When to Start Seeds in a Greenhouse

In an unheated greenhouse, start seeds about two to four weeks earlier than you would outdoors -- not months earlier. The structure traps solar gain during the day and loses almost all of it overnight, so by dawn an unheated greenhouse is usually within a couple of degrees of the outside air. Your sowing date is set by the night temperature your seedlings can survive, not by how warm the greenhouse gets at two in the afternoon. With a heat mat under the trays, or a thermostatically controlled heater holding a frost-free minimum, you can go six to eight weeks ahead of outdoor sowing.

A greenhouse in March can hit 80 degrees on a sunny afternoon and 33 degrees before sunrise the same day. That daytime figure is what persuades people to sow in February; the night figure is what kills the result. Polycarbonate and glass are poor insulators and there is no thermal mass in an empty hobby greenhouse to carry heat through the night, so the inside tracks the outside within a degree or two by the small hours.

This is why the first thing to buy for a greenhouse is not a heater but a min/max thermometer. One week of recorded overnight lows tells you more about what you can actually sow and when than any general advice, including this page. Until you have that number you are guessing.

Find your own average last frost date first -- it is the only input that makes these numbers mean anything, and it varies by weeks across short distances. Then work backward. The full indoor version of this timetable, with the germination temperatures crop by crop, is in when to start seeds indoors; the greenhouse numbers above are that table shifted for a structure that is cold at night.

Seeds germinate on soil temperature, not air temperature, and the spread between crops is wide. Lettuce and brassicas will germinate in soil at around 50 degrees. Tomatoes want 70 or more to come up promptly. Peppers and eggplant want 75 to 85 and will simply sit, sulk and sometimes rot in a cold tray -- which is why they are the one group we would start indoors even if you own a greenhouse.

A heat mat solves this for a few dollars of electricity because it heats the one cubic foot that matters instead of the whole building. That is the single highest-value purchase for greenhouse seed starting, and it is covered properly in seedling heat mats. Take the trays off the mat once germination is through, though -- continued bottom heat on emerged seedlings produces stretched, weak growth.

Quick picks

#	Product	Best for	Score	Price
1	Top pickMin/Max Greenhouse ThermometerRecords the high and low since last reset	Knowing what actually happened overnight	8.6★★★★★	$13.95 - View on Amazon
2	VIVOSUN Seedling Heat MatWaterproof mat sized for 1020 trays	Peppers, tomatoes and slow germinators	8.8★★★★★	$30.99 - View on Amazon
3	Burpee Self-Watering Seed Starting KitCell tray, dome and watering mat in one box	First-time seed starting	8.4★★★★★	Check price on Amazon
4	Agribon AG-19 Floating Row Cover0.55 oz/sq yd fabric, about 85% light transmission	A few degrees for almost no money	8.4★★★★★	Check price on Amazon

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Scores are our own editorial judgment, formed from published specifications and independent sources. They are not aggregated customer ratings, and they are not the result of hands-on testing, which we do not currently do. See how we choose.

This is the genuine advantage over a windowsill. A greenhouse gives seedlings light from every direction at close to full outdoor intensity, which is the one thing an indoor setup cannot match without a grow light. Seedlings raised in a greenhouse are typically stockier and better colored than windowsill seedlings of the same age, and they need less hardening off because they have already had real light and some temperature variation.

In midwinter, though, day length is still the limit no matter how clear the glazing is. Seeds sown in late December and January in an unheated greenhouse germinate slowly and then grow very little until the light returns in February, which is why sowing earlier often produces no earlier harvest. If you want to start genuinely early, you need light as well as warmth -- see grow lights vs windowsill for what a light actually buys.

1. Group the trays together on a bench rather than spreading them along the greenhouse. A block of trays holds heat better than scattered ones.
2. Away from the glazing. The air at the panel edge is several degrees colder on a cold night than the middle of the house.
3. Throw row cover fabric over them. A layer of lightweight floating row cover draped over trays overnight is worth a few degrees for almost nothing, and it is removed in seconds in the morning.
4. Add thermal mass. Dark water containers under the bench absorb daytime heat and release it overnight. Cheap, and it measurably flattens the swing.
5. Use the heat mat at night, not the day. If you only have one source of heat, nights are when it earns its keep.

The opposite failure is just as common. A closed greenhouse on a bright March day cooks seedlings, and the trays dry out between morning and evening. Vent whenever the day is mild enough -- an automatic vent opener that works off temperature is the one accessory that makes a greenhouse safe to leave for a working day. Greenhouse gardening tips covers the ventilation, watering and hygiene routine in full.

Late winter: onions, leeks and the first salad sowings, on a heat mat if you have one. Early spring: brassicas, then tomatoes. Mid spring: squash, cucumbers and beans, which germinate fast in warming conditions and resent being started too early. Early summer: direct sowings for the autumn and winter crops, which is the half of the year most greenhouse owners never use -- fall seed starting covers that timetable.

And if the overnight lows you record turn out to be the thing standing between you and a six-week head start, that is the honest signal to look at heating rather than at sowing dates. Do I need a greenhouse heater? answers it with the running costs attached.