Should I keep plants in unheated greenhouse with huge temperature swings?

Question

I'm in zone 7B and the greenhouse is shaded until around 9, then still partially shaded until 10 or 11 and then it gets the full brunt of the afternoon sun. It's just starting to warm up here, with nights around 40 F and midday temps around 50 or 60 now. The greenhouse goes to the 40s or high 30's at night like outside, but is always at least a couple of degrees warmer than outside even on cloudy days, around 55 F. Sunny days, it's at least 10 degrees warmer than outside and if its sunny, and the outside temp is mid 60's , it can reach 87F inside, and remain at 80 F even with all doors and vents open. That's almost a 50F temperature gradient!

I'm on spring break, so I could check on them them on weekdays, but I only come back on weekends from college, so I may not be able to ventilate if it gets sunny. I have 2 potted blueberries, a couple of salad greens in a raised bed, some swisschard just seeded in pots, several pots with sown seeds, and two potted grapes in there right now. Should I move them out or is it okay to leave them inside? Will the high heat and cold nights damage plant growth? Blueberries seem to put on more leafs than blueberries outside and grapes inside the greenhouse have already had budbreak while grapes outside haven't even had bud swelling..

machotti1 · Accepted Answer

It may worth it to purchase a thermostat for the greenhouse to turn fans on automatically when u r away. Temps 75-80 will be to hot for most plants u listed & will be exponentially higher inside the greenhouse, up to 100'. A mist system for hot temps will protect plants.
Temps with the variables mentioned are of concern. A greenhouse cover outside over the top & sides to shade the plants can help. The temps should be within a stable range.To leave plants in conditions without a better solution as well as watering plants during high temps , just in 1 day will dry many plants out in 80 temps.Fans that keep the temps at a reasonable levels with a greenhouse shade cover could be an alternative solution. Try for a day or two when u r home.
Maybe while u r away at school the plants would be better off in another environment that may not be as risky as the greenhouse where they will not dry out to fast. Drip irrigation on timers is a water solution . Also warming trays on thermostats are also available for plants. I don't recommend leaving them with no care exposed to the temp levels in the greenhouse. Just leaving the door open will not be enough to keep the temps in a safe range. Plan ahead according to weather. 
A garage space inside near a window with light & water set up may be a better alternative just until it's safe to plant them outside. Blueberries can be planted out now in alkaline PH soil between 6.0-6.5 . This will  let the plant put more energy into a strong root system and time to grow healthy berries.

Karen L · Answer

87F is going to be too hot for some things. I wouldn't worry about the temperature swings as much as that high heat. Might be better to open part of the greenhouse and leave it open so it doesn't get that hot.

Marduk · Answer

You might look into getting shade cloth for the greenhouse to keep it cooler.

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Should I keep plants in unheated greenhouse with huge temperature swings?

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