Reusing Compost Grow Bags For Winter Salads

9 November 2012, written by Benedict Vanheems

Grow bags, or growing bags, have revolutionised the way many of us raise greenhouse or patio crops such as tomatoes, capsicum and cucumbers. Self-contained, orderly and ever-versatile, they're the kitchen gardener's ultimate flexible friend.

Grow bags are more popular in some countries than others, with British gardeners perhaps most familiar with these helping hands. For those who aren't familiar, a grow bag is simply a long, compost-filled plastic sack into which (usually) two or three season-long fruiting vegetables are planted. Holes are cut into the top of the bag to home each plant and to allow water and feed to be delivered to the roots.

The bags are certainly convenient but the potential downside is the waste that comes with disposing of them at the end of the season. As gardeners I'm sure we all try to be a little greener than most, working where we can to minimise our impact on the environment. In this respect grow bags less than ideal: there's all that plastic to get rid of laden with the added guilt that the majority of bags are filled with peat-based compost, which inevitably damages fragile peat bog habitats.

A greener alternative

But grow bags and their derivatives needn't be an environmental no-no! I for one find them a huge boon to my growing and would be seriously compromised without them. So there are two things I do to minimise their impact and boost my green credentials. Firstly I avoid like the plague any grow bags that aren't completely peat free – there's just no excuse when perfectly adequate peat-free alternatives exist. Secondly I reuse my bags at the end of the season to grow a winter crop of salads, enjoying two crops from the same volume of compost. This is a no-brainer as far as ecological (and economical)-minded gardeners are concerned!

While this article is primarily concerned with grow bags, what follows would apply to any compost that's been used once for fruiting vegetables. The golden rule is simply this: don't be in a hurry to throw away all that compost – there's life in it yet.

Winter salads

Hardy and relatively quick-growing winter salads are the primary candidates for once-used grow bags and are just the ticket for sowing in autumn after the tomatoes and such like have been cleared away. But before any seed packet is so much as opened the old compost has to be given a new lease of life. Start by completely opening out the top of the bag with a sharp knife so that the compost is fully exposed. Now vigorously fork it over, removing as much debris and old roots as you can. With the compost loosened you can now tickle in some controlled-release fertiliser granules to give the exhausted compost a boost of nutrients for the next crop. If the compost is dust-dry, give it a thorough watering to completely re-wet it before allowing to drain.

You can now treat your prepped bag as a compost-filled window box or trough – just get sowing. Suitable salads for sowing in autumn include rocket, radishes, lamb's lettuce/corn salad, stump-rooted baby carrots such as 'Parmex' or 'Atlas', plus any of the cut-and-come-again salad leaf mixes sold as suitable for winter growing.

Sow your salads in short rows along the width of the bag as per seed packet instructions. Leafy salads can also be scattered thinly over the surface of the compost before lightly covering with more. Head-forming winter lettuces like 'All the Year Round' and 'Winter Density' (the clue's in the name!) can be started off in modules or small pots before planting out into the grow bag at the exact spacing required.

Aftercare

Don't expect salads sown this late in the season to grow as fast as they do in spring. The days are getting much shorter and temperatures will be ticking lower. Plants will quietly do their thing, reaching maturity in their own time. Keep the compost moist but avoid the temptation to overwater – too much water left hanging about in cool weather can weaken plants and encourage fungal diseases.

Gardeners in temperate and colder climates will need to keep their second-life grow bags in a protected structure such as a greenhouse, porch, conservatory or cold frame to ensure growth continues over the coldest period. If the winters are severe where you are you can always delay sowing until early spring to gain a bit of a head start on outdoor sown crops.

Other uses

If you haven't got a protected space for grow bags there are a few alternatives. Use the prepared compost to start off module trays of peas and beans or plant up individual cloves of garlic before setting them outside into their final growing positions in spring. Spent compost can also be mixed with a little horticultural sand and organic fertiliser to make a well-drained planting medium for standard carrots. Use about one third sand by volume, fill deep containers with the mix and sow directly into 1cm (0.5in) deep drills. Cover and keep the compost moist, thinning once the seedlings are up to their final spacing.

Growing salad leaves in used potting soil

Once your second crop is harvested the compost can then be added to raised beds, borders and around fruit bushes and trees, either forked in or left as a mulch for the worms to dig in for you. Alternatively add the compost in layers with other materials to your compost bin where it will help to create a well-structured end product. I also use the compost to cover seeds sown in drills in those instances where a fine tilth is difficult to prepare. As well as making it easier for seedlings to push through, it helps to mark out the rows for hand weeding.

And what about the plastic? Use it as a mulch in early spring, black side facing up, to warm up the ground for early sowings. Or try using it as a cheap, water-retentive lining for hanging baskets – just make sure you spike a few holes in the plastic for drainage. Who said grow bags had to be wasteful?

By Benedict Vanheems.