Growing coriander / cilantro in a greenhouse
Bloomcabin growing notes · Malta
Growing coriander / cilantro in a greenhouse
One fast annual herb, two very different harvests: cut young in cooler conditions for fresh cilantro leaves, or let it bolt, flower and dry for fragrant coriander seed.
The green harvest
Cilantro is the quick, fresh side.
Leaf coriander is Coriandrum sativum and is picked for its soft divided leaves and tender stems. In a greenhouse, sow a small patch directly, wait for sturdy rosettes and cut only what you will use soon. The leaves taste best freshly picked, before cooking or long storage.
- Aim: repeated harvests of fresh leaves.
- Best method: sow densely and overlap short batches.
- Climate priority: avoid hot root zones and stagnant air.
- Growing rhythm: several weeks rather than a whole season.
The spice harvest
Coriander is the slower, spicier side.
It is the same plant, now allowed to grow tall, make pale umbels and ripen round fruits. The foliage becomes finer and its culinary role changes. Rather than fighting flower stems, give selected plants space, protect ripening umbels from shedding and let the crop finish dry.
- Aim: fragrant, fully ripe spice seed.
- Best method: wide spacing with free air movement.
- Climate priority: ordinary warmth is welcome once seeds form.
- Growing rhythm: one complete life cycle to maturity.
Route to leaves
Cut young.→
Route to seed
Let it grow.This split is the key decision in greenhouse growing. A crop for cilantro can stand fairly close and be picked repeatedly, while a crop for spice seed needs light, branching space and peace. Expecting both from every plant often gives too little tender leaf early and few seeds later. Divide the bed, bench or trough deliberately. Two labelled strips are enough: keep the leaf area moving with small succession sowings, while strong plants in the seed area are no longer cut once they begin to grow upwards.
Before the first leaves
Sow for the harvest you want.
Direct sowing is usually safest because coriander makes a taproot and dislikes needless disturbance. Use viable seed and loose, free-draining herb or vegetable compost. A short drill in a prepared greenhouse bed works equally well. If you raise plants in modules, transplant them very young with the root ball completely intact.
01
Choose seed for the harvest goal
For leaf harvest, varieties described as slow-bolting or suitable for cilantro are useful. They extend the picking window but do not remove the natural response to heat, long days and drought stress. For seed, the variety matters less than reliable seed and enough room for sturdy branching plants. In both cases, choose fresh tested seed with a clear variety description.
02
Make a shallow, firm seed drill
Sow the round fruits about 1 to 2 cm deep. Cover evenly, firm the surface lightly and water with a fine rose so seed is not washed together. In a greenhouse bed, short rows across the narrow side are practical: each can be watered, cut, labelled and cleared separately. Coriander does not need fresh manure or heavily enriched compost; moderately fertile, well-drained growing medium is enough.
03
Thin leaf and seed crops differently
Leaf coriander may be sown fairly densely; later thin it to around 5 cm if seedlings crowd each other. For seed, allow about 20 to 25 cm between plants. Light can then reach the base and branched flower stems can develop without shading one another. Where access allows, around 35 to 40 cm between rows makes care, checking and harvesting easier.
04
Wait patiently, do not waterlog
Germination may take two to three weeks and can be slower in cool compost. Keep the top layer evenly moist until emergence, never saturated. A clear cover can briefly help in very dry greenhouse air, but remove it after emergence so seedlings do not grow in still, humid air. Label every sowing with the date and intended harvest: leaves or seed. This makes later succession sowings much easier to track.
The greenhouse paradox
Give plenty of light without letting heat take over.
A greenhouse buffers wind and heavy rain, but on sunny days it can drive coriander into flower quickly. Leaf coriander prefers bright, relatively mild conditions. Aim for balanced daytime conditions, dependable ventilation and a root zone that does not repeatedly overheat. A thermometer at plant height gives a truer reading than a glance through the door. Open roof vents and the door as the house warms in the morning, not only when plants visibly wilt.
