The recent dip in nighttime temperatures had many people (including me) scurrying around outside at night to protect their tender plants.
It is that time of year, when we play chicken with Mother Nature. Will it really get that cold? Am I in a little pocket that’s warmer/colder/somehow different than the forecast? Some of us are in perpetual denial, while the rest (like me) run around like Henny Penny thinking the sky is falling.
Depending on the source you check, the average first frost in Austin is said to be anywhere between November 28 and December 5. That means the time to prepare is now.
What’s the difference between a frost and a freeze? Frost occurs when temperatures drop and moisture in the air condenses to dew and freezes on plants when it reaches 32F. (Plants lose heat faster than air, so it’s important to remember that plants can be damaged even if air temperatures don’t actually reach freezing.)
Frost may then turn into a freeze when temperatures fall below 32F. At that point, ice crystals may form inside plant cells, rupturing them. When the temperature rises again, the cells leak, turning to mush and the resulting black foliage.
A “hard” freeze occurs when the temperature drops to 28F or below. Long, hard freezes, such as those we experienced last winter, are almost impossible to protect against.
But, in an average Central Texas winter, there are many things you can do protect plants from frosts or freezes.
First, water moderately before the freeze. Water loses its heat more slowly than air throughout the night. Combined with covering plants or even a heat source, watering can help make a real difference by a few critical degrees.
Sheets, blankets and heavyweight row cover can all help protect plants from a freeze. But it’s important to note that it’s not the cover that keeps the plant warm, it’s the radiant heat coming up from the ground that is held in by the cover. Drape the cover all the way down to the ground and secure it like a tent with rocks, bricks or my favorite – canned vegetables (lighter than rocks, easier to find in a pinch, and they don’t mess up your sheets and blankets).
Do not, however, drape something over the top of the plant and then tie it around the trunk like a giant lollipop. This is pointless, because you are actually keeping the heat away from the plant. If you have plants that can’t withstand the weight of a blanket or sheet, you can plan ahead and use tomato cages, large boxes or PVC hoops or frames – really, anything to hold up the cover.
For particularly tender plants or a really cold night, you can also add a droplight or the large-bulb Christmas lights under the cover to create some additional heat. Be careful not to let the bulb touch either plant or cover.
If temperatures rise above freezing – remove covers the next day to allow the plants to absorb the next day’s heat and recover as necessary.
Protecting container plants is a little trickier. Their roots are much less insulated than plants in the ground and will get much colder. To protect them, you can group them against your house and use the same techniques as you would for in-ground plants. Even the littlest radiant heat from the house can help make a difference on a cold night.
But what do you do when we have a colder winter, or you have larger or more tender plants, that just need more protection.
If you have one, you can put them in a greenhouse. There are many on the market – from traditional glass structures to mini houses made of plastic. But beware, in the cold of winter, the temperature inside is usually the same as it is outside. A greenhouse will require a heater to protect plants. And, with our wildly fluctuating day and night-time temps, you will frequently have to open it all up and water daily when the days are nice and warm. Even though the nighttime cold doesn’t change the inside temperature, the daytime sun most definitely does! My greenhouse can be 20 degrees hotter inside than the outside temperature on a really warm day – so be prepared to tend to the plants often and open and close the greenhouse daily.
Having spent years putting big pots of plants in the garage, this is also a great way to overwinter them, with a few conditions. Remember, plants need light, and overhead light won’t cut it. If you don’t have windows in your garage, make sure you open the garage door to let in sunlight and fresh air when temperatures allow. In the garage, some plants will go dormant for the winter reserving their energy in their roots for the next spring. Water them sparingly and let them rest for the winter. Once indoors, these plants require less water since there is no wind, and winter
daylight hours are shorter and lighting levels lower. More houseplants are lost to over watering than under watering. Water only when the top 1/2-inch of soil is dry to the touch. When their leaves drop, don’t worry and don’t fertilize them to try to push them into growth while they are inside.
How do you decide what to cover? Different plants have varying cold hardiness – more established and mature plants have stronger, deeper root systems and can withstand more than tender plants or newly planted plants. You may want to take extra care if you’ve put in a new fall bed or have recently planted new perennials.
Let’s say you weren’t watching the weather forecast and you lost some perennials. What do you do now? Don’t prune them. I know it’s tempting to take that dead stuff off so you don’t have to look at it. But, those freeze-damaged stems provide some protection for the plant for the rest of the winter. And pruning encourages growth – the last thing you want now when there are several more freezes to come. A repeat cycle of growing and freezing is hard on the plants. Just consider them items of sculptural interest in your garden!
So, plan now – collect your sheets and blankets, find some tomato cages, lights and canned goods and you’ll be ready to go when a surprise weather forecast sets you scurrying at 5 p.m.
Frost tolerance of fall vegetables:
Possible damage by light frost: Beans, cucumbers, eggplants, cantaloupe, Okra, peppers, pumpkins, summer squash, tomatoes
Can tolerate light frost: Artichokes, beets, carrots, cauliflower, celery, Chinese cabbage, lettuce, peas, Swiss chard
Can tolerate hard frost: Broccoli, Brussels sprouts, cabbage, collards, kale, kohlrabi, onions, parsley, peas, radishes, spinach, turnips.
And remember, frost can make leafy greens and root vegetables sweeter, so leave some of your chard and carrots in the ground until you are ready to eat them.