01 • Choosing varieties

The best greenhouse lettuce variety depends on how you want to harvest it

For Maltese greenhouses, loose-leaf and cut-and-come-again lettuces are particularly straightforward: they grow quickly, need little room and can be picked leaf by leaf or harvested young as whole rosettes. Butterhead and head lettuces form closed heads but need steadier moisture. Romaine grows upright and suits narrow beds, while iceberg needs more room and consistently cool management.

Choose for the season as well as flavour. Cool-tolerant varieties suit the cooler months, while bolt-resistant, more heat-tolerant types help as spring temperatures rise. Seed packets usually give seasonal guidance, which is more useful than relying on a supposedly universal greenhouse lettuce.

For beginners, a mix of two loose-leaf varieties and one head or romaine lettuce works well. You will soon see which type performs most reliably in your own greenhouse microclimate while spreading the harvest.

02 • Sowing calendar

When should you sow lettuce in a Maltese greenhouse?

In Malta, the most dependable lettuce season in an unheated greenhouse usually starts as summer heat recedes and runs through winter into early spring. Sow small batches while daytime conditions remain mild; overheating becomes increasingly likely as late spring approaches, and sunny summer conditions are usually the hardest. Conditions vary with exposure and location, so use readings from your own greenhouse together with local forecasts rather than a single timetable.

Do not rely on the calendar alone: measure the real greenhouse climate. A bright winter day can become unexpectedly warm indoors, while the following night may be much cooler. For germination, moderately cool, evenly moist compost is preferable to soil that has overheated in the sun.

A useful routine is to sow a small amount every 10 to 21 days. This spreads the risk: if one batch struggles during an unusual hot or cool spell, younger plants are already coming along behind it.

03 • Temperature

Lettuce succeeds when you control heat, not when you warm the greenhouse as much as possible

Maltese summers are intensely sunny and can bring long hot spells. In exposed gardens, greenhouses need early shading and generous ventilation; coastal wind may help air exchange but can also increase water loss. For lettuce, the practical upper temperature limit is often more important than the minimum. On sunny days, indoor temperatures can rise far above outdoor air, so open doors and roof vents before heat builds up.

Excess warmth does not simply speed growth. It can cause loose plants, bitter leaves, poor head formation and premature bolting. Young seedlings are especially sensitive when the growing medium overheats.

A thermometer at plant height is more useful than one fixed to a wall. If possible, also check root-zone temperature and note which areas become hottest in the morning or afternoon.

04 • Direct sowing or transplants

Sow directly or raise plants first? Both methods work in a greenhouse

Direct sowing saves work and is very practical for loose-leaf lettuce. Sow shallowly, keep the surface evenly moist until emergence and thin crowded rows early. Seedlings that stand too close remain wet longer and develop weaker plants.

Raised transplants give you more control over space. You can grow a seed tray in a bright, cooler place and plant into the bed only when plants are sturdy. This is useful when another crop still occupies the greenhouse or when you want to fill gaps accurately.

For a continuous household harvest, combine small directly sown rows with a few transplants raised alongside them. Find further sowing basics in the Bloomcabin seed guide.

05 • Spacing

Give full-sized heads room; grow closer only when you deliberately harvest young

For standard head and romaine lettuces, around 20–30 cm is a sensible starting point, depending on variety and desired final size. Loose-leaf lettuce for baby leaves can stand much closer because it is harvested sooner.

Do not choose spacing only for the highest possible plant count. Air needs to move between plants and every leaf needs sufficient light. Overcrowded crops hold humidity in the canopy, make aphid checks harder and encourage rot.

Also allow a narrow working route. Lettuce is low-growing, but regular harvesting, watering and inspection become awkward if you must reach over plants every time.

06 • Soil and nutrients

A loose, fertile root zone matters more than heavy feeding

Lettuce grows well in humus-rich, free-draining soil or quality vegetable compost. The aim is a root zone that retains water without staying permanently wet. Improve heavy ground with organic matter and structure-building materials; containers need reliable drainage holes.

Excess nitrogen is not a quality benefit. It can encourage very soft, lush growth and make the crop harder to manage. Use balanced base nutrition instead, then watch leaf colour, growth rate and root health.

