Quick Answer: Most greenhouse flowers want 65–75°F days, 55–60°F nights, and 40–60% relative humidity — lower humidity than a vegetable house needs, because flowers are more prone to fungal bloom damage in overly wet air. High-value cut flowers like anemones, ranunculus, Dutch iris, and freesias tolerate an unheated or minimally heated structure well, while tender annuals like zinnias generally need more warmth and struggle in cooler unheated houses. Beginners usually start with zinnias, cosmos, or snapdragons — all reliable, prolific “cut and come again” bloomers — and add support netting once stems get tall enough to flop.
Every other guide in this series covers a food crop — tomatoes, peppers, herbs — where the goal is yield and flavor. Flowers change the calculus: humidity that’s perfectly fine for a tomato plant can invite gray mold on a rose or dahlia, and the crops most worth protecting in a greenhouse (bulb-grown cut flowers) aren’t the ones beginners usually think of first. Roses in particular have their own temperature, humidity, and disease-prevention needs worth a closer look — see our dedicated greenhouse roses guide for the full breakdown.
Greenhouse flowers by the numbers:
- 65–75°F days / 55–60°F nights — the general target range for most flowering plants, according to greenhouse growing guidance; roses want it a touch warmer (60–75°F), while cool-season cut flowers like lupine and stock tolerate night temperatures down to 35°F in an unheated house.
- 40–60% relative humidity — noticeably drier than the 60–90% range our tomato, pepper, and orchid guides call for, because flowers are more susceptible to botrytis and other fungal bloom damage in wet air.
- 4 bulb/corm crops commonly grown in unheated greenhouses for cut-flower quality: anemones, ranunculus, Dutch iris, and freesias — all rewarded with longer stems and a longer harvest window than an open-field planting.
- 3 easiest beginner flowers: zinnias, cosmos, and sunflowers — all fast from seed, prolific, and “cut and come again” bloomers that produce more the more you harvest.
Flowers vs. the rest of this series
| Factor | Fruiting veg (e.g. tomatoes) | Flowers / cut flowers | Orchids |
|---|---|---|---|
| Daytime temperature | 70–80°F | 65–75°F | 60–90°F (by group) |
| Humidity target | 65–90% | 40–60% | 60–80% |
| Biggest failure mode | Blossom drop from heat/humidity swings | Botrytis (gray mold) from excess humidity | No blooms from a missing night-temperature drop |
| Unheated-house friendly? | Rarely, outside mild climates | Yes, for bulb crops (anemones, ranunculus) | No — most orchids need consistent warmth |
That humidity gap is the one number worth internalizing: a greenhouse tuned for tomatoes or peppers is often too damp for flowers, and botrytis on a rose or dahlia bud can wreck a bloom overnight in a way that doesn’t have an equivalent in a vegetable house.
Cut-Flower Support Netting
- Suspended flat over the bed, plants grow up through the mesh squares and stay supported on all sides — the standard fix for zinnias, snapdragons, and dahlias that flop once stems get tall.
- Keeps flower heads off the soil surface, reducing contact with the soil-borne fungal spores that cause botrytis and other bloom rot in a humid structure.
- UV-resistant polypropylene mesh is reusable for multiple seasons — roll it up between plantings instead of replacing it.
Between support netting, a hygrometer to keep humidity out of botrytis range, and a bulb order every fall, a cut-flower setup pulls together a lot of small purchases — Kindle Unlimited is worth a free trial if you want a cut-flower-farming or floral-design reference to read while gear ships in. Sponsored link — we may earn a fee if you sign up. No extra cost to you.
Which flowers tolerate an unheated greenhouse
Bulb and corm crops are the standout performers in an unheated or minimally heated greenhouse: anemones, ranunculus, Dutch iris, and freesias are routinely grown this way, and the protection pays off with longer stems, fewer pests, and a longer harvest window than the same crop planted directly in open ground. Cool-tolerant annuals like lupine and stock can handle night temperatures as low as 35°F. Tender annuals are the opposite case — zinnias in particular are easy to grow once conditions are right, but they and most celosia can perform poorly in an unheated greenhouse in cooler climates, since neither the soil nor the air warms up fast enough for them without at least some supplemental heat. If your structure isn’t heated, lean toward the bulb crops and cool-tolerant annuals first.
Managing humidity to avoid botrytis
The 40–60% humidity target for flowers is meaningfully drier than what our vegetable and orchid guides recommend, and the reason is botrytis (gray mold) — a fungal disease that thrives in stagnant, humid air and can turn a healthy bud into a rotted one within days. A greenhouse fan running continuously to keep air moving, combined with vents sized using our greenhouse ventilation guide, does most of the prevention work. If you’re sharing a structure with more humidity-hungry crops — say, a greenhouse humidifier running for orchids or seedlings — zone the flower bed away from that humidity source rather than running the whole house at one setting.
Light and shade for flowers
Daylength (photoperiod), not just light intensity, matters more for flowers than it does for most food crops — some varieties won’t initiate blooms under the short days of winter even with otherwise ideal growing conditions, though they’ll still put on useful vegetative growth in the meantime. Full sun works for most cut-flower crops, but delicate blooms and petals can scorch or fade faster under intense unshaded summer sun, where a lighter shade cloth than the 60–70% orchid guideline — closer to the 30–50% range used for vegetables — helps preserve bloom color and vase life.
Getting started: the easiest picks
- Zinnias, cosmos, and sunflowers — the standard beginner trio. Fast from seed, prolific, and “cut and come again,” meaning the more you harvest, the more they produce.
- Snapdragons and ranunculus — close behind zinnias for ease, and often the better choice in a cooler greenhouse climate where zinnias and celosia can underperform.
- Anemones, Dutch iris, and freesias — bulb/corm crops best suited to an unheated or minimally heated house, rewarding the protection with longer stems and a longer season than open-field growing.
- Add support netting once stems get tall. Zinnias, snapdragons, and dahlias are the most common candidates for a horizontal grow-through mesh to prevent flopping and stem breakage.
- Watch humidity before you watch anything else. The 40–60% target is drier than most of the rest of this series — a hygrometer catches drift before botrytis sets in.
The bottom line
Flowers ask for a drier greenhouse than most food crops in this series — 40–60% humidity against botrytis, versus the 60–90% our tomato and orchid guides recommend — paired with a moderate 65–75°F/55–60°F day/night range. Beginners get the fastest wins from zinnias, cosmos, and snapdragons, while an unheated structure is still worth planting with cold-tolerant bulb crops like ranunculus and anemones rather than left empty over winter. Any of our greenhouse kit or small greenhouse picks handles a flower bed well once humidity and airflow are dialed in.
Growing food crops in the same structure? Our greenhouse for tomatoes and greenhouse for herbs guides cover the higher-humidity settings those crops want, and our greenhouse for orchids guide breaks down the one other crop family in this series that also needs its own humidity zone.