COMPARISON OF GROWING BAGS
Growing bags are simply plastic bags filled with multi-purpose compost. They are used for growing a wide variety of vegetables, herbs and plant. In general growing bags are best suited to cultivating low growing plants. Originally developed for greenhouses they are now commonly used outdoors as well.
Your average growing bag measures roughly 90cm / 3ft by 35cm / 14in. It is surprising how little difference there is in the standard size over the different brands. So it seems size is irrelevant when looking for a growing bag, most are the same. A possible reason for this is that there are several accessories sold for growing bags and these require the bag to be roughly the standard size.
The plastic used to hold the compost tend to be the same quality as well, certainly we have not heard of grow bags splitting open when they are in their final position. Some manufacturers claim the plastic of their growing bags is better insulated compared to most but there is no data to prove that this makes any significant difference.
The final variable for a growing bag is the compost it contains. It's impossible to quantify this accurately because the manufacturers do not specifically declare the nutrients held with the compost. Growing tests by the RHS and other independent organisations could find no detectable difference in the quality of plants grown between the major manufacturers of growing bags. So it's probably safe to assume that if you stick to the major manufacturers the quality of peat based compost will be roughly the same.
Which did conduct tests on peat-free growing bags and in this case they performed slightly less well compared to peat based growing bags. The difference however was minimal.
So that leaves us with the final variable which is price and here there are significant differences. Our recommendation therefore is to stick to the major manufacturers (see list below) and select the cheapest you can find. Westland growing bags, available from many diy stores and independent garden centres consistently stands out as being excellent value.
If you are shopping at Wyevale's, Dobbies, Notcutts or other chains of garden centres then their own brands are normally good value. J Arthur Bowers also represent good value in many garden centres.
HOW TO USE GROWING BAGS
As with everything in the garden there is a proper way to use something and
the same goes for growing bags. First, before you do anything, roll the bag
around on the floor a bit to loosen up compacted compost.
Next cut two or three x-shaped slits in the top of the bag. For growing tomatoes we strongly recommend having only two plants per bag and using grow rings for the very best results. See here in our main tomato article for more details.
Pierce the base of the bag (through the compost to avoid turning it over) several times so that excess water can easily drain away. If the compost is at all dry then water it well and leave for a couple of hours to allow the water to be absorbed. Now you are ready to plant up.