In general, young seedlings would not be able to handle temps below freezing, much less in the teens. That would be considered a hard freeze. Frost is not the same as freeze. You can have frost when the temps are above 32ºF. More mature plants that have been properly hardened off will be able to withstand light frosts but cilantro may get some damage. I have had cilantro leaves turn purple in 40ºF temps. The row covers generally provide about 4ºF of protection and would likely protect more mature plants from a light frost but not a hard freeze.

Most seeds won't even germinate until the soil is warm enough. You can warm up the soil by laying plastic over the area until the soil is at least 60ºF a couple of inches below the surface.

Peas and carrots should be directly sown in the garden, but everything else could be started indoors and transplanted later. Leafy greens could easily be grown in flower boxes and would be ready for harvesting at micro or mini stage in less than a month, provided they don't get slowed down by temps in the low 40's. Be sure to sow successive crops every couple of weeks in order to have a continuous supply.

I grow lettuce in pots for resale beginning in mid to late March. I sow seed in mid to late February. Germination in 3-5 days. Transplant into small pots 1 to 2 weeks later. Ready for sale 2 weeks after that. Ready for harvest 1 to 2 weeks later. Everything is started in my basement under lights and after transplanting, grown in a greenhouse heated to 60ºF. The lettuce is moved outside at about 2 weeks after transplanting in order to harden off and get the reds to color up better. I do bring them inside the greenhouse when the temperatures will drop into the 40's at this early stage but later will leave them out unless a frost or freeze is imminent just so their growth won't slow down.

Thanks for the advice francescod,

>> Most seeds won't even germinate until the soil
>> is warm enough. You can warm up the soil by laying
>> plastic over the area until the soil is at least
>> 60ºF a couple of inches below the surface.

I've seen charts that list the minimum, maximum and optimal germination temperature for seeds. How does this work in a real world situation where the soil temperature fluctuates? Under a plastic tunnel the soil temperature around the seed (near the surface) may easily reach the optimal temperature during a sunny day but then drop below the minimum germination temperature at night. Is there a certain number of hours at the minimum temperature that a seed needs to germinate or does the soil need to be at least the minimum germination temperature around the clock?

Your plastic tunnels will not only warm the air inside but also begin to warm the soil. You should see some residual warming of the soil as the days go forward with the tunnel in place. You can keep some straw nearby to fold up around the tunnel if you expect a big temperature dip. If your beds are raised also place the straw around the raised frame to retain the solar heat in the soil. Continue to use the tunnels until a week past your last average frost date -- but be sure to open the plastic and ventilate if the temps inside grow warmer than 75F. Water will lower soil temps, so water as necessary to keep plants growing without interruption but don't keep the soil overly wet until the tunnels come off.

Here is a link that might be useful: HarvestToTable.com

You'll need some heat in those tunnels and close off the ends if the temp drops that low. February in N VA can get very chilly during the day and at night. We are lucky to have some Spring like weather during the day for the next couple days but the nights are still very cold for new seedlings. Might try placing some Xmas lights inside the tunnels.

George

Here is some advice from ten years experience with hoophouses (tunnels) for personal use.

a) seal them well for the night, but leave them open during the day in sunny weather
b) no mulch - soil must be bare for maximum thermal ballast
c) some seeds (notably arugula and spinach) really germinate at low temperatures (in the 40s). You should consider them for your first wave of vegetables
d) in Zone 7a, the best way to have a head start on the competition is to plant winter lettuces in September. They will grow, go dormant, and give you a head start in the spring. I use the Territorial Seeds winter mix to great success.
e) there are other greens, such as corn salad and siberian kale, that you should consider for overwintering. There is a lot of money in being the first to market in the spring.
f) finally, there are a number of small veggies that are rated at between 35 and 45 days. These will be your meal ticket until mid May. Radishes, arugula, corn salad, various asian salad greens, such as mizuna, and spinach all qualify. Scan the catalogs for such things.
g) in Zone 7, you might be able to sell siberian kale and radicchio through the winter

Thanks for all the great advice! I guess I need to hold off a few more weeks yet. I could probably plant spinach now but it grows fast enough that there isn't such a hurry to get it in the ground. I was just hoping for some way to accelerate my peas and carrots in order to have something more substantial than salad crops for the opening market on April 4th.

Maybe if I started a small crop of the Sugar Ann snap peas inside for transplant I could possibly have them bearing by the 4th. Johnny's catalog says they should reach harvest size in 52 days. That would mean starting them on Feb. 11th. I could grow them along indoors for about 2 weeks and then transplant them into a tunnel outside in early March. I think I'll try it as an experiment.

I am also realizing, as glib mentioned in point d, that I should have planted more crops in the fall aiming to harvest them in the spring. I already have a little spinach, bunching onions and collards overwintered but I only planted enough for myself. Next year I'll plant a whole lot more for the market. I've also been reading Four Season Harvest and realize now that I could have planted lots of carrots in late summer and then harvested them all winter and spring with just a bit of frost protection. Ah well, it's my first season and I certainly have a lot to learn! ;)

For an extra early start you're already 1 1/2 months late. And in a z7 as we were when we grew for market you always start transplants inside under lights and transplant out when they're ready. We had frosts into early May and grew in an unheated hoop house without problems. If you start salad greens and spinach in trays starting in mid-December you can have a salad mix and spinach for your first market. We started most vegetable varieties inside for the entire season. A book I think you would enjoy and find helpful is "The Small Commercial Garden" by Dan Haakenson. Seeing and copying his grow light stand and market stand are worth the price of the book. This is your first market season so you'll have a lot to learn and mistakes to make. We were sure to make new mistakes every year and not keep repeating the old ones. So have fun and KEEP RECORDS. You should know how much of what you sold every market all season and how much you made from each variety you sell. It will really help when you are planning your plantings for this coming December. Tom