I put out a few sacrificial seedlings last week, and a freeze of about 29 did them in.

Mine have been wiped out at 17F(-12c) & the plants were 12 inches(30cm) high.

That doesn't sound good! I put 38 transplants under a low tunnel and my temps are suppose to drop around 20F for several nights later this week. I also have a frost blanket I plan on putting over them inside the tunnel. I have another tray of transplants, but i hope to not have to use them as replacements.

Frank, tarp them for the next several nights, then uncover them in the morning to store heat. A weighted down tarp, perhaps with a little gap between it and the tunnel (I use rolls of old rabbit fence to maintain an air gap), insulates things well. Use the blanket as well, but the tarp is a lot faster as you don't have to open the tunnel. Two layers will give you 20F easily (again, no gaps).

Tarp them with Christmas lights underneath (but not touching) the tarp. I use the little white lights and leave the plastic cloches (cut plastic gallon water bottles) over them but under the tarp. It will work!!

I have to do this almost every year. I plant early, but check the weather for frost free days right after planting. Approximately 2 hours after I have the plants in the ground, the weather forecast changes to extreme frost for the next day. I'm at work, so I don't have the temperatures that happened in past years, but I think it was down to 21F one year.

The tarp/Christmas light set up works for tomatoes in frost weather, too.

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Good luck!
Bellatrix

Very cool suggestion Bellatrix, gonna give that a try...and I'll definitely be using a tarp...thanks to all!

Also very pretty, quite festive looking.

@Denninmi - Quite festive! I always wonder what my neighbors think when they see the lights at night. Probably that I am nuts, but they may think that any way with the size of my garden :).

Bellatrix

Also, not sure how efficient. Heat flow from the soil, 10F degree difference, has to be of order 1 kiloWatt for 100 sqft, 2kW if very wet. The lights are, what, 200W? I think a 5 gallon metal bucket of water gives you more than that in passive heat over a night, holding a steady 32F temperature until frozen.

I'm doomed.. My 7 day forecast is horrible. Very low teens to around 10F at night, tonite and then again this weekend. I've got snow on the ground right now and 32F. If we clear out tonite, it could be game over. I found some soil heating cables that I may just give a shot tonite and see what happens, along with a frost blanket and of course the low tunnel. We'll see what they can take!

Glib, would you recommend putting warm water bottles in my low tunnel...

Getting worse every time I look...

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Without heat, i'm never protecting these seedlings. I may have to dig them.

Frank, I wouldn't throw in the towel just yet. Last spring, my broc seedlings took 14*F in the open and were none the worse for wear. Instead of putting the frost blanket over the seedlings, put a layer (or two) over the entire tunnel. Tuck it around the tunnel. burying or weighing down the sides. Although it will cut out a lot of light, your seedlings won't need much light if it stays below 40 F inside. If your blanket is spun woven poly, it should let in enough light and heat during the day to maintain it over 20 F at night. This morning it was 8*F outside and 26 F inside my low tunnel (2 mil poly + 2 layers of light row cover). Soil temp. just around freezing - I'd expect more than 26 F if the soil was warmer.

Its ok... They were just planted so I plucked them right back into 18 cell trays (they were in 38)... I really couldn't afford to lose them (they were graffiti and cheddar). I'll let them recuperate before putting them back in two weeks or so..

I think i'm going for high tunnels in the future. Working inside a 3ft high low tunnel isn't for a 6ft+ guy.

Not warm water, any water. It will release most of its heat on freezing, so keeping the temps at 32F. Seedlings freeze at 28F unless they are exposed, so it would keep them safe. It works for cabbage, but not for eggplants. To give numbers: if you put water at 50F under the tarp, it will release 11% of its heat going from 50F to 32F, and 89% of its heat going from 32F (water) to 32F (ice). But this only works if the bed is well insulated, so the water has time to release its heat. Hence the tarp suggestion.

glib - it doesn't seem like the Christmas lights would make that much difference, but it does. I cover pretty low, though, it might not work as well with higher tunnels and row covers. I like the water idea and have used in with the lights some years, but usually I am too closely planted to fit a one gallon, let alone a 5 gallon.

adrock430 - if you have the room, I would do both the lights and the water.

