Reusing Compost Grow Bags For Winter Salads

, written by Benedict Vanheems gb flag

Growing tomatoes in a compost grow bag

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!

Rejuvenating an old compost grow bag

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.

Adding fertiliser to reuse a grow bag

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.

Sowing into a reused grow bag

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.

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Comments

 
"You don't want to be growing you veggies or flowers in plastic bags, because the heat of the sun BAKES the soil into bricks! (I know, it's happened to me) And I would certainly worry about the chemicals from the plastics possibly leaching into the soil you grow the veggies in."
Susan on Friday 9 November 2012
"Susan, that must depend on your climate because in the (more maritime) climate where I live I find that grow bags actually help to preserve the soil moisture, stopping it drying out quickly. I have often achieved a better crop of tomatoes or peppers from the enclosed soil bags than from more open soil in my greenhouse. However, I do have to keep my greenhouse door wide open during the hottest weather to ensure that everything doesn't bake as you mention."
Jeremy Dore on Friday 9 November 2012
"Winter Salads...Compose Grow Bags... Interesting! I never heard of "grow bags"! Are they made of a special plastic? I wonder if they are sold "only" during garden season and if sold "only" at Garden Centers?"
gaia on Saturday 10 November 2012
"Interesting! I have not seen anything called a "compost grow-bag" at any of the places where I buy plants/seeds/gardening products: WOULD A REGULAR BAG OF PURCHASED COMPOST WORK IN THE SAME WAY?"
I'm here in Virginia...USA on Saturday 10 November 2012
"For our US readers, these 'grow bags' are very popular in Europe and are basically bags of potting soil, usually a little wider and flatter. They're especially useful in greenhouses where often the same plants are grown every year (think tomatoes!) and the soil therefore needs replenishing because otherwise blight can carry over, or soil nutrients be depleted. - Jeremy"
Jeremy Dore on Thursday 15 November 2012
"I have reused my grow bags to grow over wintering broad beans. Thay have all germinated and sending up shoots. Also as I still used the tomatoe frames they will give added support to the beans as they grow, I hope. Another plus is that they will be easier to protect come winter."
David Pointer on Friday 16 November 2012
"i make my own version of "grow-bags" that i use outside,about 2 years ago i acquired a bail of used "silage wrap" from a local farmer.its basically a large roll of black agricultural grade plastic about 8 feet wide "miles" long. i prepare an area of ground,put a pile of well rotted manure,garden compost etc on this area cover it with a length odf tthis plastic then leave it for a month or two.when i am ready to plant i just cut holes in it pop in the plants. i have used this method now very successfully for the past 2 years for my courgettes, runner beans tomatoes,strawberries a whole host of other plants.when done with itss simply a matter of removing the plastic lightly forking the compost underneath to make a new seedbed cheers Dave"
Dave Barber on Monday 19 November 2012
"I used 5 gallon grow bags last summer. A total of 800 in which I grew crops I was unable to get into the ground due to a severely windy and cold Spring. Tomato's, Peppers and Cucumbers performed quite well. I will use them again when necessary outdoors. I plan on them to be the primary containers for my greenhouse. They were purchased without any medium. I used a compost,vermiculite combination that I mix into the garden. When the season was over, all the bags were emptied into a patch in the garden and tilled in. All organic. Bags were cleaned and stored for upcoming seasons. Theses are very durable and I suspect they will last several years. They can be purcased at greenhouse supply companies online. Hope this was a help. W"
Wayne on Friday 18 January 2013
"please make it plain and simple. can grow bags that grew tomatoes last year in a greenhouse be re-used this year if u can please txt back 'YES its ok' or 'NO its a bad idea' thanks if u can advise me"
gordon mcdonald on Monday 23 February 2015
"Gordon, I believe if you use fresh growing medium you can grow again in your greenhouse. If it were me, I'd use a compost and vermuculite mix with a good tomato fertilizer."
Wayne on Monday 23 February 2015
"Hi Gordon. Yes, grow bags that grew tomatoes last year can be re-used - but primarily for salads and root veg, as described in the article. The compost/potting soil should NOT be used for growing more tomatoes or potatoes, as the compost will be exhausted for these crops and could carry disease over."
Benedict Vanheems on Tuesday 24 February 2015
"Hi Wayne and Benedict. Thanks for your advice about not reusing grow-bags but using the older soil elsewhere. Hope others take advice due to risk of disease. Ive only just started growing tomatoes but how it passes the time and its so lovely to come in from work to pick ripe fruit every night. "
Gordon McDonald (Wishaw) on Tuesday 17 March 2015
"Tomatoes are fantastic Gordon - so many varieties to try - enough for a lifetime of growing! Keep up the good work."
Benedict Vanheems on Wednesday 18 March 2015
"FYI. Grow bags of various sizes in durable, well draining fabric and several colors are widely available online at catalogue or garden supply sites in the US and Canada. (We all know how tomatoes love the color red.) I am reusing mine for the 3rd year, after washing them and refreshing the soil with various nutrients, depending on the plant needs. I've grown potatoes, tomatoes, herbs and salad greens in these bags. I've also heard that gardeners use them on decks etc, to grow flowers."
Sheila on Thursday 26 March 2015
"Has anyone used fabric like burlap or other sturdy but porous material to construct their own grow bags? Do grow bags need to be placed on any types of pedestals if they're used on wooden decks to prevent the water from rotting the wood? Thanks!"
Debra Messick on Sunday 7 June 2015
"Hi Debra. I'd be inclined to place them onto some sort of plastic sheeting, just to avoid staining the wood."
Ben Vanheems on Monday 8 June 2015
"Can I re use grow bags that have been infected with mildew plants.stan"
Stan on Thursday 16 June 2016
"Hi Stan. My concern with reusing disease-laden compost is that spores of the mildew fungus may still be lurking in it, ready to infect the next crop. I would suggest just adding this compost as a mulch around fruit bushes or similar and using fresh compost for winter salads."
Ben Vanheems on Monday 27 June 2016
"Thanks Ben . now I know where Im at. Stan"
Stan on Monday 27 June 2016

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