Blackberry Winters and Thermal Masses : 5 Tips To Protect Your Summer Garden From Spring Frosts

Blackberry Buds

As the warming sunny days of April enchant us into delusions of summer, many gardens are planted before May. In some lucky years, we get away with it. But nearly as often, a late frost comes to claim our tender plants. Midwestern folk tales call this the Blackberry Winter, because it usually comes around the time blackberries flower. That time is now. And that frost is tonight. The forecast predicts a frigid 26 degrees by tomorrow morning. In this article, I will share five tips you can use to help protect your tender plants from frosts- whether the frost be blackberry, or other.

I like to wait until after Mother’s Day weekend to plant my tomatoes and peppers (unless I plan to protect them), because these plants are very tender and blackberry winters have bitten me before. But my potatoes are already growing in the ground, and although they can withstand light frosts, the young vines can be damaged by temperatures below 29 degrees. I also planted a few cucumbers last week against my better judgement, and they’ll need serious protection tonight. In my cool season garden I have onions, kale, collards, kohlrabi, and cabbage. Those plants should be mostly fine, but 26 degrees is right on the borderline of cabbage’s cold hardiness, so I’ll add a light covering to that bed just to be safe. A quick google search should inform you of the frost tolerances of any other plants you might have in your garden.

Tip #1: Water Well. This strategy is important for all types of plants. Wet soil can retain heat better than dry soil, so by ensuring your soil is well watered before a frost, you can add a layer of protection. I suggest watering before adding any sort of frost covering unless your soil is already moist.

Tip #2: If only light protection is needed, use a fabric covering. You can use an old sheet, or you can purchase floating row cover fabric. Floating row cover fabric is sold specifically for farm and garden purposes, and is usually rated to a specific temperature. Fabric coverings can be draped directly over the plants and anchored to the soil with rocks, bricks, or earth staples.

Tip #3: For more frost protection, I use 6 mil clear plastic sheeting, such as might be found at a hardware store in the vapor barrier section. But if you use a plastic covering, it’s important that the plastic is raised above plants instead of resting directly on them. Use hoops, buckets, or anything else you can find to elevate the plastic above the level of your tallest plant. Plastic is also easily carried off by a strong wind, so make sure it’s weighted down very thoroughly.

A low tunnel I built in one of my previous gardens from PVC pipe, 6 mil clear plastic sheeting, and 2×4 bottom weights. I should mention that the 2×4 weights weren’t strong enough to keep the plastic in place during wind storms, and I ended up adding rocks to anchor it more solidly.

Tip #5: Add thermal mass. A great thermal mass material will absorb heat from the sun during the day, and then release that stored heat during cool periods. In a garden situation, water is by far the best and most available thermal mass material. I save empty gallon jugs to fill with water and place throughout my garden for this purpose, but I’ve also used 5 gallon buckets with great results. There’s a product called “Wall O Water” that creates 360 degree water-based thermal mass around a single plant, and the manufacturer claims it will protect a plant down to 16 degrees! I don’t think I’ve ever tried it in weather quite that cold, but I have used these for years and I can say that they’ve handled whatever the weather has thrown at them and they have the benefit of being much more convenient and user-friendly than any of the other protection options. They work from exactly the same principles as the gallon jugs of water though, so don’t feel like you have to go out and buy fancy items just to have a successful garden. For more information about thermal mass, this article is very educational! Note that the water must be placed out during the day to give it a chance to store warmth. In a pinch, if you don’t have time to set them out in advance, you can fill the jugs with very warm water from your tap.

Gallon jugs of water are added under the plastic covering for extra thermal mass
Gallon jugs of water are placed around potato plants before adding the plastic covering. These potato plants are too numerous and too closely spaced to benefit from “Wall O Water”, so they will be protected by gallon jugs of water + 6 mil plastic sheeting. The gallon jugs also have the benefit of lifting the plastic away from the tender plants.
Plastic covering is placed over four gallons of water and potato plants.
These “Wall O Water” plant protectors are made up of many narrow tubes. Each tube is filled with water, and then the protector is placed around a single plant. The thermal mass of the water protects the plant inside very effectively, even in very low temperatures. This is one of the very few “garden gimmick” products that I actually use and recommend (perhaps the only one). If they’re not in your budget, or they’re not the right fit for your plants, then the gallon jugs of water under plastic covering work similarly well.
I’ve added 6 mil plastic sheeting over the trellis in this bed that contains the Wall-O-Waters. The plastic will seal in the heat of the Wall-O-Waters for the benefit of the other plants in that bed that would be otherwise unprotected. This extra layer of protection shouldn’t be necessary for the cucumber plants directly protected by the Wall-O-Water.

Tip #5: For even more protection, add an extra layer. Have you ever noticed that if you go out into the depths of winter wearing a t-shirt and leggings under a big bulky coat, you actually feel colder than if you had a tight layer of wool thermals under jeans and a lighter coat? Two layers of plastic with an air gap between them will far surpass the protection of a single layer. The air gap can be created with a layer of bubble wrap, or by adding a physical separation like an outer hoop structure. If you also add thermal mass inside, your plants will ultra-protected. That level of protection isn’t needed tonight for our 26 degree cold snap, but it’s a useful strategy that can help you grow plants all winter long.

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