Which? Gardening has tested the mostly widely-available grow bags so you can give your plants the best growing conditions.
Great plants start with great compost, and this is particularly true when growing summer veg, such as tomatoes, cucumbers, peppers and chilies.
We tried 11 grow bags, and grew cucumbers in them. These produce a quick crop, but they are hungry plants. As a result we can clearly see from the number and size of cucumbers which were happy in their bags and which were starved of nutrients.
We found four excellent Best Buy grow bags for you to try.
Peat-free grow bags
We were disappointed by the number of peat-free grow bags available to buy in garden centres. Almost all contained some peat, and most of them had much a higher percentage peat than your average bag of multipurpose compost. Thankfully one of the peat-free bags was good enough to be a Best Buy.
We hope more peat-free grow bags will be on sale in 2021 as manufacturers come under increasing pressure to reduce the amount of peat they use in all their composts. The government has asked compost manufacturers, garden centres and plant nurseries to come up with a timetable for phasing peat out of horticulture. Meanwhile some countryside and wildlife organisations are calling for an end to peat in compost by 2025.
Learn why peat is a problem in our guide to peat-free compost.
How to use grow bags
You can use a grow bag in the greenhouse or on a patio or balcony. If you’re growing outside, keep your plants protected indoors after the last frost, usually sometime in mid to late May.
Roll the bag gently to fluff up the compost, then cut slits in the bottom to allow excess water to drain away. Most bags have markings on the top to show where to cut to plant your veg. They usually have space for three tomato, aubergine or pepper plants, and two cucumber plants.
Tomatoes and cucumber plants will need to be supported with a string tied to the greenhouse roof, or a bamboo cane, so put this in when you plant.
Most growing bags only have enough feed to last a few weeks, so you will need to add some fertiliser. This can either be a controlled-release fertiliser added when you plant up, or a liquid feed mixed into a watering can every two weeks once fruits start to form.
Watering is essential. Grow bags dry out quickly as your plants gets larger and the summer starts to heat up. You may have to water two or even three times a day in the height of summer, so you might want to set up an automatic watering system. These attach to an outside tap and use a timer attached to a hose and drippers. There are plenty to choose from at the garden centre, but work out how many drippers you will need before you buy.