Spinach is the highest-yielding leafy crop an unheated greenhouse can produce through winter, and it is the crop that most rewards understanding what actually makes it bolt. It is not simply heat. Spinach is a long-day plant, and increasing day length is the primary trigger, with warmth accelerating it.
That single fact reorders the growing year. A spring sowing races toward lengthening days and bolts almost regardless of how cool it is kept. An autumn sowing grows into shortening days, and it will stand for months, sweetening as the cold deepens, until the light returns in late winter. Spinach in a greenhouse is fundamentally an autumn and winter crop.
| Measure | Figure | Why it matters |
|---|---|---|
| Germination soil temperature | 45 to 68 degF | Spinach germinates well in cool medium and poorly above 75 degF, which is the reverse of most vegetables. |
| Days to emergence | 7 to 14 days | At the optimum soil temperature, in a moist starting medium. |
| Optimum day temperature | 50 to 65 degF | Where growth is fastest. Above the range, specific processes start failing. |
| Optimum night temperature | 40 to 50 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 | 75 degF | Growth slows and bolting accelerates, particularly when combined with lengthening days. |
| Target daily light integral | 12 to 17 mol/m2/day | Moles per square meter per day. Below this the crop holds rather than grows. |
| PPFD needed at 16 hours | 208 to 295 umol/m2/s | What a fixture must deliver at the canopy to reach that daily total. |
| Spacing under cover | 4 to 6 in each way | Wider than outdoor spacing, because still air makes disease the limit. |
| Floor area per plant | 0.15 to 0.25 sq ft | Includes each plant share of shoulder and path space. |
| Sowing depth | 0.5 in | Unlike lettuce, spinach seed needs proper burial and darkness, and shallow sowing gives patchy emergence. |
| Soil pH | 6.5 to 7.5 | Outside this band, nutrient availability rather than nutrient supply becomes the limit. |
| Days to first harvest | 35 to 45 days in warm conditions, 70 or more from an autumn sowing | From sowing, under cover, at good light. Winter timings run considerably longer. |
| Humidity preference | 0.6 to 1.0 kPa VPD | Expressed as vapor pressure deficit, which accounts for temperature as relative humidity does not. |
How do you start spinach under cover?
Sow half an inch deep into a cool medium at 45 to 68 degrees Fahrenheit. Unlike lettuce, spinach seed needs proper burial and does not want light, and shallow sowing is a common reason for patchy emergence. Germination is slow, commonly a week to a fortnight, and slower still in cold medium.
Above about 75 degrees Fahrenheit germination becomes poor and erratic, which is the standard obstacle for late summer sowings intended for winter harvest. Sow into a cooler spot, water the bed thoroughly before sowing to drop its temperature, and sow slightly thicker to allow for losses.
Direct sowing usually beats transplanting. Spinach makes a taproot and resents disturbance, so a transplanted plug frequently bolts earlier than a direct-sown plant beside it. Where a tray is unavoidable, use deep cells and transplant young, at the first true leaf.
What temperature does spinach need in a greenhouse?
The working range is 50 to 65 degrees Fahrenheit by day and 40 to 50 by night, which is cooler than almost anything else grown under cover. Spinach is genuinely comfortable in conditions that hold most crops static.
At the cold end it is remarkable. An acclimated plant under cover survives well below freezing, and the leaves can freeze solid and recover when they thaw. Cold also improves it: the plant accumulates sugars as an antifreeze response, which is why winter spinach tastes noticeably sweeter than spring spinach.
That cold tolerance is what makes the double-cover method so effective. Row cover over hoops inside an unheated greenhouse holds a spinach bed through nights 8 to 10 degrees Fahrenheit colder than the structure alone would, and the approach is covered in winter greens in an unheated greenhouse.
At the warm end, growth slows above about 75 degrees and bolting accelerates. Combined with lengthening days it is decisive, which is why spring greenhouse spinach is a short crop and summer greenhouse spinach is not worth attempting.
How much light does spinach need?
Spinach wants 12 to 17 moles per square meter per day, the same low requirement as lettuce, which is what makes it viable in low winter light. Over a 16 hour photoperiod that is roughly 208 to 295 micromoles per square meter per second at the canopy.
The complication is that light does double duty here. It drives growth, and it drives bolting. Long days push the plant toward flowering regardless of temperature, so extending the photoperiod with supplemental lighting, which helps almost every other crop, actively harms spinach.
The practical conclusion is that supplemental lighting for spinach should raise intensity rather than extend hours. In the depths of winter, adding intensity over a short natural day increases growth without triggering the bolting response. In practice most growers simply accept slow winter growth and harvest progressively.
Spacing, support and training
Space at 4 to 6 inches each way for full leaves, and considerably tighter, 2 to 3 inches, for baby leaf harvested young. That is around a fifth of a square foot per plant, so spinach is among the most productive crops per square foot in a greenhouse.
Thin ruthlessly rather than leaving a thick stand. Spinach sown thickly and left produces small leaves, poor airflow at the base and a downy mildew problem, and thinnings are perfectly good baby leaf so nothing is wasted.
