Quick Answer: Microgreens want cooler and drier conditions than almost every other crop in this series — 60-75°F and 50-70% relative humidity, versus the 70-85°F and 60-90% range that tomatoes, peppers, and tropical crops elsewhere in this series need. The bigger difference is speed: most varieties finish in 8-14 days (fast brassicas like radish and broccoli in as little as 6-10, long-cycle crops like sunflower and pea in 10-14), a turnover rate 5-10x faster than anything else you’d grow in a greenhouse. That combination of cooler air and constant tray turnover means microgreens do best on their own dedicated shelf rather than sharing a zone tuned for slower, warmth-loving crops.
If you’ve read our guides on greenhouse tomatoes, herbs, or seedlings, microgreens will feel familiar and completely different at the same time — familiar because you’re still managing temperature, humidity, and light, but different because the entire crop cycle is measured in days, not weeks or months.
Greenhouse microgreens by the numbers:
- 60-75°F — the target temperature range for most microgreens, per Virginia Tech Extension’s guide to microgreen production; run it warmer and stems stretch thin instead of staying short and dense.
- 50-70% — the target relative humidity, drier than the 60-90% range most fruiting vegetables in this series want, because dense seeding leaves little airflow to dry out excess moisture.
- 8-14 days — the typical seed-to-harvest window for most varieties, per Penn State Extension, out of a full range of 7-21 days depending on the crop.
- 200-400 µmol/m²/s, 12-16 hours a day — the light target for supplemental lighting, positioned 6-12 inches above the tray, per Virginia Tech Extension.
Why microgreens don’t fit your existing greenhouse zone
Every other crop in this series shares a common shape: plant it, tend it for weeks or months, harvest over an extended window. Microgreens break that pattern entirely. A tray of radish microgreens goes from seed to scissors in about a week — faster than most seedlings even germinate. That speed is the appeal, but it’s also why running microgreens on the same bench as your tomatoes or peppers usually backfires: the warm, humid zone that ripens a tomato is exactly the combination that pushes microgreens into leggy, weak-stemmed growth and invites the mold that ruins a tray days before you’d otherwise cut it.
| Factor | Microgreens | Full-size crops (tomatoes, peppers, herbs) |
|---|---|---|
| Temperature | 60-75°F | 70-85°F daytime |
| Humidity | 50-70% | 60-90% |
| Seed-to-harvest | 7-21 days (most 8-14) | 60-90+ days |
| Airflow need | High — dense seeding traps moisture | Moderate |
| Best bench placement | Coolest, driest corner | Warm, humid zone near irrigation |
Bootstrap Farmer 1020 Mesh Deep Extra Strength Tray
- Same tray our seed starting tray guide recommends for soil blocks — open mesh base solves the standing-water problem that causes most microgreen mold and rot.
- Sits inside a solid 1020 flat for bottom-watering, then lifts out to drain freely — no soggy roots sitting in a puddle overnight.
- Extra Strength #5 polypropylene, reusable across many cycles instead of a one-season disposable tray.
Running a microgreen habit means restocking trays, seed, and growing medium every week or two — the kind of small, recurring order Amazon Prime’s free shipping quietly pays for itself on. If you’d rather read your way into it first, try Kindle Unlimited free for the growing guides and variety references that make seed-density guesswork a lot less common. Sponsored link — we may earn a fee if you sign up. No extra cost to you.
Seed density: the mistake that ruins more trays than temperature does
Microgreens are grown far denser than any other crop in this series — seed practically shoulder-to-shoulder across the whole tray, not spaced individual plants. That density is what makes the harvest work, but it also means airflow and drainage matter more here than almost anywhere else in a greenhouse. Too little seed wastes tray space and your growing medium; too much seed creates the exact overcrowded, low-airflow conditions that cause damping-off, according to Penn State Extension’s guide to microgreen production.
| Variety | Days to harvest | Typical seeding rate (per 10"x20" tray) |
|---|---|---|
| Radish | 6-8 days | ~0.8 oz (22.5g) |
| Broccoli | 8-10 days | ~0.5-0.8 oz |
| Mustard | 8-10 days | ~0.8-1.2 oz |
| Sunflower | 10-14 days | ~1.5 oz (40-50g) |
| Pea shoots | 10-14 days | ~4-6 oz (soak first) |
The fast brassicas — radish, broccoli, mustard — are the easiest entry point precisely because they’re the most forgiving of a slightly-off seed density or a humidity swing over a shorter growing window. Sunflower and pea take longer and germinate more slowly at the start, which gives temperature and humidity mistakes more time to compound before you’re at the cutting board.
Light: less intense than you’d guess, but on a stricter schedule
Microgreens don’t need the highest light levels in this series — a greenhouse in full sun growing tomatoes runs closer to 400-800+ µmol/m²/s at midday, well above the 200-400 µmol/m²/s microgreens target. What microgreens are stricter about is duration and distance: 12-16 hours a day, and the light source held close, 6-12 inches above the tray. Too far away and the seedlings stretch upward chasing light instead of staying short and dense — the same leggy-growth failure mode that running the temperature too warm causes.
VIVOSUN T5 Grow Light Fixture (2ft)
- T5 fixtures run cooler than most LED grow panels — useful on a microgreen shelf where you want light without adding to the humidity-trapping heat load.
- Sized for a 2ft tray footprint, close enough to hang 6-12 inches above the crop without scorching seedlings.
- Closes the gap on an overcast week or a winter cycle when a shaded or north-facing structure can't hit 200-400 µmol/m²/s through the glazing alone.
A greenhouse humidifier or misting system tuned for the rest of the structure will push humidity well past what a microgreen tray wants — another reason a dedicated shelf, away from the main misting zone, works better than sharing the bench.
A dome for germination, then off
The two-stage humidity trick most microgreen growers use: a clear dome or cover traps moisture during the first 2-4 days while seeds germinate, then comes off once the first shoots appear. Leave the dome on past germination and you’ve recreated the same standing-moisture, low-airflow conditions that a dense seed bed already risks — a compact tray-and-dome kit works well for the germination stage specifically, not as a permanent cover.
How to choose
- Give microgreens their own shelf, not a shared bench. The 60-75°F/50-70% target is drier and cooler than what tomatoes, peppers, and most herbs in this series want.
- Match your seed density to the variety, not a one-size-fits-all guess — too dense is the single most common cause of moldy, rotted trays.
- Pull the humidity dome once shoots emerge. It’s a germination tool, not a growing-cycle cover.
- Start with fast brassicas (radish, broccoli, mustard) if you’re new — shorter cycles mean fewer days for a temperature or humidity mistake to compound.
- Add supplemental light in winter or shade rather than assuming ambient greenhouse light covers the 200-400 µmol/m²/s, 12-16 hour target year-round.
The bottom line
Microgreens are the fastest-turning crop in this series by a wide margin — most varieties go from seed to harvest in under two weeks, running cooler and drier than the tomatoes, peppers, and herbs that dominate the rest of a typical greenhouse. That mismatch is the whole story: give microgreens their own cool, dry, well-ventilated shelf with the right seed density and a dome that comes off on schedule, and a tray turns over every week or two instead of rotting halfway through.
Still setting up the growing space itself? Our seed starting trays guide covers the same mesh-tray, standing-water logic in more depth, our grow lights roundup covers fixtures sized for a dedicated microgreen shelf, and our greenhouse shelving guide covers staging that keeps airflow moving underneath every tray. Growing herbs or seedlings on the same bench? Our greenhouse for herbs and greenhouse for seedlings guides cover the warmer, more humid conditions those crops want instead.