I checked some of my (missing) notes and the only temp information I could find was 28F for the freshly planted cole crops, but I know it went lower one year. The year with the tomatoes, it went to 30F on May 19th. Very shocking. The tomatoes were fine but I about had a heart attack.

Hope everything makes it,
Bellatrix

consider saving milk jugs. I have nearly 100 (I have an attic in the garage). The lights give you some power regardless, but the jugs (again) will work only if you have enough insulation to use their low quality heat.

I save glass apple sauce jars (50oz),. I'd love to cut off an end of them and use them as cloche...

I would pour some dry leaves over them and then invert things like cardboard boxes on them. What you want to do is to slow down heat loss/flow/transfer and cooling.
Remember, When the forecast calls, say for a low of 20F, but the air in the tunnel had been 40(during the day), Your tunnel should not get down no where near 20F, unless it has a lot of openings and the winds just blow through it.
For those in WI, I recommend to put all those seedlings under a big cardboard box. Additionally, you can heat couple of bricks/rocks in the oven, wrap them in newspaper and put them under the box, to release the heat slowly all night.

Your right/// yesterday with clouds and snow I read 41F at one point and it stuck around 40F almost all day. When I woke up this morning, the outdoor showed 12.5F while the low tunnel showed 27F.

I hope to use this year as a trial run.

I am confused! Reading all of your post above I feel like I am in a different gardening world. How try so hard against mother nature? Plant broc and cauli under 20F? Taking challenges is one thing playing against the limit of nature is quite an other. There are much more things to challenge and keep the gardeners busy especially in these early spring days!

I'm growing under plastic and have a frost blanket. So far the coldest inside the low tunnel was 26F.. I think my plants would have been fine so far. The NWS upped the low temps so no more single digits. Last night the low temp was 18F and the night before was 14F (at the airport)...

homegro-You have to remember that during the day, the temps in the low tunnel easily exceed 70F, so growth can really take off on these cold, sunny days. That is why some of us risk planting so early.

homegro - sometimes the longrange forecast looks favorable, then once the plants are in the ground, conditions change. If you're lucky, you get earlier crops, or a crop before hot weather ruins it. If you're not lucky, you hopefully have backup plants.

So far, so good...have the plastic cloche up, covered by a tarp overnight, with some water bottles underneath. Made it through last night, but tonight and the next are the real tests, 24 back to back.

One saving grace is that it's been getting up to mid-40s during the day.

adrock-

I'm starting to think, with a low tunnel, I could protect down to 10F under optimal conditions (sun the day before), 2 layers of plastic (or tarp), frost blanket inside.

Extended forecast now looks good.

Outside temp is 33F and inside the hoop its 74F at seedling level (probably much warmer up higher).

I left all my seedlings in the garage last night and they were fine. I'm sick of bringing them inside.

Plan on planting in the ground tomorrow if the forecast looks good. I'm erecting another 20ft long tunnel too.

I have been told that I should have used a tunnel.
My collards went to seed in late June & seedling came up in the bed. A few grew in the late Fall & are still growing. They most have harden off as it got cold.
Who care what your neighbors think,bella,I think you are wonderful & I will use your ideal next year.
I have a device that plugs into my ac outlet on the water pump that switches a light bulb on at 40F, to keep my pipes from freezing.

Looks like we made it through the cold spell...the broc and cauli were touched lightly by frost, but it looks like they made it. Thank you for all the help.

I just hope that the frost doesn't make them button, but it looks like they're doing fine.

Mine have some white on the leaves// Think they may have got burned because the temp hit 103F in there after a morning low of 21F/// oops!

I planted out my seedings today. They've been well hardened, I'm hoping for the best.