unvented greenhouse -- any way to make it work for seed starting?

[habitat_gardener]

The good news is that I have access to a greenhouse this year.

The bad news is that the greenhouse is unvented. It's at a community garden and is used basically as a locked shed for tool storage, but I can start seeds there if I can find a way to make it work.

The question: is it worth trying?

It's convenient enough that I could possibly stop by twice a day (to take them outside and put them back inside), but I know from experience that it doesn't take much time for seedlings to fry, if I got there too late one time or skipped a day.

Shading the flat won't help, because (1) they need light and (2) a hot box is still hot if you add a sunshade.

If I place my flats at floor level (=the coolest spot, next to the glass), do they have a chance? A fan might help, but I don't know if there's electricity and I wouldn't want to leave a fan running unattended. But maybe there's a way to get some passive airflow?

I could buffer temperature swings by surrounding the flat with gallons of water, or placing a wall-o-water around a few containers, but will that protect them enough?

The alternative is to do what I've done before: start them outdoors in cold frames. If I start early, growth is glacially slow because nights are still in the 40s until May, and if rainfall is anywhere near normal, they also need to be protected from rainstorms. If I start late, then I end up getting plants for my main crop and growing only earlies and cherries from seed (and have only a short time to harvest them).

I'll probably end up trying a batch of seeds in any case, but it'd help to get some more ideas on how to make this work.

[Redbaron]

You can do it, as long as you have a spot you can leave them outside on warm days, and you watch the weather reports religiously. Then when the days start being more warm than not, switch to the vented cold frames you used in the past.

Basically when the nights are still in the 40's or occasional frosts, and cold frames are glacially slow, use the greenhouse. Then when it warms up a bit use the cold frames. It is a lot of moving of seedlings though. Do you have many usually?

[Doug9345]

If I'm depending on manual methods for temperature control in a greenhouse I want it to be right next to me and not down the road. As you said get there an hour late and you seedlings are toast. I think the effort would be better put into increasing the temperature in the cold frames than chancing a greenhouse or should I say transparent tool shed down the road.

[Cole_Robbie]

Plants die from heat when they run out of water to transpire. I would want my flats to sit in a larger tray of water that they could wick up as needed. That's the only way I can keep greenhouse plants alive towards the very end of my spring season when it starts to get hot. You can experiment with water depth, cell tray size, and media to see what works best. People will say that roots rot if they're wet all the time, but that's not exactly true; hydroponics roots thrive that way. I don't think you would want any compost in media that stayed wet all the time, though. It would need to be a pro-mix of peat,perlite, vermiculite, pine bark fines, and other inert media. You can lower the moisture content of the media by making it wick the water farther.

Without air movement, you will get leggy plants. A tiny solar fan is better than nothing. Sometimes legginess is ok if you just plant the seedling deeper, or lay part if the stem in a trench.

[habitat_gardener]

Those are great ideas -- a tray with water and possibly a solar fan.

I looked more closely at the greenhouse and it does actually have a roof vent. But I also talked with someone who's been at this garden for a few years, and she said the greenhouse is useless for seed starting because it gets too cold at night and too hot during the day. It looks like it's made from plywood! The tall south side is all glass, as are a third of the east and west sides, all vertical, and the shorter 4th side is wood. The sloped roof is wavy fiberglas (like a sine curve) and has a manual vent in the middle. Near the door, at floor level, is a small "window" cut into the plywood covered with hardware cloth.

The roof is covered with debris, so if you're inside and you look up, you see brown. Some shrubs are also growing over it from the north (no glass) side. The gardener I talked to said it's probably good that the roof is covered with debris -- keeps the temperature down!! (But the roof slope faces north, so I don't think a clean roof would add much heat. If I have time I'll wash it down the next time I'm there and cut back some of the weedy shrubs.)

I'll have to do some research on wicking media.

This might work as a nighttime refuge for one small cold frame, assuming I can get there twice a day every day for a couple months.

[Doug9345]

Can you build a cold frame by your house and put a bulb in it for some heat. If you are only trying to raise 40 degrees to 6 it's not going to take too much. You can through a piece of insulation on it at night and the it becomes much easier to keep warm all night and the cold frame is starting from a warm level as soon as the sun comes out.

[LoreD]

If I needed to take seedlings in and out, I would prefer to be close to the coldframe or greenhouse. If you have a cold frame, you could put some bottles of hot water in there to keep them warm at night. Also, I have found that putting a fairly thick layer of straw on the bottom would help insulate them from the cold ground.