Airflow matters as much here as with lettuce. Downy mildew is the defining spinach disease under cover and it needs leaf wetness in still air, so a circulation fan running continuously is worth more than any spray.
Rows rather than a broadcast stand make harvesting and thinning far easier, and they let a soaker line sit between rows delivering water at the root rather than over the leaf.
Watering and feeding
Spinach uses about 0.15 gallons per square foot per day at peak and roughly a fifth of that in winter. It is shallow rooted, and drying out is one of the checks that accelerates bolting, so consistency matters more than volume.
Winter overwatering is the more common failure. A cool, barely transpiring spinach bed in a cool medium takes up very little water while the medium dries slowly, and a timer left on a warm-season program saturates it. This kills more winter spinach than frost does.
Water at the root zone in the morning. Spinach leaves are broad, they hold water, and downy mildew needs several hours of leaf wetness. Drip emitters or a soaker line under mulch remove the problem completely.
Feed moderately with nitrogen early and stop as the plant approaches size. Spinach accumulates nitrate in the leaf, and heavy late feeding in low light conditions increases that accumulation while doing very little for growth.
What goes wrong with greenhouse spinach?
| What you see | Usual cause | What to do |
|---|---|---|
| Plant sends up a flower stem and leaves become narrow and bitter | Bolting, triggered primarily by increasing day length and accelerated by heat | Sow in autumn into shortening days rather than in spring, and use bolt-resistant varieties for spring sowings. |
| Yellow patches on the upper leaf with greyish growth beneath | Downy mildew, which needs leaf wetness at moderate temperatures | Water at the root zone, vent in the morning, run a fan continuously, and choose resistant varieties. |
| Pale, yellowing leaves with the veins staying green | Nutrient availability rather than supply, commonly pH below 6.5 in a greenhouse bed | Test the bed pH and correct it. Spinach is unusually sensitive to acidity compared with other vegetables. |
| Winding pale tunnels inside the leaf | Leaf miner, whose larvae feed between the leaf surfaces | Remove and destroy affected leaves promptly, and use fine mesh over vents to exclude the adults. |
| Seed sown in late summer emerges patchily or not at all | Medium above about 75 degF, where spinach germination becomes erratic | Water the bed thoroughly to cool it before sowing, sow in the evening, and sow slightly thicker. |
| Plants collapse at soil level shortly after emerging | Damping off in cold wet medium with poor drainage and still air | Improve drainage, sow thinner, and get air moving over the bed. |
| Growth stops entirely in midwinter | Daily light integral below what the crop needs, which is normal below about ten hours of daylight | Expected rather than a fault. Grow to size in autumn and harvest progressively through the dark weeks. |
| Leaves look thick and blistered | Savoy leaf type rather than a problem, or occasionally cold response | Nothing needed. Savoy types are naturally crinkled and are among the hardiest for winter. |
Which types work best under cover?
| Type | Suits | Note |
|---|---|---|
| Savoy types | Winter production under cover | The crinkled leaf is the hardiest form and stands the longest in cold, at the cost of being harder to wash. |
| Semi-savoy | General greenhouse use | A compromise with good cold tolerance and easier cleaning, which is why most modern varieties are semi-savoy. |
| Smooth or flat leaf | Baby leaf and quick cropping | Fast and easy to wash, but generally the least cold tolerant and quickest to bolt. |
| Bolt-resistant varieties | Spring sowings | The only way to get a useful spring crop, since day length rather than temperature is driving the bolt. |
| Downy mildew resistant varieties | Any humid greenhouse | Resistance is race-specific and listed by number on the packet, and it is worth choosing on rather than ignoring. |
| Perpetual spinach and chard | Growers who want spinach flavour without the bolting | A different species entirely, far more heat tolerant, and it crops for months rather than weeks. |
The last row is worth taking seriously. Perpetual spinach, which is a leaf beet rather than true spinach, is much more forgiving in a greenhouse: it does not bolt on day length, it tolerates heat, and a single sowing produces for months rather than weeks. It is coarser in flavour and texture, which is the trade.
For true spinach, the two labels worth reading are bolt resistance and downy mildew resistance. Mildew resistance is race-specific and listed by number, and in a humid greenhouse it makes a visible difference to how long a stand lasts.
Harvesting and what comes next
Harvest outer leaves progressively rather than cutting whole plants. The growing point at the centre survives and produces again, which matters enormously in winter because a replacement sowing will not grow until the light returns.
Pick in the middle of the day when leaves are turgid and not frozen. Frozen spinach leaves handled will collapse into mush when they thaw, so wait for the house to warm rather than harvesting at dawn.
Take no more than about a third of the leaves from any plant at once. Stripping a plant leaves it with too little leaf area to recover, particularly in low light, and it will sit for weeks rather than regrowing.
Clear plants once they begin to bolt rather than trying to keep them. A bolting spinach plant produces bitter narrow leaves and occupies space a succession sowing should have. Cut at the base and leave the roots to break down in the bed.
Equipment for growing spinach under cover
Grow spinach into shortening days rather than lengthening ones, keep the leaves dry, and harvest a third at a time. Done that way it is the most productive thing an unheated greenhouse does through winter, and it sweetens with every cold night.