The short answer: keep the root zone of a coir slab between about 18 and 25 °C for the main fruiting crops, and treat 15 °C and 30 °C as the edges where you start losing money. Tomato grows normally with roots anywhere from 20 to 30 °C, cucumber does best at 19 to 22 °C, pepper tolerates warmer roots than either, lettuce wants its solution at about 24 °C and never above 25 °C, and strawberry is the odd one out because published trials disagree. A wet coir slab is mostly water, so it follows the temperature of whatever touches it most: the floor, the gutter, the irrigation water and the sun on the film.
The slab is a different climate from the air the computer controls. It sits below the air on a cold floor in January and above it on a raised gutter in July. Cold roots take up water slowly and leaf expansion stalls even under a warm canopy. Hot roots run short of oxygen, and Pythium turns from passenger into killer once the slab passes the mid-twenties.
Optimum root-zone temperatures by crop
| Crop | Root-zone range where growth is good | What happens outside it |
|---|---|---|
| Tomato | 20 to 30 °C | Below 20 °C growth drops at any time of year, mainly through smaller, thicker leaves. Above 30 °C it drops in summer only. Heating the root zone to 20 to 21 °C lifted yield from 5.7 to about 7 kg per plant in early plantings, but did nothing for late plantings. |
| Cucumber | 19 to 22 °C in winter, 22 °C in summer | In a winter heating trial, 19 and 22 °C gave 3.4 and 3.7 kg per plant against 2.3 kg at 16 °C. Running at 19 instead of 22 °C saved about 30% of the heating energy. In summer, cooling the root zone to 22 °C gave 180 to 220 fruits per m² against 101 to 133 at 33 °C. |
| Sweet pepper | 24 to 30 °C for growth, cooler for earliness | Shoot weight and leaf area peaked between 24 and 30 °C and fruit weight at 30 °C, but warm roots delayed first harvest. On coir in summer, cooling the slab from 29 °C to about 26 °C brought flowering forward 4 to 5 days and raised fruit number. |
| Strawberry | Trials conflict: 10 °C in one deep-flow trial, 29 °C in another | At 30 to 35 °C flower number falls, roots stop growing and most plants in one trial died within two months. Cool roots slow ripening and push the plant towards flowering rather than leaf. |
| Lettuce | 21 to 24.5 °C, never above 25 °C | Holding the solution at 24.5 °C in summer gave 47% more shoot dry weight than letting it float between 25 and 32 °C. At 21 °C plants were 15% heavier than at ambient but sugar content fell by a quarter compared with 18 °C. |
Two patterns stand out. The upper limit is sharper than the lower one: a slab at 16 °C costs you growth, a slab at 33 °C costs you the crop. And the optimum for size is not the optimum for earliness or quality. Pepper fruit gets heavier at 30 °C but comes later. Lettuce gets bigger at 21 °C but less sweet.
How a coir slab gains and loses heat
A coir slab at normal running water content is roughly two-thirds water by volume. Water takes a lot of energy to change temperature, so the slab warms and cools slowly and lags the greenhouse by hours. In a coir pepper trial, uncooled slabs peaked at 7 to 8 pm, three hours after the cooled ones and long after the air had started to fall.
What sets slab temperature, in rough order of influence:
- What the slab sits on. On concrete or soil the slab is coupled to the ground, which barely moves across the day. On a raised steel gutter it's coupled to the air and the sun and swings far more.
- The irrigation water. You pour several litres a day through a slab that holds a few litres. If the water is 12 °C and the slab is 20 °C, every shot cools the roots.
- Radiation on the film. In nursery containers, switching from black to white pots cut peak root-zone temperature by up to 7 °C and cut the time above 38 °C from 21% to 3%. In field tomato, black or grey mulch raised midday root-zone temperature by up to 4 °C. Slab film behaves the same way, which is why slabs are white outside and black inside. See grow bag film: white, black and UV.
- Water content. A drier slab has less water to hold heat and more air to insulate, so it warms faster in sun and cools faster at night. The trade-off is explained in water holding and air porosity.
- The canopy. Once the crop closes over, the swings shrink. The dangerous weeks are right after planting.
