Growing coriander / cilantro in a greenhouse

17.08.2026.

Bloomcabin growing notes · South Africa

Growing coriander / Cilantro in the greenhouse

One fast annual herb, two very different harvests: grown cool and cut young, it provides fresh cilantro leaves. Let it bolt, flower and dry, and it produces fragrant coriander seeds for the spice cupboard.

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. Greenhouse growing is wonderfully direct: sow a small patch where it will grow, wait for sturdy rosettes, and cut only what will soon reach the kitchen. The leaves are best used fresh, before cooking or extended storage.

Goal: repeated harvests of fresh leaves.
Best method: sow densely and overlap short batches.
Climate priority: avoid hot root zones and stagnant air.
Growing rhythm: weeks rather than a whole garden season.

The spice harvest

Coriander is the slower, spicier side.

It is the same plant, now allowed to grow tall, make pale flower umbels and ripen rounded fruits. The foliage becomes finer and its culinary role changes. Instead of fighting flower stems, give selected plants space, protect ripening umbels from shedding, and allow the crop to finish dry.

Goal: fragrant, fully mature spice seed.
Best method: wide spacing with unrestricted airflow.
Climate priority: ordinary warmth is welcome after seed set begins.
Growing rhythm: one complete life cycle to maturity.

Route to leaves

Cut young.

Route to seed

Let it grow.

This separation is the key decision in greenhouse cultivation. A crop for cilantro may stand relatively close together and be harvested repeatedly, while a seed crop needs light, branching space and peace. Expecting both from every plant often gives too few tender leaves early on and few seeds later. Divide the bed, bench or trough deliberately. Two labelled strips are enough: keep the leaf section moving with small succession sowings, while several strong plants in the seed section are no longer cut once they start growing taller.

Before the first leaves

Sow for the harvest you want.

Direct sowing is usually the safest route because coriander makes a taproot and dislikes unnecessary disturbance. Use viable seed and a loose, stable herb or vegetable mix with good drainage. A short row in a prepared greenhouse bed works equally well. If you do raise plants in modules, transplant seedlings while very young and keep the entire root ball intact.

Choose seed for the harvest goal

For leaf harvests, varieties described as “slow bolt” or suitable for cilantro are useful. They extend the usable leaf window, but do not remove the plant’s natural response to heat, long days and drought stress. For seed harvest, variety matters less than a reliable seed source and enough room for sturdy, branching plants. In both cases, choose fresh tested seed with a clear variety description.

Make a shallow, firm seed drill

Sow the rounded fruits about 1 to 2 cm deep. Cover evenly, firm the surface lightly, then water with a fine rose so the seed is not washed together. In a greenhouse bed, short rows across the narrow side are particularly practical: each can be watered, cut, labelled and cleared separately. Coriander does not need fresh manure or heavily enriched compost; moderately fertile, well-drained soil is sufficient.

Thin leaf and seed crops differently

Leaf coriander can be sown relatively densely; later thin to around 5 cm apart if seedlings crowd one another. For seed, around 20 to 25 cm between plants is more useful. This lets light reach the base and allows branching flower stems to develop without shading each other. Where access permits, row spacing of roughly 35 to 40 cm makes care, checks and harvesting easier.

Wait patiently, do not waterlog

Germination can take two to three weeks and may be slower in cool growing medium. Keep the top layer evenly moist until emergence, but never saturated. A clear cover can briefly help in very dry greenhouse air; remove it after emergence so seedlings do not grow in stagnant humidity. Label every sowing with the date and harvest goal—leaf or seed. This makes later succession sowings much easier to manage.

The greenhouse paradox

Give plenty of light without letting heat take over.

A greenhouse softens cold nights, wind and heavy rain. Yet the same structure can push coriander into flower very quickly on sunny days. Cilantro prefers bright but relatively mild conditions. A balanced daytime climate, dependable ventilation and a root zone that does not repeatedly overheat are therefore essential. A thermometer at plant height gives a truer picture than a glance through the door. Open roof vents and the door as the house warms in the morning, rather than waiting for plants to wilt visibly.

Light

Give coriander a bright position. During hot periods, light shade on the sunny side is better than excluding light altogether. Gentle morning sun remains valuable; dark corners instead produce long, pale and less sturdy stems.

Temperature

Mild days and cooler nights favour leaf harvests. As heat rises, the leaf window becomes noticeably shorter. Above all, avoid a hot root zone and sharp 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-flow. A small circulation fan can help in still corners. Moving air is not decorative: it keeps plant tissue drier and evens out temperature differences.

Water

Water directly onto the growing medium, preferably in the morning. Foliage should dry before evening. Persistently wet surfaces combined 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 signals

01Rosette
Cut now

02Centre rises
Harvest generously

03Fine foliage
Choose a goal

04Flower umbels
Decide

05Seed set
Dry

Read the growth stages and act. A low, rounded rosette is the best stage for leaf harvesting. Once the centre rises, harvest generously but leave the growing point intact. Narrower, finely divided upper leaves show that the plant is putting more energy into height and flowering. When branched stems and umbels appear, decide whether to remove early buds to gain a little more leaf time or allow marked plants to continue consistently for seed. The initially green fruits later turn yellowish, straw-coloured and light brown; leaf harvest is then over. Bolting rarely has one cause. Long days, heat, dry roots, erratic watering, crowding, root disturbance and normal maturity all work together. Ventilate earlier, lightly shade intense afternoon sun, water before wilting and harvest on time. Above all, sow the next batch before the current one is exhausted.

