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23 November 2020

Best grow bags

Before planting your cucumbers and tomatoes, look at our round-up to find out which grow bags will give them the best chance of growing big and healthy.
Growbag main
Jade Harding

Growing bags are popular with gardeners - and for good reasons. You don't need a pot as you can plant directly into the bag and for tender crops they're a great alternative to planting in a greenhouse border. They're a bargain option, too - often costing less than bags of multipurpose compost.

But not every bag is as great as it sounds, which is we decided to test them for you. We trialled 10 peat and peat-free growing bags to find the best for growing summer veg, such as cucumbers, tomatoes, aubergines, chillies, peppers and melons. The best produced lots of large, healthy summer vegetables, while the worst made inadequate yields with pale leaves.

Unfortunately, half of these bags have since been reformulated, although most of our Best Buys are still available. Find out which ones made the cut in our table below.

New test results will be available in March 2021. 

Best grow bags

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Growing bags Our verdict Harvest weeks 1-3 (kg) Harvest weeks 4-6 (kg) Harvest at end of test (kg) Overall rating

Peat content: 0%

M 4.9 3.4 0.3 75%

Great peat-free bag

This peat-free planter is approved by the Organic Soil Association as both the compost and the feed are organic. The instructions on the bag recommend liquid feeding after three to four weeks. 

Our plants rooted very well into these bags and produced a good number of large cucumbers throughout our test, with plenty more to come at the end of it, too.

Find out which compost we're talking about - log in or join Which?

Peat content: 45%

M 5.6 2.7 0.3 74%

Large, deep bag

This planter is larger than many others we tested and is labelled as ‘deep fill’, meaning roots can grow down as well as outwards. The compost contained in the bags is a mix of peat and wood fibre. However, the peat content is less than half, which is far less than when we tested it two four years ago. 

We found that in the first two weeks of our test, the plants growing in these bags produced a heavier crop than any of the others and lasted well to the end of our test.

Find out which compost we're talking about - log in or join Which?

Peat content: 80%

M 5.7 1.3 0.3 65%

Sold with controlled-release feed

This is the first time we’ve tested this growing bag. As with the standard compost, it comes with a sachet of controlled-release feed to add to the compost when you plant. As a result, we didn’t liquid feed these bags. The compost in the bags is peat-rich with added wood fibre. 

The bags produced a huge number of large cucumbers in the first three weeks, but ran out of steam by the end of the test.

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Peat content: 50%

M 3.5 3.3 0.3 65%

Bargain alternative

This growing bag, the cheapest in our test, contains a mix of peat and wood fibre, which we’ve found to be a free-draining yet moisture-retentive mix. 

For the first three weeks, we harvested a good but not outstanding number of cucumbers. However, in the last three weeks of the test, we harvested more cucumbers growing in these bags than we did from most of the others in the test.

Find out which compost we're talking about - log in or join Which?

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Before testing, we send out secret shoppers to buy all our grow bags so we know we're getting the same product as you. We also buy two of each to see if there are any variation in the quality of each bag.

We then sowed seeds of cucumber ‘Carmen’, a standard-sized cucumber, which can be harvested within a few weeks after planting. We potted the plants on after 10 days, then planted them into our growing bags in late May. We grew two cucumber plants in each bag.

We linked each growing bag to an automatic watering system, so all the bags received the same amount of water.

We grew and picked the cucumbers twice a week. We weighed and counted the harvest from each bag over a six-week period. At the end of the test, we harvested, weighed and graded the remaining cucumbers for size.

To find out more, head to how we test compost. Alternatively, browse our best controlled release fertilisers

How to use grow bags

  • Once young plants reach around 15cm high and the bottom leaves have developed, they're ready to plant into your growing bag.
  • Before you plant, shake the bag to fluff up the compost and make some cuts in the bottom of the bag to allow excess water to drain out.
  • Cut along the marked lines on the top of the growing bag to position your plants. If you only want to put two plants in each bag, use the two outer-planting positions on the bag.

How do you water plants in a grow bag?

  • Water regularly so the bag doesn’t dry out. In hot weather, this may need to be up to three times a day.
  • An automatic watering system with a timer will help to avoid dry plants.
  • Most summer veg will be in the bags for many weeks, so you’ll need to feed them. Either add controlled-release feed by mixing it into the compost where you have cut holes for the plants, or liquid feed when you water. Follow the dosage and frequency advice on the bottle.

For more advice on choosing the best compost for you, head to our compost buying guide.