Cold? No Problem.

Brrrr! The mercury plummeted to nine degrees Fahrenheit in my garden a couple of weeks ago. Yet I was still harvesting fresh salad greens. And I don’t mean kale and Brussels sprouts; they’re tasty and still available in my “back forty,” but tender and succulent they are not. Likewise, I don’t mean turnips, carrots, or other root crops that can nestle in the relative warmth of the earth. (My root crops anyway were pulled and packed away into a box for winter storage.)

What I am talking about is lettuce, endive, and Chinese cabbage. These vegetables, which ARE tender and succulent, must have antifreeze in their cells to be able to remain so in the face of such cold temperatures. Actually, that’s not far off: With gradual exposure to increasingly colder temperatures, cold-hardy plants are able to move water out of their cells into the spaces between the cells, where freezing would cause less damage. Moving water out of the cells also concentrates the solution within the cells and -- if you remember from your high school chemistry -- concentrating a solution lowers the temperature at which it freezes. Warming weather reverses the process.

Mother Nature had a little hand from me, in the form of row covers, which are diaphanous blankets thrown over plants to offer them additional frost protection. Spun-bonded row covers let light and water pass

through. I’ve used these materials in spring and autumn for many years, but looking through the Harris Seeds catalog (www.harrisseeds.com), I came across a “point bonded row cover” which was said to give plants an additional 8 degrees or more of cold protection. That’s a lot.

A few weeks ago, I set metal arches (made from 5-foot lengths of concrete block truss reinforcement) over the rows, cut the row cover to 6 foot widths, and laid it over the hoops secured by additional metal hoops over the row cover. The material evidently is very effective; my guess is that endive and lettuce are cold-hardy to the low 20s and the row cover would bring protection down to the low to mid ‘teens. But this was 9° F.!

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Row covers represent a trade-off between cold protection and light transmittance. Generally, the  heavier the row cover material, the warmer the temperature under the cover and the less light reaches the plants.

So in early spring, I’ve use a 1.25 ounce cover to speed along growth of new plants. This cover lets about 70% of the sunlight penetrate and keeps plants about 3° warmer than outside the cover. In early summer, I use an even lighter weight material, 0.55 ounce, to cover my eggplants so flea beetles don’t ravage them. Eighty-five percent of sunlight makes it through this lightweight material.

The point-bonded row cover is a heavier material than row covers I’ve used in the past. Endive, lettuce, and Chinese cabbages are now snuggled under 2 ounce fabric. Only about 30% of the sunlight, which is sparse anyway this time of year, makes it through this material. But the plants are fully grown, so don’t need to grow. To just stay alive, now, they need cold protection more than light.

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So I’m driving along here in New York's Hudson Valley and what do I see growing wild along the

roadside? A cactus. A cactus growing wild outdoors wouldn’t be an oddity in Arizona, but New York doesn’t have the climate and soils usually associated with cactii.

The Eastern Prickly Pear or Indian Fig (Opuntia humifusa) actually grows wild throughout the eastern parts of North America. You’re most likely to come upon the plant growing in a sunny spot in well-drained soil. Not the one I found, though. This plant was growing on a rock face, which is well drained, on an east-facing slope right at the edge of woods. Not particularly sunny.

Prickly pear cactus can be oddly attractive, even edible. The pads, once the spines have been rubbed off (not with bare hands) can eaten be raw or cooked. The red fruits are also edible. The Opuntia species usually eaten is O. ficus-indica, which is not hardy in cold climates. Even that species never tasted that good to me so I wasn’t anxious to try eating any of the roadside plant.

Also, in New York, Eastern prickly pear is classified as an “‘exploitably vulnerable species,’" which is a plant likely to become threatened in the near future throughout all or a significant portion of their ranges within the state if causal factors continue unchecked.”

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I lied; I did pick off one fruit for tasting. It was seedy and flavorless. All was not exploitive, though, because I am planting the seeds.