A fresh row is the best insurance

Plan a rhythm rather than one date.

One large sowing creates a short glut. Small portions every two to three weeks provide a more useful rhythm: young leaves, harvest-ready rosettes, a replacement row already germinating and an intentionally uncut section for flowers and spice seed.

Example seasonal sequenceMarAprMayJunJulAug

Batch A · leavesSowing and young leaves

Batch B · leavesstrong leaf harvest

Batch C · leavesSowing and young leaves

Batch D · leavesstrong leaf harvest

Reserved section · seedflower, ripen, dry

Young-leaf and replacement window Leaf harvest window intentionally maintained seed crop

This March-to-August calendar illustrates a cool-season rhythm, not a fixed promise. Across South Africa, timing differs between cooler coasts, high-altitude interiors, winter-rainfall districts and warmer summer-rainfall regions. Sow under cover when light is adequate and temperatures are mild. In intense summer heat, the leaf window contracts; ventilate very well, add light shade, or pause until cooler weather returns. Cold inland winters can also slow growth in an unheated greenhouse. Adjust each batch to local nights, daytime heat and the conditions inside your structure.

Leaf harvest

Cut so that regrowth remains.

Turquoise markings show productive cuts. The red line indicates the growing point that must remain for further regrowth.

Three good ways to harvest

Selective cutting: Take the outer stems first and allow the centre to keep unfolding. This method is especially suitable for small handfuls used immediately in 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 into the heart: A hard cut directly at soil level ends the batch. That is right when clearing an old row completely, but not when you expect another flush.

Preserve flavour: Harvest in the morning or another cool part of the day once leaves are dry. Wash stems only immediately 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. When dried, leaves lose much of their bright, citrus-like aromatic edge.

A considered layout makes this routine easier. Place leaf coriander along the front edge of a growing bench or near the door, so cutting, watering and resowing stay convenient. Find guidance on usable space, airflow and a clearly organised herb zone in the Classic aluminium greenhouse.

The warm, dry finish

Let one section flower. Then be patient.

Once you choose seed production, stop regular leaf cutting in that section. Plants form airy, flat umbels with pale flowers, followed by green rounded fruits. Flowering coriander may remain in the greenhouse while it does not shade younger leaf crops and air circulates freely. With doors or vents open, hoverflies and other beneficial insects can reach the flowers. Keep ventilation generous so foliage and fruit heads dry quickly after watering. Support tall plants gently if necessary, without repeatedly moving delicate umbels.

01Green

02Colour change

03Partly dry

04Dry in a bag

05Clean and store

Green: The soft fruits are still filling and do not yet have the dry, warm spice aroma. Leave them to mature on the plant.
Colour change: Watch umbels shift from green to yellow-beige, straw and light brown. As flowers open in sequence, ripening is uneven too. Check the crop regularly.
Partly dry: When a large proportion of umbels is straw-coloured to light brown, cut whole stems on a dry morning before the first fruits shed in quantity.
Dry in a bag: Hang stems upside down inside a clean paper bag or another breathable covering. The place should be dry, warm and well ventilated; the bag reliably catches released seed.
Clean and store: Rub fully dry umbels gently, remove plant debris and store whole seeds in a tightly closing jar, protected from warmth and direct light.

Do not wait until every umbel on the plant has turned dark brown. In a warm greenhouse, the earliest fruits can already fall while later ones remain green. Cut when the majority is clearly changing colour, then complete drying under cover. Grind seeds only shortly before cooking to retain their warm, citrusy, woody fragrance 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, major watering fluctuations or harvesting delayed too long.

Response: Open vents and the door earlier, lightly shade the most intense afternoon sun and water before plants wilt. Sow a new small batch straight away. Removing an early flower stem may buy some time, but succession sowing is more reliable than trying to reset a bolting crop permanently.

Pale, weak and stretched leaves

Likely cause: insufficient light, an overcrowded crop or permanently wet medium that limits root function.

Response: Move containers to a brighter position, thin crowded rows and allow the upper layer of growing medium to dry slightly before watering again. Feed sparingly only after correcting light, air and watering. Too much nutrient encourages fast but often soft, less fragrant growth.

Yellow lower leaves and a tired centre

Likely cause: waterlogged soil, lack of oxygen around roots or an old crop naturally preparing to flower.

Response: Check drainage holes, saucers and the structure of bed soil. Reduce watering frequency when wet and clear over-aged plants instead of forcing another cutting cycle. A fresh sowing is the better foundation for the next harvest window.

Aphids gather on young tips

Likely cause: soft, lush growth and a sheltered greenhouse climate where unnoticed pests can establish quickly.