Light
Give coriander a bright position. In hot months, light shading on the sunny side is better than removing all light. Gentle morning sun remains valuable; dark corners produce long, pale and weak stems.
Temperature
Mild days and cooler nights favour leaf harvest. The leaf window shortens sharply as heat rises. Avoid overheated roots and abrupt temperature swings; early ventilation works better than a late emergency response.
Air movement
Open roof vents and the door early on warm days to create a gentle through-draught. In still corners, a small circulating fan can help. Moving air keeps plant tissue drier and evens out temperature differences.
Water
Water the compost directly, preferably in the morning. Leaves should dry before evening. Permanently wet surfaces with warm, still air encourage soft growth and increase the risk of leaf and seedling problems.
From leaf to flower
The harvest window in five signs
01Rosette
Cut now
Cut now
02Centre rises
Harvest generously
Harvest generously
03Fine foliage
Choose the goal
Choose the goal
04Flower umbels
Decide
Decide
05Seed set
Dry
Dry
Read the stages and act. A low, rounded rosette is the best stage for leaves. When the centre lifts, harvest generously but leave the growing point. Narrower, feathery upper leaves show the plant is putting energy into height and flowers. When branched stems and umbels appear, decide whether to remove the first shoots for a little more leaf time or let marked plants continue for seed. Later, green fruits turn yellowish, straw-coloured and light brown; leaf harvest is then over. Bolting rarely has one cause: long days, heat, dry roots, uneven watering, crowding, root disturbance and natural maturity work together. Vent earlier, lightly shade intense afternoon sun, water before wilting and harvest in time. Above all, sow the next batch before the current one is exhausted.
A fresh row is the best insurance
Plan a rhythm, not one date.
One large sowing creates a short glut. Small portions every two to three weeks create a more useful rhythm: young leaves, harvestable rosettes, a replacement row already germinating and an intentionally uncut patch for flowers and spice seed.
Example seasonal patternOctNovDecJanFebMar
Batch A · leavesSowing and young leaves
Batch B · leavesStrong leaf harvest
Batch C · leavesSowing and young leaves
Batch D · leavesStrong leaf harvest
Reserved patch · seedflowering, ripening, drying
Young-leaf and replacement window Leaf harvest window deliberately retained seed crop
This calendar shows a principle, not a fixed promise. Malta’s bright light and warm conditions can make coriander bolt quickly, especially from late spring into summer. In a greenhouse, sowings are often most successful through autumn, winter and early spring, while the hottest months may require strong ventilation, light shading and short leaf batches. In exposed coastal locations, protect young seedlings from drying wind while keeping air moving. During summer heat, pause leaf production if needed and restart when conditions become milder.
Leaf harvest
Cut so that regrowth remains.
Turquoise markings show productive cuts. The red line marks the growing point that must remain for new growth.
Three good harvesting methods
- Selective cutting: Take the outer stems first and allow the centre to keep unfolding. This is especially useful for small handfuls that will go straight into the kitchen.
- Light row cutting: Cut a dense young row above the growing point and leave a short leafy base. It is quick and tidy, but works only with vigorous, adequately supplied plants.
- Never cut the heart: A hard cut at soil level finishes the batch. That is right when clearing an old row, but not when you expect another flush.
Preserve flavour: Harvest in the morning or another cool part of the day once foliage is dry. Wash stems only just before use where possible. For short storage, keep them loosely in a glass with a little water in a cool place. Freeze surplus chopped in portions. Drying removes much of the leaves’ bright citrus-like flavour.
A thoughtful layout makes this routine easier. Place leaf coriander at the front of a growing bench or near the door so cutting, watering and resowing stay convenient. Find advice on usable space, airflow and an organised herb zone with the Classic aluminium greenhouse.
The warm, dry finish
Let one patch flower, then be patient.
Once you choose seed harvest, stop regular leaf cutting in that patch. Plants make airy, flat umbels of pale flowers followed by green round fruits. Flowering coriander can remain in the greenhouse if it does not shade younger leaf batches and air circulates freely. With doors or vents open, hoverflies and other beneficial insects can reach the flowers. Keep ventilation generous so foliage and seed heads dry quickly after watering. Support tall plants gently if needed, without repeatedly moving delicate umbels.
