Coco coir works well in an unheated polytunnel or high tunnel, and it's often the fix for a tunnel whose soil has gone sick after years of tomatoes or strawberries. Lay grow bags on a clean ground cover, run one drip line on a timer, feed through a simple proportional injector and check the drain by hand once a week. For a 30 m tunnel that's roughly 85 to 110 bags of tomatoes or about the same number of strawberry bags, depending on how many rows you fit.
This page is for seasonal growers in the UK, Ireland, northern Europe, the northern USA and Canada who plant in spring, crop through summer and leave the tunnel cold in winter. It doesn't cover heated glasshouses with climate computers.
Why bags beat tunnel soil after a few years
A tunnel keeps rain off the soil. That's the whole point, and it's also the problem. Two things build up when the same ground is cropped year after year under plastic.
Salt. Without soaking rain, fertiliser and compost salts stay in the root zone. Drip irrigation doesn't wash them down either. A white crust on the surface is the visible sign. Compost is a big contributor: 2.5 cm (1 inch) of compost a year tripled soluble salts in one year and raised them six-fold in two. At 5 cm (2 inches) a year, salts rose 13-fold in two years, to a level classed as strongly saline. The usual soil advice is to take the cover off every third or fourth year so winter rain can leach it, which costs you a season and a sheet of plastic.
Soil-borne disease. Tunnels that grow the same crop in the same place for years build up progressively heavier disease loads. In one state-wide survey of tomato high tunnels in Ohio, the share of farms carrying each pathogen was:
| Pathogen | Disease | Farms where found |
|---|---|---|
| Colletotrichum coccodes | Black dot root rot | 97% |
| Verticillium dahliae | Verticillium wilt | 75% |
| Root-knot nematodes | Root galling | 56% |
| Pyrenochaeta lycopersici | Corky root rot | 50% |
The soil answers are grafting onto resistant rootstocks, soil disinfestation or moving the tunnel. Each helps against some of these pathogens, and none fixes the salt. Bags of fresh coir sidestep both problems at once. The roots never touch the tunnel soil, you control every gram of fertiliser that goes in, and the old bag leaves the tunnel at the end of its life.
What you give up is the soil's buffer. A bag holds far less water than a bed, so irrigation has to be reliable. That's the trade, and the rest of this page is about making it easy.
Bags on the ground, on legs or on tabletops
| Layout | Cost | Spring warmth | Winter risk | Best for |
|---|---|---|---|---|
| Bags on ground cover | Lowest | Slower to warm, steadier at night | Lowest | Tomatoes, cucumbers, peppers, strawberries in cold regions |
| Bags on legs or low wires | Low to moderate | Faster to warm in sun | Moderate | Strawberries where slugs or splash are a problem |
| Tabletops at waist height | Highest | Fastest to warm, fastest to chill | Highest | Strawberries where picking labour matters |
On the ground, lay a woven ground cover over level, free-draining soil and run the bags along it. Level matters more than people expect. A bag on a slope drains to one end and stays wet at the low corner. Some trough systems are sold on legs specifically to keep the substrate off the tunnel floor, which is worth it if you're moving into a tunnel with known root disease.
Tabletops save your back at picking time. They also expose the root zone to air on every side. Bags and troughs raised in the air suffer more cold injury than those on or near the ground, and tabletops freeze faster and catch more wind.
Water and feed without a computer
You don't need a climate computer for a tunnel of bags. You need four things.
A timer with enough start times. Coir in bags wants several small waterings a day in summer rather than one big soak. Pick a controller that takes six or more start times and lets you change the run length quickly when the weather turns.
One dripper per tomato plant, several per strawberry bag. In UK coir strawberry trials, five 1.2 L/h drippers per 1 m bag spread water better than one 6 L/h dripper split four ways, and they cut the damage when one emitter blocks. For tomatoes, give each plant its own dripper. The worked example below assumes 2 L/h.