Coir and rockwool are close here. Coir has more dry mass per slab, so it has slightly more thermal inertia, which helps under root-zone heating and hurts a little when you want to cool fast. The wider comparison is in coco coir vs rockwool.
Winter: cold water and cold floors
Cold irrigation water. Water stored outside arrives at the dripper far colder than the slab in a northern winter, and with short cycles the slab never recovers between shots. Warm the day tank or run the feed through a heat exchanger so water reaches the dripper within a few degrees of the slab target. Don't try to heat the slab through the water, because you heat the drain too.
Cold floors. A slab on an unheated floor is coupled to ground that never warms up. Lifting slabs onto gutters or polystyrene is often the cheapest root-zone fix in a floor-grown crop. Where slabs stay down, a heating pipe laid along the slab under the gutter film does the same job as a grow pipe higher up.
The cucumber figures show what this is worth: 16 to 19 °C was worth about a kilogram per plant, 19 to 22 °C a few hundred grams at a third more energy. In most winters the right target is the bottom of the range.
Summer: overheating on raised gutters
Raised gutters are good for drainage and bad for slab temperature. The slab hangs in warm air, takes direct sun until the canopy closes, and has nothing cool to lose heat to at night. In the coir pepper trial, uncooled slabs averaged 29.1 °C through late July and August and were above 25 °C on all 40 days of that period. Cooling pipes beside the slab brought the average down to 25.6 to 26.1 °C and cut the days above 25 °C by a third to nearly half.
If you can't run chilled water, the passive steps in the order they pay:
- Keep the film white and intact. Faded or torn film that exposes the black layer is a heat collector.
- Run the slab wetter through the hottest weeks and push the last irrigation later so it goes into the evening holding water. Settings are in irrigating coco coir slabs.
- Shade or whitewash the roof. The slab only overheats because the canopy and gutter do.
- Saturate new slabs the evening before planting in hot weather, not the morning of.
- Irrigate with the coolest water you have, usually from an underground tank.
Root-zone cooling alone doesn't rescue fruit set when the air is over 30 °C. In the pepper trial hot afternoons still aborted flowers on cooled plants. Cool the air and the roots together.
Root-zone heating options
| System | Where the heat is | Strength | Limit |
|---|---|---|---|
| Pipe-rail | Large pipes near the floor, doubling as trolley rails | Warms the air under the canopy and the gutter from below. Standard in heated glasshouses | Indirect. On raised gutters little heat reaches the slab |
| Grow pipe | Small pipe hung in the crop or fixed beside the slab | Heat next to the slab and lower stems, dries the canopy, cheap to add | Modest slab effect unless run close to the slab |
| Heated gutter | Pipe or hot water channel in or under the gutter | Direct conduction, most even slab temperature, lowest water temperature needed | Highest cost, has to be designed in |
| Warm irrigation water | Heat exchanger on the feed line or heated day tank | Removes the cooling shock at each shot, simple retrofit | Can't heat the slab alone without heating the drain |
| Cooling pipes or chilled feed (summer) | Cool water beside the slab or in the feed | Dropped the coir slab average by 3 to 3.5 °C and brought pepper flowering forward 4 to 5 days | Energy cost, condensation on pipes |
Root-zone heating pays back in early plantings, cold-season crops and crops on cold floors. In late spring plantings the tomato trials found no benefit because the slab was already in range.
How to measure slab temperature properly
- Probe in the middle of the slab, mid-depth, between two drippers. Under a dripper you read irrigation water. At the surface you read air. At the slab end you read the gutter.
- Log, don't spot-read. The slab lags the air by hours, so a 2 pm reading misses the evening peak and the pre-dawn trough. A cheap logger at 15-minute intervals for a week beats a month of hand readings.
- Measure the water too. Dripper and drain temperatures bracket what the roots see. A 10 °C dripper and a 17 °C drain means the feed is cooling the slab.
- Compare with air at slab height. That difference tells you whether the problem is the floor, the water or the sun.
For a single alarm, use 15 °C low and 28 °C high for fruiting crops and tighten from there.