Response: Check leaf undersides and young shoots twice weekly. Isolate or remove badly affected plants, gently wash off small colonies with water and improve air movement. Yellow sticky traps can indicate winged insects but do not replace direct plant checks. If a plant-protection product is considered, use only a product registered in South Africa for the intended use on edible herbs, exactly as its label directs.

Seedlings collapse at soil level

Likely cause: damping-off encouraged by overwatering, old medium, poor hygiene, sowing too densely, or warm, still and persistently humid air.

Response: Discard affected seedlings, clean used containers and start again with fresh growing medium. Sow less densely and water the soil rather than continually wetting foliage. Do not close the greenhouse at night with wet surfaces and warm, humid air.

Planning for South Africa

A greenhouse moderates climate—it does not replace it.

Coriander timing shifts noticeably across South Africa. A protected structure gives greater control, but does not remove the plant’s preference for bright, mild weather. Available light, cold nights, wind and seasonal heat should set the pace.

Cooler coast and winter-rainfall areas

Protected direct sowing can begin after the darkest, coldest weeks, although cool conditions may slow early growth. Coriander often stays leafy longer in mild weather. Use sunny mornings, protect young seedlings from cold wind and ventilate as the greenhouse warms. Avoid cold, continuously wet roots. Without added warmth and plant lighting, a strong winter harvest is not always dependable.

Highveld and inland elevations

Early batches work well under cover when light, water and ventilation are balanced. After a cold night, plants do not need a tropical burst of heat; let them start the day gradually. In the warm season, even an unheated greenhouse can become hotter than outdoors by afternoon. Place leaf crops near well-ventilated areas, shade lightly when needed and sow only small quantities. A second reliable leaf phase often begins after the hottest period.

Warm inland and subtropical areas

Start in the cooler part of the year and expect faster flowering as heat builds. Ventilate generously, lightly shade intense sunny sides and focus leaf harvests on cooler months and milder transition periods. During very hot weather, a pause is often wiser than a constant battle against bolting. A selected seed crop can, however, use protected airy warmth to ripen fruits dry. Continue watering evenly, but keep ripening umbels as dry as possible.

For a concentrated herb area in limited space, the aluminium mini lean-to greenhouse is a useful choice. The same principle applies: 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, easily organised growing space.

Questions from the herb bench

Greenhouse coriander FAQ

Can coriander grow in a greenhouse all year?

In principle it can with frost-free growing and supplementary plant lighting. Without extra light, winter may still be too dim for strong continuous leaf production, especially in cooler regions. In a cold unheated greenhouse growth slows markedly; in an overheated summer greenhouse plants bolt quickly. Use the protected space chiefly for an early start, dependable cool-season batches and protected seed ripening. Treat coriander as a succession crop, not one permanent plant.

How long does coriander take to germinate?

Allow about two to three weeks in suitable conditions. Cool growing medium can delay emergence. Keep soil evenly moist but never waterlogged, and do not repeatedly dig for seed. Uneven sowing depth or fluctuating moisture often produces uneven germination.

Should coriander seeds be split before sowing?

The rounded fruit usually contains two smaller fruits and can be divided carefully, but this is not necessary for reliable home growing. Fresh seed, even sowing depth, gentle watering and a loose growing medium matter much more. Do not crush the fruits, as damaged portions germinate poorly or not at all.

How deep should a coriander container be?

Use a container with an open drainage hole and at least about 20 cm of usable depth so the taproot can develop. A broad trough is usually more practical than many small pots: it allows short rows, labels for each harvest goal and sowings at intervals. Avoid standing water in saucers and use a stable, well-drained growing medium.

Can coriander grow beside tomatoes?

Yes, if you allow for its own climate needs. In summer, the tomato area is often the brightest and hottest part of the greenhouse, making coriander bolt sooner. Position herbs toward a cooler edge with good airflow, but not in a strongly 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 allow the harvest to dry completely in a paper bag or other breathable covering.

Sources and further guidance

Practical foundations for this guide

DLR Rheinland-Pfalz – growing guidance for leaf coriander: German crop and variety guidance focused on leaf production and bolting risk.
Oregon State University Extension – growing coriander: guidance on growing for leaves and seed, sowing and heat response.
Utah State University Extension – cilantro/coriander in the garden: practical information on sowing depth, spacing, watering, flowering and drying seed.
NC State Extension – Coriandrum sativum crop profile: a botanical profile covering position, growth and uses of coriander.
University of Wisconsin–Madison Extension – cilantro and coriander: an overview of the life cycle, leaf harvest, bolting, flowers and ripe-fruit use.
Royal Horticultural Society – growing coriander successfully: practical notes on sowing, care, repeated harvesting and seasonal growing.

Adapt every recommendation to your greenhouse type, variety, available 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 the crop has bolted.

Classic aluminium greenhouse

Route to seed

Choose spice.

Give several strong plants room to grow, let umbels ripen quietly with good airflow and store fully dried fruits in a jar. Your own coriander spice can then serve the kitchen through the season.