A proportional injector. Positive-displacement dosing pumps inject stock solution in proportion to the water passing through, so the dilution holds steady when line pressure changes. Venturi injectors are cheaper and have no moving parts, but their ratio shifts with pressure, so the feed strength wanders. A ratio of 1:100 means one litre of stock makes 100 litres of feed. That's the common setting.
An EC pen and a drain tray. Measure the water before the injector, then the feed at a dripper. The feed should read the water's EC plus the fertiliser's EC. If your tap water reads 0.8 and the fertiliser label gives 1.2 at your dilution, the dripper should read about 2.0. If it doesn't, the injector is off. Once a week, catch the drain from two or three bags and compare it with the feed. The targets and corrections are in our pH and EC management guide.
How much water? Mature tomatoes take roughly 1.7 to 3 ml per square metre for every J/cm² of outside radiation, so a bright day can mean several litres per square metre and a dull one under a litre. A timer can't read the light, so you change it by hand. Fit a water meter on the line. It turns "the bags look dry" into a number you can compare day to day. Fill in a cheap notebook with the meter reading, the timer settings and the drain, and the next season gets much easier.
Size your bags for the timer, not the catalogue. Timer-watered crops need more coir per plant as insurance against a missed cycle, and the volumes are set out in grow bag sizes and plant density.
Cold springs and root-zone temperature
Tunnel tomatoes go in once the soil reaches about 16°C (60°F) at 5 cm depth. A bag behaves like a raised bed. It warms faster on a sunny day than the floor does, which brings planting forward, and it loses heat faster on a clear night.
A few things help in an unheated tunnel:
- Wet the bags up a week before planting and let them warm in the tunnel. Planting into a bag filled with cold mains water stalls the roots.
- Keep bags on ground cover rather than raised for the earliest plantings.
- Water early in the day so the bag isn't cold and wet going into the night.
Crop targets and fixes for cold and hot slabs are in root-zone temperature in coir slabs.
Frost and overwintering
Empty bags waiting for spring need little care. Blow the lines through or drain them so they don't split, and keep fresh, unopened bags out of the sun. Bags with a crop in them are another matter.
For strawberries carried over winter, short frosts of around -5°C don't do harm. Long, deep freezes do, and plants in bags are more exposed than plants in the ground. What works:
- Lower tabletops to the ground where you can. If you can't, wind protection matters most.
- Stand bags on free-draining ground. A waterlogged bag freezes into a block of ice that holds the cold on the roots for longer.
- Push troughs and bags side by side to cut the number of cold outside edges.
- Use two layers of fleece. Two layers protect slightly better than one double-weight layer. Covering is rarely needed before late October, and the serious cold risk in the UK is usually over by mid-February.
- Put a max-min thermometer between the bags in the coldest, lowest spot.
- Check for grey mould weekly. Fleeced crowns stay damp. Uncover on mild days to dry them.
Further north the advice changes. In Nordic tunnel trials, everbearers overwintered in substrate bags under covers were badly frosted and gave very low yields, and the researchers advised against it. In those climates, treat tunnel strawberries as a single season and plant fresh each spring.
Tomatoes and strawberries, the two main tunnel crops
Tomatoes. Three plants per 1 m bag is the usual retail layout. Tie the strings to a separate support frame unless your tunnel hoops are rated for a full fruit load. Pre-wet unbuffered coir with a calcium-rich feed before planting, then feed and train as you would any bag crop.
Strawberries. Commercial coir bags of 1 m carry eight to ten plants. UK tunnel growers have used coir commercially since the late 2000s, first as a peat replacement. Bag formats, feed and drain targets are in growing strawberries in coco coir.
Reusing bags across seasons
A bag that grew one healthy crop can usually grow another, which halves the coir bill. Flush it before replanting and throw out any bag from a row that showed root disease. When and how to do it is in our article on reusing coco coir.
