Carrots are an unusual greenhouse crop because the greenhouse contributes very little to them directly. They are hardy, they do not need warmth, and they take a long time to occupy valuable covered space. What a greenhouse genuinely offers is a controlled growing medium, protection from carrot fly, and the ability to harvest through winter from unfrozen ground.
The last two are the real arguments. Carrot fly is the defining outdoor carrot problem and an insect-screened structure excludes it entirely. And a bed of mature carrots left in unfrozen medium under cover is a storage system that produces better roots than any refrigerator, right through the cold months.
| Measure | Figure | Why it matters |
|---|---|---|
| Germination soil temperature | 50 to 75 degF | Carrot germinates across a wide band but very slowly, and the medium surface must stay continuously moist for the whole period. |
| Days to emergence | 14 to 21 days | At the optimum soil temperature, in a moist starting medium. |
| Optimum day temperature | 60 to 70 degF | Where growth is fastest. Above the range, specific processes start failing. |
| Optimum night temperature | 45 to 55 degF | The day to night difference controls stem elongation and plant habit. |
| Survives down to | 20 degF | Brief exposure on an acclimated plant, not a sustained figure. |
| Trouble above | 80 degF | Root quality suffers, colour development is poorer and the tops grow at the expense of the root. |
| Target daily light integral | 12 to 17 mol/m2/day | Moles per square meter per day. Below this the crop holds rather than grows. |
| PPFD needed at 16 hours | 208 to 295 umol/m2/s | What a fixture must deliver at the canopy to reach that daily total. |
| Spacing under cover | 2 to 3 in in rows 6 to 8 in apart | Wider than outdoor spacing, because still air makes disease the limit. |
| Floor area per plant | 0.04 to 0.06 sq ft | Includes each plant share of shoulder and path space. |
| Sowing depth | 0.25 in | Carrot seed is tiny and must not be buried deeply, but it must not dry out either, which is the tension that makes germination difficult. |
| Soil pH | 6.0 to 6.8 | Outside this band, nutrient availability rather than nutrient supply becomes the limit. |
| Days to first harvest | 65 to 80 days for maincrop, 50 to 60 for early types | 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 carrots under cover?
Sow a quarter of an inch deep in rows 6 to 8 inches apart, thinly, into a fine, level, stone-free surface. Carrot seed is tiny and slow, taking two to three weeks to emerge, and the entire difficulty of the crop is concentrated in that window.
The medium surface must stay continuously moist for the whole germination period. A crust forming over the seed, or a single day of drying, kills a substantial fraction of a sowing and produces the patchy stands that carrots are notorious for. Cover the row with fleece or a board until the first seedlings appear, and mist the surface rather than watering heavily.
Do not transplant. Carrots make a taproot from the moment they germinate, and any disturbance to it produces a forked or stunted root. This is one of the few crops where direct sowing is not merely preferable but essential.
Sow thinly enough that thinning is minimal. Thinning carrots disturbs neighbouring roots and, more importantly, releases the scent that attracts carrot fly, which is the one thing a screened greenhouse is protecting against.
What temperature does carrots need in a greenhouse?
Carrots want 60 to 70 degrees Fahrenheit by day and 45 to 55 at night, and they are genuinely hardy, surviving well below freezing in the ground. Cold improves them, because the root converts starch to sugar as an antifreeze response.
The greenhouse contribution here is modest. Carrots do not need heat and they are not made better by it. Above about 80 degrees the tops grow at the expense of the root and colour development suffers, so summer greenhouse carrots are usually worse than outdoor ones.
Where the structure earns its place is at the end of the season. A bed of mature carrots under cover stays in unfrozen medium and can be lifted as needed through winter, which is both better storage than a refrigerator and less work than lifting and packing a whole crop.
In a hot climate, shading a carrot bed through summer is worthwhile. Fifty percent cloth costs the crop nothing, since carrots want only 12 to 17 mol/m2/day, and it keeps the medium cooler, which matters for root quality. Set it with the shade cloth calculator.
How much light does carrots need?
Carrots want 12 to 17 moles per square meter per day, the same as leafy crops, which a greenhouse supplies easily through the growing season.
Light is rarely the limiting factor. What limits a greenhouse carrot crop is depth, texture and moisture consistency, in that order, and no amount of light compensates for a shallow or stony bed.
In winter, carrots already in the ground do not need light at all in any meaningful sense. They are being stored rather than grown, and the greenhouse is functioning as an unfrozen root cellar. Any tops that regrow in late winter signal that the root is about to bolt and should be lifted.
Spacing, support and training
Space at 2 to 3 inches within rows and 6 to 8 inches between rows. That is a very small footprint per plant, around a twentieth of a square foot, which makes carrots efficient per square foot despite their long occupancy.
The critical dimension is not spacing but depth. A bed needs 14 to 18 inches of loose, stone-free medium, considerably more than the length of the root, because a taproot meeting resistance at its tip forks or stunts. Work out the fill volume with the bed and soil volume calculator, and expect the quantity to be substantial.
