Strawberries under cover produce earlier, cleaner fruit than outdoor plants, and they escape the slug and rain damage that ruins so much of an outdoor crop. They are also the crop where a greenhouse most easily works against you, in two specific ways.
The first is chill. Junebearing varieties set their flower buds in autumn and need a period of cold to break dormancy properly, and a greenhouse kept warm through winter deprives them of it, producing vigorous leafy plants that flower poorly. The second is pollination: a strawberry is an aggregate fruit where every seed on the surface corresponds to a separately pollinated ovule, so a still greenhouse with no insects produces misshapen berries.
| Measure | Figure | Why it matters |
|---|---|---|
| Germination soil temperature | 65 to 75 degF | Strawberries are almost always grown from runners or bare-root plants rather than seed, which is slow and variable. |
| Days to emergence | 14 to 28 days from seed, which is rarely the route taken | At the optimum soil temperature, in a moist starting medium. |
| Optimum day temperature | 60 to 75 degF | Where growth is fastest. Above the range, specific processes start failing. |
| Optimum night temperature | 50 to 60 degF | The day to night difference controls stem elongation and plant habit. |
| Survives down to | 15 degF | Brief exposure on an acclimated plant, not a sustained figure. |
| Trouble above | 85 degF | Fruit set falls off, berries soften and ripen unevenly, and the plant puts energy into runners rather than fruit. |
| Target daily light integral | 17 to 22 mol/m2/day | Moles per square meter per day. Below this the crop holds rather than grows. |
| PPFD needed at 16 hours | 295 to 382 umol/m2/s | What a fixture must deliver at the canopy to reach that daily total. |
| Spacing under cover | 12 to 15 in each way | Wider than outdoor spacing, because still air makes disease the limit. |
| Floor area per plant | 1 to 1.5 sq ft | Includes each plant share of shoulder and path space. |
| Sowing depth | Crown at medium level, never buried | A buried crown rots. This single detail causes more strawberry failures under cover than any disease. |
| Soil pH | 5.5 to 6.5 | Outside this band, nutrient availability rather than nutrient supply becomes the limit. |
| Days to first harvest | 4 to 6 weeks from flower to ripe fruit | From sowing, under cover, at good light. Winter timings run considerably longer. |
| Humidity preference | 0.8 to 1.2 kPa VPD | Expressed as vapor pressure deficit, which accounts for temperature as relative humidity does not. |
How do you start strawberries under cover?
Start from runners or bare-root plants rather than seed. Seed-raised strawberries are slow, variable and rarely worth the two seasons they take, while a runner is a clone of a known variety that fruits the following season.
Plant with the crown exactly at medium level. This is the most important sentence on this page. A crown buried even slightly sits in moisture and rots, and a crown planted too high dries out and the plant fails to establish. Set the plant so the base of the crown sits on the surface and the roots are fully covered.
Plant into a free-draining medium. Strawberries want an open mix and resent saturation, which is why they do so well in fabric grow bags and hanging containers where drainage is unavoidable. In a bed, add grit or perlite to open the fill.
Plant in autumn for a crop the following season, or in early spring for a lighter first-year crop. Remove the first flush of flowers from a spring planting so the plant establishes rather than fruiting itself into exhaustion.
What temperature does strawberries need in a greenhouse?
The working range is 60 to 75 degrees Fahrenheit by day and 50 to 60 at night. Strawberries are considerably more tolerant of cool conditions than of hot ones, and fruit quality is better at the cooler end of the range.
Above about 85 degrees, fruit set falls off, berries soften and ripen unevenly, and the plant diverts energy into runners rather than fruit. Shading through the peak of summer is therefore worthwhile, and strawberries want only 17 to 22 mol/m2/day so a 30 to 40 percent cloth costs nothing.
The chill period is the greenhouse-specific complication. Where junebearing varieties are grown, the house needs to run genuinely cold for part of the winter, which conflicts with using the same structure for anything tender. Growing strawberries in containers that can be moved outside for the cold months resolves this neatly.
Frost during flowering is the other risk. Open strawberry flowers are damaged at around 30 degrees Fahrenheit, showing as a blackened centre, and those flowers set no fruit. A greenhouse largely prevents this, which is one of its clearest advantages for this crop.
How much light does strawberries need?
Strawberries want 17 to 22 moles per square meter per day, higher than leafy crops and lower than tomatoes, which is 295 to 382 micromoles per square meter per second over a 16 hour photoperiod.
A greenhouse supplies this from spring through autumn without difficulty. In winter it does not, which is one more reason a junebearing plant in a warm house does poorly: it has neither the chill it needs nor the light to make use of being warm.
Day-neutral varieties are less sensitive to photoperiod and will crop over a longer season given adequate light and temperature, which makes them the better choice for anyone trying to extend the harvest rather than concentrate it.
Spacing, support and training
Space at 12 to 15 inches each way, giving 1 to 1.5 square feet per plant. That is generous for a plant this size, and the reason is airflow: botrytis on the fruit is the main loss in a greenhouse strawberry crop and it needs still humid air.
Keep fruit off the medium. Straw, matting or a container with the fruit hanging over the edge all achieve this, and it prevents both botrytis and the soil contact that spoils fruit. This is where the crop name comes from and the technique is still the right one.