01Green
02Colour change
03Partly dry
04Dry in a bag
05Clean and store
- Green: Soft fruits are still filling and do not yet have the warm dry spice aroma. Leave them to ripen on the plant.
- Colour change: Watch umbels change from green to yellowish beige, straw and light brown. Because they flower in sequence, ripening is uneven too. Check the crop regularly.
- Partly dry: When much of the crop is straw-coloured to light brown, cut whole stems on a dry morning before the first fruits shed heavily.
- Dry in a bag: Hang stems upside down in a clean paper bag or another breathable cover. Choose a dry, warm, well-ventilated place; the bag catches loosened seeds reliably.
- Clean and store: Rub fully dry umbels gently, remove plant debris and store whole seeds in a tightly closed jar away from warmth and direct light.
Do not wait until every umbel is dark brown on the plant. In a warm greenhouse, the earliest fruits may shed while later ones remain green. Cut when the majority has clearly changed colour, then complete drying under cover. Grind seeds only shortly before cooking to retain their warm, citrus-woody aroma for as long as possible.
Correct calmly and precisely
Read common problems as cause and response.
Plants bolt at only 12–15 cm
Likely cause: hot roots, sudden temperature peaks, drought stress, erratic watering or harvesting delayed too long.
Response: Open vents and the door earlier, lightly shade the strongest afternoon sun and water before plants wilt. Sow a fresh small batch immediately. Removing an early flower stem may gain some time, but succession sowing is more reliable than trying to reset a bolting crop permanently.
Pale, weak, stretched leaves
Likely cause: too little light, overcrowding or permanently wet compost that restricts root function.
Response: Move containers to a brighter place, thin crowded rows and let the compost surface dry slightly before watering again. Feed sparingly only after correcting light, air and watering. Too much nutrition encourages quick but often soft, less aromatic growth.
Yellow lower leaves and a tired centre
Likely cause: waterlogged compost, lack of oxygen around roots or an old crop naturally preparing to flower.
Response: Check drainage holes, saucers and the structure of bed compost. Water less often in wet conditions and clear over-aged plants rather than forcing another cutting cycle. A fresh sowing is the better basis for the next harvest window.
Aphids gather on young tips
Likely cause: soft, lush growth and a sheltered greenhouse climate where unnoticed pests establish quickly.
Response: Inspect leaf undersides and young shoots twice each week. Isolate or remove badly infested plants, rinse small colonies away carefully with water and improve airflow. Yellow sticky traps can reveal winged insects, but they do not replace direct inspection. Before using any plant-protection product, check Malta’s current register for the exact crop, use and user category, then follow the authorised label precisely.
Seedlings collapse at compost level
Likely cause: damping-off encouraged by overwatering, old compost, poor hygiene, dense sowing or warm, still, constantly humid air.
Response: Discard affected seedlings, clean used containers and begin again with fresh compost. Sow less densely and water the soil rather than constantly wetting foliage. Do not close the greenhouse at night with wet surfaces and warm humid air inside.
Timing in Malta
A greenhouse moderates conditions – it does not replace the climate.
Coriander timing varies across Malta and Gozo. The protected structure gives more control, but does not change the plant’s preference for bright, mild weather. Light levels, coastal wind and summer heat should set the pace.
Coastal and exposed sites
Direct sowing can work through the milder months, but exposed gardens may dry quickly in wind. Use bright mornings, shelter young seedlings from harsh gusts and ventilate once the greenhouse warms. Avoid both dry roots and cold, wet compost. In summer, keep leaf crops short and provide gentle shading where necessary.
Sheltered central locations
Early and late-season batches do well under cover when light, water and ventilation are balanced. An unheated greenhouse can become hotter than the outside air during sunny afternoons. Place leaf crops close to well-ventilated areas, shade lightly if needed and sow only small amounts. A second reliable leaf phase often begins when summer heat eases.
