Growing lettuce in a greenhouse

Lettuce is the crop a greenhouse most reliably improves, and it is also the crop most often ruined by the greenhouse itself. It grows fast, tolerates cold far better than most people expect, and produces a harvest within a month of sowing. What it does not tolerate is heat, and a greenhouse is a heat-producing structure.

That tension defines the whole subject. In the cool months, lettuce under cover is close to effortless and produces when nothing outdoors will. In the warm months it needs shading, ventilation and cool-season varieties to avoid bolting, and it is frequently easier grown outdoors. Setting a shade cloth correctly matters more for summer lettuce than anything else on this page.

How do you start lettuce under cover?

Sow into cells or blocks at 60 to 68 degrees Fahrenheit in the medium and press the seed onto the surface rather than covering it. Lettuce is one of the few common vegetables that requires light to germinate, and burying seed a quarter inch deep is the single most common cause of a sowing that never comes up.

Above about 80 degrees Fahrenheit in the medium, lettuce seed enters thermodormancy and will not germinate until it cools. This is why late summer sowings fail on a warm bench. The fixes are sowing in a cooler place, sowing in the evening so the seed takes up water overnight at the coolest point of the day, or shading the propagation bench specifically. Bottom heat is not merely unnecessary here, it is actively harmful, so keep lettuce off the heat mat that the tomatoes are using.

Sow into a fine sterile medium such as Burpee Organic Coconut Coir Seed Starting Mix, 16 qt Expanded in a standard tray. Lettuce transplants readily at the two to three true leaf stage, which is around three weeks from sowing, and it is one of the few crops where a slightly root-bound plug still establishes without a serious check.

What temperature does lettuce need in a greenhouse?

The working range is 60 to 70 degrees Fahrenheit by day and 45 to 55 by night, and lettuce is far more tolerant at the cold end than the warm one. It survives brief exposure down to about 20 degrees on an acclimated plant, and cold actually improves the eating quality by concentrating sugars.

The ceiling is what matters. Sustained temperatures above roughly 75 degrees trigger bolting: the plant switches from producing leaves to producing a flower stem, the leaf turns bitter within days, and the process is irreversible. Nothing recovers a bolted lettuce, which is why prevention is the only strategy.

Shade is the primary control. Fifty to sixty percent shade cloth over the roof through the peak season typically drops peak house temperature by 10 to 15 degrees Fahrenheit, and lettuce wants a daily light integral of only 12 to 17, so removing half the light costs the crop nothing. Set the percentage properly with the shade cloth calculator rather than by how hot the house feels.

Ventilation is the second control, and it has a hard ceiling: a fan can bring the house toward outside air temperature and never below it. On a genuinely hot afternoon, shading plus damping down the floor does more than a fan. A vent opener matters most in spring, when a clear morning takes a closed house past the bolting threshold before anyone is awake.

How much light does lettuce need?

Lettuce wants 12 to 17 moles per square meter per day, which is among the lowest requirements of anything grown for food. Over a 16 hour photoperiod that is roughly 208 to 295 micromoles per square meter per second at the canopy, and a greenhouse supplies it comfortably from early spring to late autumn.

That low requirement is what makes lettuce a genuine winter crop. Even in the shortest weeks, a mid-latitude greenhouse often delivers enough for slow but real growth, and a modest supplemental fixture such as linked LED strips closes the gap where it does not. Check what your own house delivers with the DLI calculator before assuming you need lighting.

Below about 10 mol/m2/day the plant holds rather than grows, producing thin pale leaves that never fill out. This is the normal condition in deep winter, and the correct response is to have grown the plant to near full size in autumn light and to harvest it progressively rather than expecting it to grow through.

Spacing, support and training

Space heading types at 8 to 10 inches each way, which gives roughly half to two thirds of a square foot per plant. Cut and come again sowings go far tighter, at 2 to 4 inches, because they are harvested as leaf rather than grown to a head.

Greenhouse spacing runs wider than outdoor spacing for the same reason it does with every crop under cover: still air keeps leaf surfaces damp, and a dense lettuce canopy in a humid house goes down to botrytis at the base within days. Airflow between plants is worth more than the extra plants a tighter spacing would fit.

A small circulation fan running continuously does more for a lettuce bed than almost any other intervention. It breaks the still saturated layer over the leaves, keeps the crown drying after watering, and produces noticeably sturdier plants.

Plant slightly proud rather than deep. A lettuce transplant set with its crown below the medium surface sits in moisture and rots at the base, which is the most common transplant failure in this crop.

Watering and feeding

Lettuce uses about 0.15 gallons per square foot of growing area per day at peak and roughly a fifth of that in winter. It is shallow rooted and has a small reserve, so it is less forgiving of a missed day than a deep-rooted crop, and it responds to a check by bolting.

Water at the root zone rather than over the foliage. Lettuce leaves hold water in the crown and in the folds between leaves, and that water sits for hours in still air, which is precisely the condition botrytis needs. A soaker hose along the row under a light mulch, or drip emitters, removes the problem entirely.

Water in the morning so anything that does land on foliage dries before dark. Evening watering in a lettuce bed is the most reliable way to produce basal rot.

Feed lightly and stop feeding nitrogen once the head begins to form. Excess nitrogen produces soft, fast growth that is more susceptible to tipburn and to aphids, and lettuce is not a hungry crop to begin with. In a greenhouse bed that never leaches, feed less than an outdoor label suggests.

What goes wrong with greenhouse lettuce?

Which types work best under cover?

The single most useful variety decision is matching the type to the season rather than to preference. A summer butterhead sown in autumn grows slowly, rots in the humidity and never heads. A winter variety sown in spring bolts early because it was bred for cold rather than for speed. Seed suppliers label these clearly, and following the label is worth more than any cultural technique on this page.

For cut and come again, loose-leaf types are the correct choice because the growing point sits at the base and survives cutting. A head type cut the same way simply stops, and the stump rots.

Harvesting and what comes next

Harvest heads whole by cutting at the base rather than pulling, which leaves the root in place and avoids disturbing neighbouring plants in a tight bed. Cut in the morning when leaves are turgid; a head cut in the heat of the afternoon wilts before it reaches the kitchen and does not fully recover.

For cut and come again, cut about an inch above the crown, leaving enough tissue for the plant to regrow. Most loose-leaf types give three or four cuts before quality declines, and each cut regrows faster in warm bright conditions and much slower in winter.

Remove every yellowing outer leaf as you harvest rather than leaving it. Dead lettuce tissue in a cool humid greenhouse is the ideal substrate for botrytis, and a bed cleared of debris runs noticeably cleaner than one that is not.

Clear a finished lettuce bed promptly rather than letting bolted plants stand. A bolting lettuce hosts aphids, sheds seed, and occupies bench space that a succession sowing should be using.

Lettuce rewards a schedule more than a technique. Sow small amounts often, shade the house before the weather demands it rather than after, and clear each bed as soon as it is finished. Done that way it is the crop that keeps a greenhouse producing something in almost every week of the year.

Working out the figures for your own house and season? The Greenhouse Build & Growing Planner is the paid version of these pages: 8 printable worksheets you fill in with your own numbers, plus the full PDF, $29.