If you grow lettuce in the same bed each year, renew compost or parts of the growing medium, remove plant debris and rotate with other plant families where possible. Clean growing areas are especially valuable under cover.

07 • Watering

How often should greenhouse lettuce be watered?

There is no fixed watering schedule in days for Malta. Young plants with a small root zone need more frequent light watering, while established crops use much more on bright days. Test the soil with a finger or moisture meter and water when the root zone starts to dry, before plants visibly wilt.

Consistency matters more than large volumes. Moving from dry compost to flooding stresses plant tissue and can worsen quality problems. Water the soil in the morning where possible, allowing leaves and paths to dry before night.

Drip irrigation can help with longer rows but still needs checking: blocked emitters or uneven pressure show up quickly in lettuce. A brief manual inspection remains worthwhile.

08 • Ventilation

Ventilation controls heat and helps keep leaf surfaces dry

Ventilation is not only a summer task. High humidity can develop even in cooler weather when wet soil, plants and condensation share an enclosed space. Open early on mild days and create gentle air movement.

Roof vents are particularly effective because warm air rises. An open door or opposing opening completes the air exchange. Check automatic vent openers regularly: lettuce reacts to a single very hot day faster than many tougher ornamental plants.

At the same time, avoid a constant strong draught directly over young plants. The goal is exchange, not a forceful airflow across damp foliage.

09 • Bolting

Why lettuce bolts and how to delay it

Bolting is the move from leaf growth into flowering and seed production. It is accelerated by rising day length, high temperatures and stress. Once the central stem starts to extend, heads loosen and flavour can become noticeably bitter.

Prevention starts with seasonal planning: choose more heat-tolerant varieties for late spring batches, use shade during hot periods, maintain even watering and harvest promptly. Do not expect extra fertiliser to reverse a crop that has already started bolting.

If high summer is regularly too hot, a crop break is not a failure. Use the space for heat-loving plants and restart lettuce when conditions become cooler again.

10 • Succession planting

Do not sow 40 heads in one day if your household cannot eat 40 heads at once

Lettuce is ideal for succession planting. For a household, small sowings roughly every two weeks are usually more useful than one large bed. You harvest over many weeks rather than all at one weekend.

Divide the bed mentally into four zones: freshly sown, young plants, almost ready and just harvested. When a zone becomes free, prepare it again. This simple rotation keeps the crop clear and helps you spot pest or watering issues early.

Combine varieties with different maturity speeds too. Fast loose-leaf lettuce bridges the wait for romaine or head lettuce.

11 • Pests and hygiene

A greenhouse excludes some pests but can multiply introduced ones rapidly

Aphids matter particularly in protected lettuce because mild conditions can let them multiply quickly and hide deep among young leaves. Check new plants and transplants before bringing them in, and inspect beneath leaves regularly.

Remove heavily infested plants early. Yellow sticky traps can indicate flying pests but do not replace visual checks. Clean pots, tools and old plant material so pests and diseases are not carried from one batch to the next.

Biological control with beneficial insects can work well under cover, but requires the correct species, temperature and timing. Use only plant-protection products authorised in Malta for the crop and purpose; check the Maltese register for the product, use, user category and current label, and observe any waiting period.

12 • Tipburn and quality

Brown leaf edges do not automatically mean the soil lacks calcium

Tipburn often develops when rapidly growing young leaves do not receive enough calcium, even where the root zone contains sufficient calcium. What matters is transport through the plant with the water flow.

In practice, water evenly, avoid extreme heat, do not overfeed with nitrogen and improve air movement through the crop. Big growth surges after drought stress are less favourable than steady, even development.

If the problem always occurs with the same variety, trial another alongside it. Varieties differ in sensitivity, and the best answer is often a combination of climate, watering and genetics.

13 • Hydroponics

Can lettuce be grown hydroponically in a home greenhouse?

Yes. Lettuce is a classic crop for NFT, deep-water and other hydroponic systems. European research shows that lettuce can grow efficiently in controlled systems. Hydroponics suits home growers willing to monitor pH, conductivity, nutrient solution, oxygen supply and hygiene regularly.