Texture matters as much as depth. Carrots want a fine, open, sandy medium with no stones and no lumps. A heavy or compacted bed produces short, forked, misshapen roots regardless of how deep it is.
Thin to final spacing as early as possible and do it in the evening, watering afterwards to settle disturbed soil and damp down the scent. Late thinning damages neighbouring roots and gains little.
Watering and feeding
Consistency is the entire watering story for carrots. Split roots are caused by a dry period ended by a heavy watering: the core takes up water faster than the outer tissue can stretch, and the root cracks. Nothing repairs a split carrot.
A soaker hose along the row on a timer is the correct answer, because it delivers the same volume every day without anybody being present. Carrots are a long crop, and a single missed week in the middle of it shows in the harvest.
During germination the requirement is different: frequent light misting to keep the surface from crusting, rather than deep watering. Switch to deep infrequent watering once the seedlings are established, which encourages the root downward.
Do not feed with nitrogen. Carrots in a well-prepared bed need very little, and nitrogen produces large tops and small forked roots. If the bed needs anything, potassium supports root development without pushing top growth.
What goes wrong with greenhouse carrots?
| What you see | Usual cause | What to do |
|---|---|---|
| Roots fork into two or more branches | Stones, compaction, fresh manure, or transplanting a taproot crop | Deep, fine, stone-free medium with no fresh organic matter. Never transplant carrots. |
| Roots split lengthwise as they mature | A dry period followed by heavy watering, so the core expands faster than the skin | Put the bed on a timer and a soaker line. Consistency matters more than total volume. |
| Fine white or rust-coloured tunnels through the root | Carrot fly larvae, which is the defining outdoor carrot problem | Fine insect mesh over vents and openings excludes the adult entirely, which is a greenhouse advantage. |
| Patchy or very poor germination | Surface dried out or crusted during the 14 to 21 day germination window | Cover the row with fleece or a board, mist rather than water heavily, and keep the surface continuously moist. |
| Short stumpy roots that stop abruptly | Insufficient depth of loose medium, or a compacted layer beneath | Fill to 14 to 18 in of loose medium and break up anything hard beneath the bed. |
| Hairy roots covered in fine rootlets | Excess nitrogen or fresh organic matter in the bed | Do not feed nitrogen and do not manure a carrot bed. Prepare it the season before. |
| Green shoulders on the exposed top of the root | Light reaching the crown as the root pushes up | Draw a little medium over the shoulders as the roots swell. Cosmetic rather than serious. |
| Tops regrow strongly in late winter | The root is about to bolt as day length increases | Lift the remaining crop. A bolting carrot becomes woody and inedible within a couple of weeks. |
Which types work best under cover?
| Type | Suits | Note |
|---|---|---|
| Nantes types | Greenhouse beds and containers | Cylindrical, blunt ended and relatively short, which suits a bed that cannot be made 18 inches deep. |
| Chantenay types | Heavier or shallower medium | Short and broad-shouldered, and the most tolerant of a bed that is less than ideal. |
| Imperator types | Deep, sandy, stone-free beds only | Long and tapered, and the most likely to fork in anything short of perfect medium. |
| Amsterdam and early types | Quick crops and succession sowing | Fast at 50 to 60 days and small, which suits growing between other crops. |
| Round or Paris market types | Shallow beds and containers | Almost spherical, so they need only a few inches of depth. The answer where a deep bed is impossible. |
| Coloured varieties | Interest rather than yield | Purple, yellow and white types grow identically to orange ones, and some hold their colour better cooked than others. |
The variety choice should follow the bed rather than the preference. If the bed is 18 inches of fine sand-based medium, anything grows. If it is 10 inches of ordinary bed fill, Nantes or Chantenay types will produce good roots where an Imperator type produces short forked ones.
Round varieties are the genuinely useful option nobody considers. In a container or a shallow bench bed with six inches of medium, a round carrot produces a perfect root where every long type fails.
Harvesting and what comes next
Lift when the shoulders reach the diameter the variety describes, usually visible at the medium surface. Carrots do not have a narrow harvest window and will hold in the ground for weeks, which is the main advantage of growing them under cover.
Water the bed before lifting if the medium is dry. A carrot pulled from dry firm medium frequently snaps, leaving the tip behind, and the tip then rots in the bed.
Twist off the tops immediately after lifting rather than leaving them attached. The foliage continues to draw moisture from the root and softens it within a day.
For winter storage, leave the crop in the bed and lift as needed. Unfrozen medium under cover is better storage than any container, the roots continue to sweeten in cold, and lifting on demand is far less work than lifting and packing an entire crop at once.
Clear the bed before the roots bolt in late winter. Strong new top growth is the signal, and a bolted carrot becomes woody and inedible within a fortnight.
Equipment for growing carrots under cover
Carrots earn a greenhouse bed for two specific reasons: carrot fly exclusion and winter storage in unfrozen medium. Give them depth, keep the surface moist through the slow germination, never manure the bed, and put the watering on a timer. Everything else about the crop follows from those four.