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5 May 2021

Best grow bags

See our round-up of the best grow bags for growing everything from cucumbers to tomatoes
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Adele Dyer

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 11 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.

Best grow bags

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Growing bags Our verdict Harvest of best-quality cucumbers Harvest of misshapen cucumbers Mean fruit weight Overall rating

Peat content: 45%

M 10.9kg 0.8kg
94%

Large bag

The compost mix in this growing bag contains less peat than many bags on test this year – the rest is made up of wood fibre, green compost, manure and seaweed.

It’s one of the largest bags on test and in past years we’ve found that larger bags often give better results. It certainly gave the heaviest harvest, with almost 12kg of cucumbers per bag. Plus it produced more good-quality cucumbers than any of the other bags on test.

Peat content: 70%

M 10.1kg 0.7kg
87%

Cheap but high peat

As the name suggests, the compost in this bag is a traditional high-peat mix, albeit with added wood fibre, manure, seaweed and green compost (made from composted green-waste collections). 

It’s also smaller than many other bags, so bear this in mind if you want to grow a lot of tomatoes or cucumbers. 

The two plants we grew in this bag were very healthy and gave us a heavy crop of straight cucumbers, but were slightly smaller than others.

Peat content: 60%

M 10kg 0.5kg
87%

Organic, but not peat-free

This compost and the feed within this bag have been certified as organic by Organic Farmers &Growers, a body that certifies organic foods and feeds. However, don’t confuse ‘organic’ with‘ peat-free’, as this bag contains 60% peat, along with wood fibre and bark.  

It’s described as a two-plant planter, but the small print recommends only one cucumber plant. 

We grew two and found the bags were plenty large enough to cope, giving us lots of excellent, supermarket-quality cucumbers.

Peat content: 0%

M 9.8kg 2.1kg
86%

Best peat-free bag

This peat-free planter is made from composted bark and coir, but with a small amount of carefully screened green waste. It’s 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. 

The bags produced a good number of cucumbers, and cumulatively they were one of the heaviest harvests. However, there were a larger proportion of misshapen cucumbers, suggesting this compost needs more watering than others.

Not found the right product for you? Browse all our compost reviews.

Why Which? grow bag reviews are better

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

We sow seeds of cucumber ‘Carmen’, a standard-sized cucumber, which grows fast and produces good harvests. In late May, when the plants each have five leaves we plant two into each of our growing bags. We grow them in a greenhouse, trained up wires and removed sideshoots below the fruit.

We link each growing bag to an automatic watering system, so all the bags receive the same amount of water, and feed them with a liquid fertiliser.

We grow and pick the cucumbers twice a week. We weigh and count the harvest from each bag over an eight-week period, noting if any are bendy, bulbous or have rough skins. At the end of the test, we harvest, weigh and grade 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

  1. Wait until your young plant reaches 15cm high. 
  2. Shake the grow to fluff up the compost.
  3. Make some cuts in the bottom of the bag to allow excess water to drain out.
  4. Cut along the marked lines on the top of the growing bag to position your plants. 
  5. 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 before you plant, or liquid feed twice a week once the first fruits have formed. 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.