Warm urban gardens
Start in the cooler part of the year and expect rapid flowering as spring warmth builds. Vent generously, shade intense sunny sides lightly and focus leaf harvest on autumn, winter and early spring. In high summer, a break is often wiser than constantly fighting bolting. A selected seed patch can use protected warmth and airflow to ripen fruits dry; continue watering evenly while keeping ripening umbels as dry as possible.
For a concentrated herb area in limited space, the aluminium mini lean-to greenhouse is suitable. Here too, deep containers suit the taproot better than shallow trays, and ventilation takes priority over trapped heat. For larger staggered rows, the Classic aluminium greenhouse offers a bright, well-organised growing space.
Questions from the herb bench
Greenhouse coriander FAQ
Can coriander grow year-round in a greenhouse?
Potentially, but only if temperature and light are managed. In Malta, excessive summer heat is usually a greater obstacle than winter darkness. Growth slows in cool spells, whereas an overheated summer greenhouse triggers rapid bolting. Use the protected space mainly for autumn, winter and spring batches and for sheltered seed ripening; strong summer ventilation and shading may still be insufficient for tender leaves. Treat coriander as a succession crop, not a permanent plant.
How long does coriander take to germinate?
Allow roughly two to three weeks in suitable conditions. Cool compost can delay emergence. Keep soil evenly moist but never waterlogged, and do not repeatedly dig for seed. Uneven sowing depth or fluctuating moisture often causes uneven germination.
Should coriander seeds be split before sowing?
The round fruit usually contains two smaller fruits and can be carefully split, but this is not necessary for reliable home growing. Fresh seed, even sowing depth, gentle watering and loose compost matter much more. Do not crush the fruits, because damaged parts germinate poorly or not at all.
How deep should a coriander container be?
Use a container with open drainage holes and at least about 20 cm of useful depth so the taproot can develop. A wide trough is usually more practical than many small pots: it allows short rows, labels for each harvest purpose and staggered sowings. Avoid standing water in saucers and use stable, well-drained compost.
Can coriander grow beside tomatoes?
Yes, if you respect its own climate needs. In summer, the tomato area is often the brightest and hottest part of the greenhouse, making coriander bolt faster. Position herbs towards a cooler edge with good airflow but not a drying draught. Keep batches young, water evenly and do not let tall tomatoes shade coriander completely.
When are coriander seeds ready to harvest?
Harvest when most umbels have changed from green to straw, beige or light brown and fruits feel firm rather than soft. Cut stems before the earliest seeds fall when touched. Then dry the harvest completely in a paper bag or another breathable cover.
Sources and further reading
Practical foundations for this guide
More from Bloomcabin: automatic greenhouse window openers · Bloomcabin aluminium greenhouse FAQ · greenhouses and conservatories from Bloomcabin
- DLR Rheinland-Pfalz – Leaf coriander growing notes: German growing and variety notes focused on leaf production and bolting risk.
- Oregon State University Extension – Growing coriander: guidance on leaf and seed use, sowing and heat response.
- Utah State University Extension – Cilantro/coriander in the garden: practical advice on sowing depth, spacing, watering, flowering and seed drying.
- NC State Extension – Coriandrum sativum profile: botanical growing profile covering position, growth and uses.
- University of Wisconsin–Madison Extension – Cilantro and coriander: overview of life cycle, leaf harvest, bolting, flowering and ripe fruits.
- Royal Horticultural Society – Growing coriander: practical guidance on sowing, care, repeated harvesting and seasonal growing.
Adapt all recommendations to your greenhouse type, variety, light and local conditions. For a compact sheltered space for deep herb pots, explore the aluminium mini lean-to greenhouse.
Route to leaves
Choose freshness.
Reserve a short row, sow the next batch before the harvest peak and make the first cut part of dinner, not a task left until plants bolt.
Route to seed
Choose spice.
Give several strong plants room to grow, let umbels ripen calmly with good airflow and store fully dried fruits in a jar. Your own coriander spice can then accompany the kitchen through winter.