It (Could Be) Cold

I see a lot of gardens under wraps this morning, plants covered with upturned buckets or flowerpots, or blanketed under . . . well . . . blankets. Day after day of balmy temperatures have made it hard to hold back finally getting vegetable and flower transplants out of their pots and into the ground.

But temperatures just below freezing were predicted for last night (May 13th) and everyone got a wakeup call: Freezing temperatures, which could kill tomato, marigold, and other tender plants, are still possible. It’s all about averages; around here, there’s about a 10 percent chance of a frost the middle of May.

The likelihood of cold, frosty, or freezing temperatures has been detailed -- see http://cdo.ncdc.noaa.gov/climatenormals/clim20supp1/states/NY.pdf -- for locations throughout the country. The closest weather station connected to that site around here is in Poughkeepsie, and in mid-May that site has a 50% chance of experiencing cold weather (36°F.) and a 10% chance of of experiencing frost (32°F.). Cold air, being heavier than warm air, sinks to low-lying spots on clear still nights, such as last night, so my garden in the Wallkill River valley is usually a few degrees colder than surrounding areas, such as Poughkeepsie. Fortunately, temperatures last night here dropped only to 31° F.

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Not that lower temperatures would have done my vegetables or flowers any harm. I took the advice I’ve been doling out to others for the past couple of (warm) weeks, and held off planting anything that could be harmed by frost. So tomatoes, peppers, melons, and the like are still in pots that I moved into the warmth of the greenhouse last night.

I’d like to plant out all these cold-tender seedlings but chilly temperatures are predicted for the next few night. Even chilly temperatures, let alone freezing temperatures, are not good for tender plants.

Still, anyone looking out over my garden this morning would have seen white blankets over some beds and overturned flowerpots over a few plants. Because my garden is in a cold spot, temperatures well below freezing were not out of the question for last night. Cold enough temperatures could damage cabbage and its

kin, lettuce, onions, and other cold-hardy transplants that have been growing out in the garden for the past couple of weeks. I had some row cover material readily on hand, so why not, methought, throw it over some of the beds anyway? Just in case.

Throwing covers over plants at 7 in the evening is a lot more pleasant than waking up at 3 am with the sinking feeling that temperatures have really plummeted and then, if they in fact did, running outdoors in the cold darkness to cover plants.

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Fruit trees, shrubs, and vines present another story. A freeze won’t kill the plants, but low enough temperatures could kill flowers or developing fruit, as it did on many fruit plants last year. One frigid night and you have to wait a whole year for the next crop. Unfortunately, not much could be done about this situation. Fruit plants here are too many or too big to cover. My tack is to keep fingers crossed.

Critical temperatures for fruit damage vary with the kind of fruit, the stage of flower or fruit development, the depth of cold, and the duration of cold. Probably other things, too, such as humidity and plant nutrition. 

An excellent table of “Critical Temperatures for Frost Damage on Fruit Trees” can be viewed at http://extension.usu.edu/files/publications/factsheet/pub__5191779.pdf. So, put simply, 25°F would spell death to 90% of my apples, which are in full bloom, and pears, which are post-bloom, and 28% would do in 10% of their fruits. Plums, also post-bloom, tolerate a bit more cold.

In addition to crossing fingers, my tack is also to grow a variety of fruits, and especially native fruits.

Pawpaw blossom, from below.

(Apples, pears, peaches, and most plums are not native.) It’s not a chauvinistic choice; it’s just that these natives -- American persimmon, pawpaw, blueberry, grape, and gooseberry, to name a few -- are better adapted to our conditions. And not just the weather here. Pests also.

This spring has been the most perfect spring in a long time, with plenty of clear, sunny days and gradually warming temperatures that kept blossoms from jumping the gun. Playing the averages, the critical cold periods should be pretty much be behind us. As with the stock market, though, “Past performance is no guarantee of future returns.”

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Update, May 17th: Warm days and nights that are not too chilly are predicted for the next few days, so I planted out tomatoes and peppers today. I’ll still keep an eye on temperatures because there’s still a 10% chance of temperatures dipping to 36° as late as May 28th according to records at the nearby Poughkeepsie weather monitoring station.