Temperature, oxygen, EC and Pythium
Oxygen. Air-saturated water carries about 10.1 mg/L of oxygen at 15 °C, 9.1 at 20 °C, 8.3 at 25 °C, 7.6 at 30 °C and 7.0 at 35 °C. Root and microbial respiration roughly doubles for every 10 °C, so a slab at 30 °C has less oxygen on offer and more demand. Growers are advised to keep the root zone above about 5 mg/L, and lettuce ponds usually run near 8. A hot, wet, poorly draining slab is the fastest route to an anaerobic root zone, which is an argument for a chip blend in warm climates.
EC. Transpiration in tomato rose steadily with root temperature from 12 to 35 °C while nutrient uptake did not keep pace, so slab EC climbs through a hot afternoon at constant feed. If slab EC spikes only on hot days, check slab temperature before you drop the feed EC. The EC side is in growing tomatoes in coco coir.
Pythium. The two species that matter behave differently. Pythium aphanidermatum, behind cucumber crown rot and most summer root rot, causes severe disease once the root zone is 23 to 27 °C or above, and in chrysanthemum got progressively worse from 20 to 32 °C. Roots can be heavily colonised at 16 to 18 °C and look healthy, then collapse within hours when the slab warms to 24 to 28 °C. Even a few hours at 28 to 34 °C followed by a return to 22 to 24 °C made later infection faster. Pythium dissotocum prefers cooler roots and in spinach was worst in winter. Neither end of the range is safe, but the warm end kills fastest. The combination to avoid is a slab over 25 °C, low oxygen and a wet slab at night.
What we supply
CVM Fiber and Industries makes coco peat blocks, grow bags and open-top slabs, husk chips and custom blends in Kanyakumari, Tamil Nadu, India. Our coco peat is washed with water to reduce EC, with no calcium nitrate or other chemicals, and isn't buffered. If you want a slab that drains and holds more air in a hot climate, ask about a chip blend when you contact us.
FAQ
What is the ideal root-zone temperature for tomatoes in coir?
Young tomato plants grow normally with roots anywhere between 20 and 30 °C. Below 20 °C growth slows at any time of year, and above 30 °C it slows in summer. Holding the slab at 20 to 21 °C through winter and early spring is where root heating has paid back in trials.
What slab temperature is too cold for cucumbers?
Below about 16 °C you lose yield quickly. In a winter trial, roots at 16 °C gave 2.3 kg per plant against 3.4 kg at 19 °C and 3.7 kg at 22 °C. Running at 19 °C rather than 22 °C saved about 30% of the heating energy for a small yield penalty, which is why many growers settle on the low end in winter.
Does a coir slab stay warmer than a rockwool slab?
Slightly. Both are mostly water when wet, but coir has more dry mass per slab and holds part of its water inside the pith particles, so it has a little more thermal inertia. It holds heat a bit longer under root-zone heating and cools a bit more slowly at night. The difference is small next to the effect of the floor, the gutter or the irrigation water.
How do I warm the slabs without a heated gutter?
Warm the irrigation water first, since each cold shot cools the roots directly. Then get the slab off the cold floor with gutters or polystyrene. A grow pipe run close to the slab helps, and a heating pipe laid along the slab under the film helps more. Only heat towards the bottom of the crop's range.
At what temperature does Pythium become a problem in slabs?
The common warm-weather species causes severe root rot once the root zone is around 23 to 27 °C or above, and disease worsens up to 32 °C. Roots can carry it at 16 to 18 °C without symptoms and collapse within hours when the slab warms into the mid-twenties. A cooler-weather species is worse in winter, so neither extreme is safe, but the warm end does the fastest damage.
Why does slab EC rise on hot afternoons?
Warm roots take up water faster than nutrients. Transpiration climbs with root temperature while nutrient uptake lags, so the solution left in the slab gets more concentrated. If slab EC spikes only on hot days, the cause is temperature and water content rather than the feed recipe.
Where should I put the temperature probe in a coir slab?
In the middle of the slab, at mid-depth, between two drippers. Under a dripper you measure irrigation water, at the surface you measure air, and at the slab end you measure the gutter. Log at 15-minute intervals for at least a week, because the slab peaks several hours after the air does.
Is white or black film better for slab temperature?
White on the outside. In nursery containers, white rather than black cut peak root-zone temperature by up to 7 °C. Greenhouse grow bags are white outside and black inside so light is reflected while roots stay dark and algae can't grow on the inner surface. Faded or torn film that exposes the black layer becomes a heat collector.