Southwest Yard & Garden; Why frost damages some plants and not others

Question: Why did some plants in my garden handle the first freeze fine, and others died back completely? Jane P., Albuquerque

Answer: Last year I was in Las Cruces when we got our first two freezes in Los Lunas. Luckily, my poor houseplants on the patio did not freeze hard enough— or for long enough—to cause permanent damage. I believe my grandmother would understand and even chuckle if she knew my spider plant that was propagated from hers 20 years ago by my aunt was one of those worried houseplants on my patio. But I should not have risked it. On those same cold nights at the Agricultural Science Center, Los Lunas, fewer than three miles away, more than 100 plants in our tomato study were practically wiped out. By the time I got to them, the droopy leaves looked as though they had been baked and burned, and the tomatoes, mostly green, looked shocked and exposed.

Every year, after the first hard frost, Facebook gardening pages from all over New Mexico fill up with posts from unlucky gardeners and photos of their frost-bitten vegetable plants full of almost-ripe fruit. Those who could harvested in a rush or covered their plants with protective frost cloth. One person commented that everything in their garden turned black after a frost, except for the oregano and parsley. That got me thinking too about how some plants can tolerate sub-freezing temperatures, and others are definitely not.

Much like the liquid in a can of soda I tried to chill quickly in the freezer, and then promptly forgot, water trapped in rigid plant cells expands when frozen and bursts the cell wall, killing it in the process. That is why the leaves and stems turn color and go limp. But solutes, like sugars and salts, build up in the cells of some plants, and that dramatically decreases the freezing point of liquid in the cells. In addition, some plants can create proteins that act as a type of antifreeze. It seems likely that some particularly flavorful herbs (I am thinking of you, parsley, and oregano) have what it takes to make it through those cold nights. One source online claims that some types of parsley are hardy all the way down to 10°F. I suppose the can that exploded in my freezer would have done so sooner if it had less sugar content. Let us test this theory in someone else’s kitchen.

Of course, it depends a lot on how long the plant tissues are exposed to freezing temperatures, and whether the plants have been preconditioned and had time to build up those helpful cell solutes. However, cucurbits (e.g., melons, squash, and cukes), corn, and nightshades (e.g., tomatoes, chile, and eggplants) are killed when temperatures drop to 31— 33°F. Many brassicas (e.g., broccoli and cabbage) might have frost burn on leaves, but not all die at temperatures down to around 26°F. Carrots, beets, spinach, and other brassicas like kale and Brussels sprouts are hardy to 20°F and even below.

This year, of course, I was home for the first frost and snow last week, so I brought all my houseplants inside safely for the winter. Of the plants in pots and planted in the ground, some braved the cold without any trouble at all. Among these are the parsley, artemisias, lettuce, dusty miller, lamb’s ear, and my collection of mints. However, the cosmos, tomatoes, basil, okra, and sacred datura were quick to turn black and slimy. That is ok with me. All that mush makes great mulch.

For more gardening information, including decades of archived Southwest Yard & Garden columns, visit the NMSU Extension Horti-culture page (http://desertblo oms .nmsu.edu/), follow us on social media (@NMDesertBlooms), or contact your County Extension office (littps //aces .nmsu. edu/c ounty).

Marisa Thompson, PhD, is the Extension Horticulture Specialist for New Mexico State University and is based at the Agricultural Science Center at Los Lunas.