost for tomatoes, peppers, or cucumbers in the following year — soil-borne diseases like tomato blight can persist.
Shop Grow Bags and Grow Bag Pots at Selections
Browse our range of grow bag pots to get the most from your greenhouse growing this season.