Basil is the herb a greenhouse most obviously improves, because it wants exactly what a greenhouse provides: warmth, shelter and no rain on its leaves. It is also the herb most often ruined in one night, because it is the most cold-sensitive common crop and the damage from a single cool night is permanent.
The other defining issue is basil downy mildew, which spread globally in the last two decades and turned basil from an easy crop into one that needs variety choice and airflow management. Both problems have straightforward answers, and both are worth knowing before sowing rather than after.
| Measure | Figure | Why it matters |
|---|---|---|
| Germination soil temperature | 70 to 80 degF | Basil germinates poorly below 65 degF and seed frequently rots in cool wet medium rather than emerging. |
| Days to emergence | 5 to 10 days | At the optimum soil temperature, in a moist starting medium. |
| Optimum day temperature | 70 to 85 degF | Where growth is fastest. Above the range, specific processes start failing. |
| Optimum night temperature | 60 to 70 degF | The day to night difference controls stem elongation and plant habit. |
| Survives down to | 50 degF | Brief exposure on an acclimated plant, not a sustained figure. |
| Trouble above | 95 degF | The plant bolts rapidly and leaf quality declines, though basil tolerates heat far better than cold. |
| Target daily light integral | 12 to 20 mol/m2/day | Moles per square meter per day. Below this the crop holds rather than grows. |
| PPFD needed at 16 hours | 208 to 347 umol/m2/s | What a fixture must deliver at the canopy to reach that daily total. |
| Spacing under cover | 8 to 12 in each way | Wider than outdoor spacing, because still air makes disease the limit. |
| Floor area per plant | 0.5 to 1 sq ft | Includes each plant share of shoulder and path space. |
| Sowing depth | Surface to 0.25 in | Basil seed benefits from light and only needs the lightest covering, or pressing into the surface. |
| Soil pH | 6.0 to 7.0 | Outside this band, nutrient availability rather than nutrient supply becomes the limit. |
| Days to first harvest | 25 to 35 days to first pinch, then continuous | From sowing, under cover, at good light. Winter timings run considerably longer. |
| Humidity preference | 0.8 to 1.2 kPa VPD | Expressed as vapor pressure deficit, which accounts for temperature as relative humidity does not. |
How do you start basil under cover?
Sow onto the surface or under the lightest covering, into a medium at 70 to 80 degrees Fahrenheit. Basil germinates poorly below 65 and seed frequently rots in cool wet medium rather than emerging, so this is a crop where bottom heat genuinely earns its place.
A VIVOSUN 10 x 20.75 in Seedling Heat Mat and Digital Thermostat Set set to 75 degrees gives fast even germination in five to ten days. Take the mat off within a day or two of emergence, as with everything else, because holding germination temperature afterwards produces stretched seedlings.
Sow in small batches every four weeks through the growing season. Basil plants decline after several cuts, and a replacement sowing keeps quality up far more effectively than trying to rejuvenate a tired plant.
Transplant at three to four true leaves into their final position, and wait until the greenhouse night minimum is reliably above 60 degrees Fahrenheit before doing so. Basil moved out too early sulks for weeks and frequently never catches up with a later planting.
What temperature does basil need in a greenhouse?
The working range is 70 to 85 degrees Fahrenheit by day and 60 to 70 at night, which is warmer than almost anything else on this site and makes basil a summer crop under cover unless the house is heated.
The cold limit is the crop-defining number. Below about 50 degrees Fahrenheit basil suffers chilling injury: the leaves blacken within a day or two and do not recover. The plant may survive, but the harvest is gone. A single cool night in late spring or early autumn ends a basil crop in a way that would barely register on any other plant.
That makes the overnight minimum the number to watch. A max-min thermometer on the bench answers the question directly, and where the minimum is marginal, thermal mass or a low cover over the plants at night is usually enough to bridge the gap.
At the warm end basil is comfortable well into the nineties, and it tolerates heat far better than any of the leafy crops. It will bolt in sustained heat, but pinching manages that. In practical terms, basil is the crop that most tolerates a hot greenhouse in midsummer.
How much light does basil need?
Basil wants 12 to 20 moles per square meter per day, at the higher end for a herb, which is 208 to 347 micromoles per square meter per second over a 16 hour photoperiod. A greenhouse supplies this comfortably from spring through autumn.
In low light the plant stretches, producing long internodes and thin leaves with noticeably less flavour, since the aromatic oil content correlates with light. Basil grown in a dim corner tastes of very little, which is a real quality difference rather than an aesthetic one.
In winter, basil under cover is a lighting problem rather than a heating one at hobby scale, because the light requirement is high and the temperature requirement is also high. A small bench of basil under supplemental lighting in a heated house works, and a whole crop rarely justifies it. Check the numbers with the DLI calculator.
Spacing, support and training
Space at 8 to 12 inches each way, which is half to a full square foot per plant. Tighter than that and airflow disappears at the base of the canopy, which is the condition basil downy mildew requires.
Airflow is the primary cultural control for the disease that most often ends a basil crop. Basil downy mildew produces yellowing above the leaf and greyish sporulation beneath, and it needs leaf wetness and still air. Wide spacing plus continuous circulation is the whole prevention strategy.