Hydroponics does not remove climate problems. Water that is too warm, low oxygen, high air temperatures or poor hygiene can damage a whole crop very quickly. Start with a small system and keep records.

If you want the simplest start, soil or compost is not a disadvantage. The greenhouse already offers its main benefit through protected conditions and manageable watering.

14 • Cooler-season growing

Can lettuce grow through winter in an unheated greenhouse?

In Malta, cooler winter conditions can favour lettuce, although limited light and stormy spells may slow growth. Lamb’s lettuce and winter varieties are hardier than summer types. An unheated greenhouse can extend the harvest window, and temporary fleece can give sensitive batches extra protection during an unusually cold spell.

It is important to distinguish survival from growth. A cool-tolerant lettuce may come through a cold period while producing almost no new leaves. Growth resumes as light and temperatures improve, so cooler-season crops should be established before the darkest period.

Water less often in winter because evaporation and growth slow down. Permanently wet soil is more dangerous than a briefly dry surface. Keep glazing clean and avoid unnecessary shade.

15 • Harvesting

Harvest lettuce before greenhouse conditions force your hand

Loose-leaf lettuce can be picked leaf by leaf as long as its centre remains intact. Head, butterhead and romaine lettuce are harvested when size and firmness suit your intended use. If hot weather is forecast, do not wait for maximum size: quality matters more than the last grams.

Harvest in the cool morning when leaves are crisp. Remove damaged outer leaves from the greenhouse at once and cool the crop promptly. Clean tools reduce the risk of carrying disease from plant to plant.

A good succession plan delivers not the largest single harvest, but the greatest number of useful kitchen portions across the season.

16 • Choosing a greenhouse

Which greenhouse suits lettuce best?

Lettuce does not need a great ridge height, but it benefits from usable floor space, reliable ventilation and easy access. A compact aluminium greenhouse can be highly productive when beds are sensibly arranged. Larger models make it easier to separate lettuce from taller, heat-loving crops.

The Bloomcabin greenhouse collection includes different sizes and designs. Plan the use first—beds, shelves, paths, water access and ventilation—and you are more likely to choose well than if you buy by external dimensions alone.

Also use the Bloomcabin beginner’s guide if you are setting up a greenhouse for the first time. Lettuce is an excellent first crop because mistakes show quickly and a new batch can be sown again within a short time.

For questions about construction, ventilation, sizes and equipment, see the Bloomcabin aluminium greenhouse FAQ.

17 • Troubleshooting

Five common greenhouse lettuce problems

Start diagnosis with three questions: how warm has it been over recent days, how evenly moist has the root zone been, and how good is the airflow? These three factors explain a large share of visible problems.

Bitter leaves often point to heat or a missed harvest window. Tall, stretching plants suggest bolting. Brown inner edges may be tipburn. Soft rotten patches call for checks of moisture, hygiene and air movement. Sticky or distorted leaves should be inspected for aphids.

Do not change five things at once. Correct the most likely stress factor first, then observe the next batch. This is how you learn your own greenhouse climate more quickly.

18 • FAQ

Frequently asked questions about greenhouse lettuce

Conclusion

The best greenhouse lettuce is not forced through peak summer heat, but grown quickly, evenly and without stress.

In Malta, lettuce is ideal for keeping a greenhouse productive during the cooler part of the year. Use the structure as a buffer against wind, heavy rain and cool nights, while remembering that sunshine can heat the interior extremely quickly.

If you keep only three rules, make them these: sow small amounts regularly, keep the root zone evenly moist and ventilate early when conditions warm. They prevent many common problems before they become visible.

Anyone planning a new greenhouse will find Bloomcabin greenhouse collection in a range of sizes and designs. For lettuce, useful space, reliable ventilation, easy watering access and a layout that separates cool and warm crops matter more than maximum height or trapped heat.

Sources and further information

1. Bavarian State Research Centre for Viticulture and Horticulture — protected lettuce growing and hydroponics
2. Bavarian Garden Academy — lettuce and protected growing in small greenhouses
3. German Weather Service — garden climate and frost probability
4. Wageningen University & Research — lettuce cultivation in controlled environments
5. Wageningen University & Research — lettuce under glass and LED lighting
6. Agroscope — strategies against aphids in greenhouse lettuce