Cold weather on the way? Colder than you had hoped or planned for? Colder than you've seen in 30 years? Here are some tips to help your plants survive.
High tunnel growers have much less to worry about. On our farm in Vermont, even when we had -20 dips plants in the high tunnel were generally fine. The larger the tunnel the more moderated the temperature. Small structures cool off much faster than large structures, so keep that in mind if you are planning a new tunnel for overwintered crops. Go as big as you can!
It's all about the roots! Most of the time if you can keep the roots alive, the plants will survive. The top growth on an overwintered plug is less important. If the roots die the plant will die. Soil temperature is much more important than air temperature. Luckily soil is a great absorber of heat and it will hold onto that heat longer than you might imagine, especially if we help it.
Frost cloth, aka Remay, aka Agribon, aka fleece. This spun fabric-like product can be a huge pain to put on and take off your rows repeatedly, but it's good to have on hand for unexpected weather, and it can be used in a tunnel or in the field. You get added warmth retention by doubling or tripling the number of layers. It also comes in various weights, so keep some of the heavy duty stuff on hand for real cold snaps. Yes multiple thick layers will reduce your light transmission but when plants are below freezing they aren't photosynthesizing, they are essentially dormant. So cover them well. In the field, make sure those edges are well weighted or secured to the ground (with sandbags, staples, etc.) and hope for the best. Frost cloth will act like a sail if the slightest bit of wind can get under an edge, so consider yourself warned.
Natural insulation. If you're lucky you will get a nice thick coat of snow before a cold snap. Snow is the best insulation there is. It's very common to dig down beneath a foot of snow and find that the soil is still loose and workable. Of course you can't guarantee good snow cover, so look around for other natural materials. Leaves are a good choice, especially if they are chopped up a bit. They create lots of little air pockets that will help insulate the soil. Straw and hay will do the same. If you're just protecting your plants for a couple of days, pile on the leaves or straw and uncover as soon as things thaw out again. A tarp or a sheet can help keep the leaves in place if you are experiencing wind. Again, plants are not photosynthesizing at these temperatures, so don't worry about light if you are just covering for a couple of days.
Adding Heat the low tech way. Of course adding heat sounds good when it's cold, but how do you do this without a tunnel? One simple method I have seen is to use Christmas lights. This works much better with the older fashioned incandescent bulbs than it does with newer LED bulbs. The LED's just don't give off heat, so look out for the old style bulbs. Some stores still sell them, or you can find them at thrift stores from time to time. Run a line or two down your rows and cover with heavy duty frost cloth. This can keep things a few degrees warmer, and sometimes that's all you need to get through an unexpected cold period.
Reduce the wind. If you're like my Mom and the many meteorologists, you like to scare yourselves and everyone around you with wind chill numbers rather than looking at actual temperatures. Yes, windchill is real and your skin and your plants will both lose temperature faster when exposed to driving winds. So cover up your plants but also consider planting wind screens. This can be a row of woody shrubs on the edge of your field, or it can be a row of tall annuals that you leave in place overwinter (think about corn and sunflowers and other tall plants that leave lots of residue overwinter). Snow fencing can work as well. You just want to keep the direct wind off of those tender leaves.
Think of this as a learning opportunity. Take notes during your cold snap. How cold did it actually get? What lived? What died? Are there more protected areas you can plant your hardy annuals, biennials and perennials in the future? What plants didn't seem to care at all? What lived with no protection whatsoever? It's easy to get wrapped up in the moment and create drama where it is not needed. You are a farmer now, and farmers experience losses. Your job is to minimize them, and to do what is within your power to control the situation. Sometimes the time and resources involved in saving a crop cost more than the plants themselves or more than the value of the eventual harvest. So don't panic. Give it your best shot, hope for the best and better prepare yourself for next time. Because there will be a next time! Invest in some thermometers to see how cold it is actually getting under that frost cloth. Get a soil thermometer to see how cold the ground actually gets. You may be surprised and may sleep better next time.
How cold is too cold? This is impossible to answer. Most hardy annuals won't mind 20F with no protection. When it gets much below that it's a matter of how cold they get and for how long, as well as what other factors they have experienced. Warm conditions before a cold snap can leave the soil warmer and help your plants make it through. Warmer conditions can also encourage tender new growth which is more susceptible to freezing damage.
Be patient. It's tempting to try to thaw out your plants as quickly as possible after a freeze, but this will do more harm than good. Allow your plants to thaw slowly and keep the covers in place until you are certain things have thawed out completely. Frozen plant tissue is easily damaged so give it time. Frozen plants may also look dead at first, but will regain their good looks once they thaw.
Good luck! Plants are tougher than you may expect!