Pinch from the first true leaf pair onward. Cut the growing tip just above a pair of leaves, and the plant produces two shoots where there was one. Repeating that at every harvest turns a single stem into a dense bushy plant, and it delays bolting because the plant never reaches the flowering stage on any given stem.
Remove flower buds the moment they appear. Once basil flowers, leaf production stops and the existing leaves toughen and lose flavour. Pinching out buds weekly extends a plant productive life by weeks.
Watering and feeding
Basil uses about 0.18 gallons per square foot of growing area per day at peak. It is a thirsty herb but an intolerant one: it wilts quickly when short and rots quickly when standing in water.
Water at the root zone and in the morning. Basil downy mildew needs several hours of leaf wetness, and overhead watering on a basil bench is the most reliable way to introduce it. Drip or a wand aimed at the medium, never a sprinkler.
Free drainage matters more for basil than for most crops. A medium that stays saturated produces root rot and stem collapse quickly, particularly in cooler conditions. An open potting mix with added perlite suits it better than a dense one.
Feed lightly with a balanced feed during active growth. Basil is not a heavy feeder, and excess nitrogen produces large soft leaves with reduced oil content, which is the opposite of what the crop is grown for.
What goes wrong with greenhouse basil?
| What you see | Usual cause | What to do |
|---|---|---|
| Leaves blacken and go limp after a cool night | Chilling injury below about 50 degF, which is permanent rather than temporary | Nothing recovers the damaged leaves. Watch the overnight minimum and cover or heat before it drops. |
| Yellowing between the veins with greyish fuzz on the leaf underside | Basil downy mildew, which needs leaf wetness and still air and spread globally in recent decades | Resistant varieties, wide spacing, continuous airflow, and never watering over the foliage. |
| Plant sends up flower spikes and leaves toughen | Bolting, which is normal at maturity and accelerated by heat and by not pinching | Pinch out flower buds weekly and harvest by cutting above a leaf pair rather than picking single leaves. |
| Stem collapses at the medium surface | Damping off in seedlings, or stem rot in older plants from a saturated medium | Use a free-draining mix, water in the morning, and improve airflow at the base of the plant. |
| Tall thin plants with long gaps between leaf pairs | Insufficient light, or germination temperature held after emergence | Take the heat mat off at emergence and increase light. Pinch hard to force branching. |
| Small holes through the leaves | Slugs at night, or occasionally caterpillars | Keep pots off a damp floor, collect after dark, and check leaf undersides for caterpillars. |
| Clusters of small insects on the growing tips | Aphids, attracted to the soft growth that heavy feeding produces | Reduce nitrogen, squash small colonies, and use insecticidal soap following the label on an edible crop. |
| Leaves lose flavour despite looking healthy | Low light, excess nitrogen, or the plant having flowered | Aromatic oil content correlates with light. Move to a brighter position, feed less, and pinch buds out. |
Which types work best under cover?
| Type | Suits | Note |
|---|---|---|
| Sweet or Genovese basil | General use | The standard culinary type, and historically the most susceptible to downy mildew. |
| Downy mildew resistant varieties | Any greenhouse in a humid climate | Bred specifically in response to the global spread of the disease, and the single most useful variety choice available. |
| Greek or bush basil | Small spaces and containers | Tiny leaves on a compact dense plant, and generally more tolerant of imperfect conditions. |
| Thai basil | Heat and humidity | Noticeably more heat tolerant and slower to bolt than sweet basil, with a distinct anise flavour. |
| Purple basil | Colour and containers | Slower growing and less vigorous, but the colour holds well and the flavour is a little stronger. |
| Lemon and lime basil | Variety of flavour | Different species in some cases, with similar cultural requirements and often better mildew tolerance. |
If you grow basil in a humid greenhouse, a downy mildew resistant variety is the most valuable single decision on this page. The disease has spread worldwide over the past two decades and it moves through a susceptible planting quickly once established. Resistant varieties are widely available and clearly labelled.
Thai basil deserves consideration on its own merits for greenhouse growing, because it is genuinely more heat tolerant and slower to bolt than sweet basil, which extends the usable season at the warm end where sweet basil starts running to flower.
Harvesting and what comes next
Harvest by cutting whole stems above a leaf pair rather than picking individual leaves. Cutting above a node causes the plant to branch there, so every harvest makes the plant bushier, while picking single leaves leaves a bare stem that produces nothing.
Take no more than about a third of the plant at once, and always leave at least two sets of leaves below the cut so the plant has the leaf area to regrow.
Harvest in the morning after the dew has gone but before the heat of the day. Aromatic oil content is highest then, and leaves cut in the afternoon heat wilt immediately.
Do not refrigerate basil. It suffers the same chilling injury off the plant as on it, and leaves stored below about 50 degrees Fahrenheit blacken within a day or two. Stand cut stems in water at room temperature instead.
Clear plants once they have flowered and been cut back several times. Quality declines noticeably in an old plant, and a fresh sowing four weeks behind will already be producing better leaf.
Equipment for growing basil under cover
Basil is straightforward in warmth and unforgiving of cold. Start it on a heat mat, plant it out only when nights are reliably above 60, pinch it from the first true leaves, keep the water off the foliage, and sow a replacement every four weeks. Done that way it produces from late spring until the first cool night ends it.