You can keep some commercial frost blankets for several days, maybe a week tops but once temps go up, I take them off. But, one can have a late frost with temps in the upper mid to upper 30s so choose when to remove them wisely. I aim for above 37-42F to tell when to remove the frost blankets but I admit that I use the frost blankets rarely (tend to forget). The National Weather Service uses 36F as a general guideline of sorts for frosty temperatures, as long as you understand that you can also get patchy frosty areas with temps higher than 36F. A little dip below freezing will not cause much pain to the plants but if they get a late frost and the soil is dry, they may lose foliage... or more important, they may lose some flower buds or stems in old wood hydrangeas. I make sure they are mulched at all times and water deeply the night before.

I was told to try Xmas Lights under the blankets but, I have not researched just how useful something like that would be. At least, I still have those old, large, non-transparent bulbs used on Xmas Lights decades ago... those sets of lights emit heat so I know those would be useful. I may try them one day. The new Xmas light bulbs (LEDs) do not emit much heat to warm things up for the plants due to the use of a heat sink or because the lights are so small but, those old fashioned large bulbs do release some heat. Not useful at -40F but 'ok' for "small" dips into freezing territory. Do not forget that covered plants need water, sun and some aeration now and then.

what do you use to cover them ....

if it gets warm during the day.. leaving them covered might make them grow even faster .. which might be a negative ..

plz always include your location ... big city name ... zone alone is not good enough ... are your urban or rural ....

have the plants been there all winter .. if so .. they should be tolerant to all but heavy frost.. or freeze .. there is a difference ... the key number is 32 ... 40 isnt really that close ...

bottom line.. try not to do harm ... plants are rather resilient .. if we dont screw it up ...

if the plants are new.. you probably should not have planted them.. next time.. hold them in the pots a few extra weeks .. until the weather stabilizes ..

and.. and this is the part that you just cant do anything about.. some years.. it just doesnt matter .. there is going to be damage ... yes.. you might lose a cycle of blooms .. but the plants rarely die ... its just part of gardening ... ma nature can be a real *itch .....

ken

Hi, thank you for the helpful advice.

I am in zone 5, suburbs, so kinda urban. These have been in place for the past 4 years, but this is the first time we had so much green and temps dropping into the low 20’s at night. Tuesday should be in the 70’s. such temperature fluctuations. Last year I didn’t cover and the plants lost a lot of buds. I wanted to prevent that this year.

You probably only have leaf buds unfurling now. Flower buds are invisible at first and look like tiny broccoli heads once they open.

Yes, leaf buds unfurling, no tiny broccoli. It looks like I made it past a heavy frost. Thank you! Will be on the look out for potential temperature drops. Again, thank you both for your quick advice!