Can I save anything in my greenhouse from frost damage?

[DaveC426913]

We moved from a house with virtually no windows to a house with a 350 square foot sunroom.
We went from 2 tiny plants to 27 medium and large plants.
We have more money than sense.

Plants were all doing great until the temp outside dropped to near 0C. (I did check that they were all hardy enough to survive down to 5C.)
I had intended to put in a space heater (oil radiator) to keep the temp at at arond 10C but I waited just a little too long.
Also, I watered them all just before this temp drop (which I now think I should not have done with the onset of dormant season. The water still has not evaporated after two weeks.)

Anyway, within 24-48 hours every one of my plants has wilted almost completely. Many have turned grey ie. no more chlorophyll. Most of the rest have turned black. Many of them are still quite wet (i.e. I guess frost got them before they could dry out). About 50% of my plants have at least some healthy green on them.I've since made a 6x4 greenhouse out of 6 mil clear "moisture barrier" over a frame to accommodate them all and have installed a couple of heaters nearby to see if I can keep them around 10C and hopefully to dry them out a little?

Presumably, I have to cut back everything that is not bright green. Even the stuff that's still somewhat turgid with water? It's not going to green up is it?

Here are my big concerns now:

• Is it too late to save these at all?
• Can it get (dramatically) worse from this point on if I do nothing more?
• Will the roots survive to regrow when it gets warm?
• Do I need to take remedial action sooner rather than later? (Or can I jut wait for spring?)
• Is all the black due to mold?
• Am I making things worse by crowding them together where a potential mold can get worse?
• If the soil is also wet (because it's not being taken up or evaporating), will it harbour a lethal bio-load of mold, killing any remaining roots?
• Would a grow light over my makeshift greenhouse help anything survive if not recover?

and finally

• Short of hauling everything into the house where it's 20C, what is my best course of action, while leaving them in the sunroom? What temp do I need to keep as a minimum? How much light? Do I need to repot them in an attempt to dry out the soil?

 

[marcusl]

Northern California had an unusual hard freeze one winter 30 years ago or so. Pipes burst and plants turned brown. The advice from horticulturists was to leave shrubs alone, care for them as usual (e.g., water them) and wait. Sure enough, a couple of sickly shrubs in my yard came back in late spring , and in June, miles of dead-looking plants along the freeways began sprouting green leaves—just as road crews were tearing them out.

Flowers and herbs aren’t outdoor shrubs with deep root systems, though, so your plants may not survive—and black is a very bad color for any plant—so “your results may vary.”

 

[Baluncore]

Prevention.
Make sure that greenhouse external ventilation is closed below 20°C.

Cold air is very dry. If you heat it without adding water, it will dehydrate the contents of the greenhouse.

On a clear night, during a frost, throw, or unroll, a rug over the greenhouse roof.

When greenhouse internal temperature reaches freezing, 0°C, automatically turn on fog or misting units. That will prevent temperatures falling sub-zero. Use warm water to prevent freezing of the misting unit jets. It may be counterintuitive, but everything that is then covered in ice, will be protected from frost damage.

 

[DaveC426913]

Oof. Things I take for granted and forget to mention:

• These are all potted plants, in 4" to 10" pots, which have been summering in my sunroom. Some are hardy, some are tropical.
• My makeshift greenhouse is indoors - in the otherwise unheated sunroom (see pic). The ersatz greenhouse is simply 6 mil moisture barrier over a 6'x4'x4' frame I built in an hour. It's not sealed, the door is just some moisture barrier stretched over a frame and leaned-to. There's a large hole in the door into which I have pointed a micro furnace - trying to ensure nothing gets blasted with too hot, dry air.
• I also have an small electrical oil-rad heater in the sunroom, set to 14C to keep out the worst of the cold. The sunroom is too big to keep the whole thing at 14C. It's probably hovering between 5 and 10 in the sunroom but I can check that.
• I want them to dry out (at least somewhat). They are soggy and moldy. (At least I think this is what I want.)

My biggest worry is that I am not doing enough, soon enough - to dry everything out. I'm afraid it will all rot. Especially the roots.

 

[DaveC426913]

I will be going to the hardware store Friday. If there's anything I can get or make, that would be a good tme to get it.

For example: would they benefit from an overhead fluorescent light fixture?
I mean, they do get 12 hours of very diffuse and doubly-filtered daylight.

Our original desire was to have them green up the sunroom over the winter. But maybe I should let them go dormant?

 

[Rive]

For example: would they benefit from an overhead fluorescent light fixture?

No. Plants require a bit different spectrum than what the usual fluorescent lights has.
If auxiliary light is necessary, then you'll need to get 'growth light' lamps (usually LEDs).
There are white(ish) ones these days, not only those magenta monstrosities...
As a temporary solution common white LED lights may not be optimal but will work too.

Our original desire was to have them green up the sunroom over the winter. But maybe I should let them go dormant?

That completely depends on the type of the plants you have. They may have different requirements for the environment (amount and length of light, temperature, moisture). You'll need to collect the right pants for your plans for your sunroom.

The sunroom is too big to keep the whole thing at 14C. It's probably hovering between 5 and 10 in the sunroom but I can check that.

Between 5-10 should be adequate for having them dormant for winter, without much growing but still green. We too have something like that, with additional growth LEDs.

what is my best course of action, while leaving them in the sunroom? What temp do I need to keep as a minimum? How much light? Do I need to repot them in an attempt to dry out the soil?

It's the hardest to just wait but that's ~ the only thing you can do.
Reducing the moisture to help the soil dry would help, and that is a bit contradicting the 'greenhouse'.
Removing the dead parts will also help a bit.
Keeping the soil clean will also help a bit.
But in fact you can only wait for spring.

No worries, it's nothing out of the range of common mistakes a beginner 'gardener' makes.
Next time you'll be better prepared.

You'll need to spend some more time to understand your plants and plan for them.

 

[DaveC426913]

No. Plants require a bit different spectrum than what the usual fluorescent lights has.
If auxiliary light is necessary, then you'll need to get 'growth light' lamps (usually LEDs).
There are white(ish) ones these days, not only those magenta monstrosities...
As a temporary solution common white LED lights may not be optimal but will work too.

Thanks for all the advice.
My question here is really do I add extra light vs. no extra light.
The kind of light is pretty secondary.

That completely depends on the type of the plants you have. They may have different requirements for the environment (amount and length of light, temperature, moisture). You'll need to collect the right pants for your plans for your sunroom.

Yes, I've got them marked with popsicle sticks for full/partial/no sun and weekly/bi-weekly water requirements.

I based on an app called 'PictureThis'. But it lies. I have to check manually anyway.