Worked example: a 30 m tunnel
Take a tunnel 30 m long and 6 m wide (180 m²). A tunnel of 6.1 m by 29.3 m holds anywhere from 125 to 300 tomato plants, depending on spacing, path width and sidewall height. Allow 1 m at each end for doors and the irrigation header, so rows are 28 m long.
| Tomatoes, 3 rows | Tomatoes, 4 rows | Strawberries, 4 rows | |
|---|---|---|---|
| Bags per row (1 m) | 28 | 28 | 28 |
| Total bags | 84 | 112 | 112 |
| Plants per bag | 3 | 3 | 8 to 10 |
| Total plants | 252 | 336 | 896 to 1,120 |
| Suggested bag | 27 to 30 L | 27 to 30 L | 15 L (100 x 15 x 10 cm) |
| Coir in the tunnel | 2,270 to 2,520 L | 3,020 to 3,360 L | 1,680 L |
| Drippers | 252 at 2 L/h | 336 at 2 L/h | 560 at 1.2 L/h |
| Flow when all run | 504 L/h | 672 L/h | 672 L/h |
The flow line is the one people skip. Check that your supply and injector can deliver it, or split the tunnel into two zones on the timer. The four-row tomato layout only works if your sidewalls are tall enough for the outer rows. In a low, round-sided tunnel, three rows is the realistic number.
Order 5 to 10% spare bags for breakages and late gap-filling.
What to buy
For a tunnel, ready-made grow bags save the most labour. You unpack, slit and wet them in place. Open-top bags suit strawberries if you want to set your own plant spacing. If you'd rather fill your own troughs or pots, compressed blocks are cheaper per litre but take time to hydrate and fill.
Our coco peat comes from CVM Fiber and Industries in Kanyakumari, Tamil Nadu, India. It is washed with water to bring the EC down, with no calcium nitrate or other chemicals. It is not buffered, so plan a calcium pre-soak before planting.
If you're planning a tunnel conversion and want bags sized to your rows, get in touch.
FAQ
Can you put coco coir grow bags straight on the polytunnel floor?
Yes. Lay a woven ground cover first so the bags sit on a clean, level, free-draining surface. That keeps the roots away from any soil pathogens and stops drainage pooling under the bag. If your tunnel has a known root disease problem, raise the bags on legs or wires.
Why do tomatoes do worse in my tunnel soil every year?
Usually a mix of salt build-up and soil-borne disease. Without rain, fertiliser and compost salts stay in the root zone, and root pathogens such as corky root rot, Verticillium and root-knot nematodes build up when the same crop goes in the same soil. Moving the crop into bags of fresh coir avoids both.
Do I need a computer to grow in coir bags in a tunnel?
No. A timer with several start times, one dripper per plant, a proportional injector and an EC pen will run a tunnel. You'll need to change the timer by hand as the weather changes and check the drain once a week.
How many grow bags do I need for a 30 m polytunnel?
In a 30 m by 6 m tunnel with 28 m rows, three rows of tomatoes is 84 bags and 252 plants, and four rows is 112 bags. Four rows of strawberries is 112 bags carrying about 900 to 1,100 plants. Add 5 to 10% spares.
Can coco coir bags stay in an unheated tunnel over winter?
Empty bags can, once the drip lines are drained. Strawberry plants in bags need protection in cold winters: lower them to the ground, keep the bags free-draining, push them together and cover with two layers of fleece. In Nordic climates overwintering bag strawberries isn't recommended.
When can I plant tomatoes into coir bags in an unheated tunnel?
When the root zone reaches about 16°C. Bags warm faster than the tunnel floor on sunny days, so planting can come slightly earlier, but they also chill faster at night. Wet the bags up a week ahead so they warm in the tunnel.
Can I reuse tunnel grow bags next year?
Usually yes, for one more crop and sometimes two, as long as the first crop was healthy. Flush the bag before replanting and discard bags from any row that showed root disease.