Containers and hanging arrangements suit strawberries unusually well. Fabric grow bags on a staging rack give drainage, airflow and fruit hanging clear of the medium in one arrangement, and they make the chill question solvable by moving them.
Remove runners during the fruiting season. Every runner is energy diverted from fruit, and in a spaced planting they simply tangle. Let a few root at the end of the season if you want replacement plants, since strawberry plantings decline after three years and need renewing.
Watering and feeding
Strawberries use about 0.22 gallons per square foot of growing area per day at peak. They are shallow rooted, so they need consistency, and they are prone to crown rot, so they must never sit saturated.
That combination makes drip irrigation close to essential. Drip emitters at the base of each plant deliver consistent moisture at the root without wetting the crown or the fruit, and overhead watering does the opposite of everything this crop needs.
Water in the morning and never over the foliage or fruit. Botrytis on ripening fruit is the single largest cause of loss in a greenhouse strawberry crop, and it needs several hours of surface wetness.
Feed with a balanced feed during establishment and switch to a high potassium feed once flowering begins, in the same way as tomatoes. Nitrogen during fruiting produces leaf and runners at the expense of berries.
What goes wrong with greenhouse strawberries?
| What you see | Usual cause | What to do |
|---|---|---|
| Misshapen berries with hard undeveloped patches | Incomplete pollination, since each surface seed corresponds to a separately pollinated ovule | Brush every open flower, run a fan for constant air movement, and open the house to let bumblebees in. |
| Grey fuzzy mould on ripening fruit | Botrytis, which needs leaf and fruit wetness in still air | Keep fruit off the medium, widen spacing, water at the root zone only, and run a circulation fan continuously. |
| Plant wilts and collapses despite moist medium | Crown rot, almost always from a crown planted below medium level or a saturated mix | Plant with the crown exactly at surface level in a free-draining medium. This is not recoverable once established. |
| Vigorous leafy plants with very little fruit | Junebearing variety that did not receive its 200 to 300 hours of chill below 45 degF | Let the house run cold in midwinter, move containers outside, or grow everbearing and day-neutral types. |
| Blackened centre in an open flower | Frost damage to the flower, which then sets no fruit | A greenhouse largely prevents this. Where the house runs cold, cover flowering plants on frost nights. |
| White powder on leaves, and leaf edges curling upward | Powdery mildew, which needs humid air and poor air movement but a dry leaf | Increase air movement, avoid water stress, and remove badly affected leaves. |
| Fine stippling and bronzing on leaves, with webbing | Two-spotted spider mite, which thrives in hot dry greenhouse air | Raise humidity, damp down the floor, and introduce predatory mites early rather than after damage shows. |
| Yield falls off after two or three seasons | Normal decline of a strawberry planting, plus virus accumulation | Renew from runners every three years, or buy fresh certified plants and start a new bed. |
Which types work best under cover?
| Type | Suits | Note |
|---|---|---|
| Junebearing | A single concentrated harvest | Highest quality and yield in one flush, but they need 200 to 300 hours of winter chill to flower properly. |
| Everbearing | Two or three lighter flushes | Lower chill requirement and a spread harvest, which suits a greenhouse used continuously. |
| Day-neutral | Continuous cropping under cover | Least sensitive to photoperiod and will crop over a long season given light and temperature. The best greenhouse choice. |
| Alpine strawberries | Small spaces and shade | Tiny intensely flavoured fruit, tolerant of lower light, and they come true from seed unlike other types. |
| Hanging basket varieties | Vertical space | Bred to trail, which keeps fruit clear of the medium automatically and uses space a bench cannot. |
| Certified virus-free stock | Any new planting | Strawberry viruses accumulate and reduce yield over years, and starting from certified stock is worth the premium. |
For a greenhouse specifically, day-neutral varieties are usually the right answer. They have a low chill requirement, so the structure does not need to run cold in midwinter, and they crop over a long season rather than in one flush, which suits a space you are visiting anyway.
Junebearing varieties give the best individual fruit and the heaviest single harvest, and they are worth growing in containers that can spend the coldest weeks outside. That arrangement gets the chill and the protected fruiting season without compromising the rest of the greenhouse.
Harvesting and what comes next
Pick when the berry is fully coloured to the tip and the shoulders have lost their white. Strawberries do not ripen further after picking in any useful sense, unlike tomatoes, so picking early costs flavour permanently.
Pick with the stem attached rather than pulling the berry off the calyx. A berry pulled from its calyx is damaged at the point of separation and deteriorates within hours.
Harvest in the morning when the fruit is cool and firm. Berries picked in afternoon heat are soft, bruise in the container and keep badly.
Pick every day or two during a flush. Over-ripe fruit left on the plant is where botrytis establishes and then spreads to sound fruit nearby, so removing it is disease control rather than tidiness.
Remove any mouldy berry immediately and take it out of the greenhouse rather than dropping it on the floor. A single infected fruit produces enough spores to affect an entire planting in humid still air.
Equipment for growing strawberries under cover
Plant the crown at exactly medium level, keep the fruit off the surface, water at the root in the morning, and make sure something is moving the pollen around. Then decide the chill question deliberately rather than by accident, because a junebearing variety in a warm house is the most common way a greenhouse strawberry crop disappoints.