Will My Vegetable Seedlings Survive This Weekend’s Cold?

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Pea seedlings

Seedlings that have emerged from the soil should be protected with mulch or row cover.

If you are an eager gardener, you may have already sown seeds of cold hardy crops such as garden and snow peas, spinach, radish, and carrots in the garden. You are not too early. In fact, NC State Extension’s Vegetable Planting Calendar recommends sowing these and several other crops in February in central NC. But now we are expecting a true arctic blast for the weekend, with temperatures in the low teens on their way. How will this affect your seeds?

If sown in the ground, cold hardy seeds should survive this weekend’s freeze without damage. Though air temperatures are predicted to drop into the teens, soil temperatures will remain much warmer thanks to the ground’s ability to store heat. If you are worried this may not be enough, you can cover the soil over any seeds you have sown with a 2-3” layer of dry, loose mulch or straw. For added protection, cover the mulch with a layer of frost protection cloth or an old blanket and stake it down to prevent it blowing away.

Seeds sown in containers will be more vulnerable to cold damage. This is because containers do not have the mass needed to retain heat and will cool several degrees below soil temperatures. Containers sitting on a raised deck will be most vulnerable as they will not benefit from warmth stored in the ground and will freeze more rapidly. To protect seeds and seedlings sown in containers, move the containers into an unheated shelter such as a garage or garden shed, or push it up against the south facing wall of your home. If containers cannot be moved, wrap several layers of old blankets around and over them.

Seeds that have already germinated, particularly those that have green sprouts above soil level, will be more vulnerable to cold damage than those that have not. Any green shoots or tissue above ground level should be protected. Lay row cover fabric, blankets, or double layers of plastic directly over crops or build a low frame or hoops out of PVC or other materials to create a mini greenhouse over crops. Uncover crops as soon as milder temperatures return.

Another way to protect young seedlings is to carefully and completely cover them with loose dry mulch such as straw or leaves. Just remember to remove the mulch when temperatures return to normal. Mulches insulating effect works both ways – it minimizes cooling during extreme cold but also slows soil warming when air temperatures rise.

Container garden

Seedlings in containers will be more prone to cold injury than those in the ground.

How Can I Tell If My Seeds Were Damaged?

If two to three weeks pass and there is no sign of germination from seeds sown before the cold snap dig a few seeds up and examine them. If they are mushy or darkened, re-sow the crop. If the seeds appear sound, give them another week, or test a few by sowing them in a container and bringing it inside, where warmth will cause the seeds to germinate much faster if they are still viable. If seeds don’t germinate within another week or when brought indoors, re-sow.

Fortunately seed are relatively inexpensive so it is worth the risk to sow crops early. Some years you will lose the early sowings and have to replant. Other years you will succeed and reap the benefits, which include earlier harvest and reduced pest and disease pressure.

Understanding Temperature Affects On Seed Germination

Germination, or sprouting, begins when a seed absorbs water, causing it to swell. This sets off a chain reaction of processes inside the seed that cannot be reversed. If a seed is damaged by cold temperatures, drowns or dries out after germination begins it is not likely to survive and grow.

Germination is directly related to moisture and temperature. Seeds cannot germinate without moisture – this is why pre-soaking seeds speeds sprouting. Soil moisture levels have certainly been adequate, and often excessive this winter so if you have already sown seeds outdoors it is highly unlikely lack of moisture has been a barrier to germination.

Temperatures are equally important for seed germination, with soil temperatures more critical to seed germination than air temperatures – though air temperatures become very important once new sprouts emerges from the soil. Seeds will not germinate until soil temperatures reach a minimum point, and germination rates will be highest within an optimum range.

The minimum and optimum range varies for each species and is documented for most vegetables. For example, garden peas will begin to germinate when soil temperatures reach 40 degrees F, with optimum germination occurring between 40 – 75 degrees F. Other vegetables that begin germination at 40 degrees F include radish, carrots, cabbage, and turnips. Onions and lettuce can even germinate at soil temperatures as low as 35 degrees! See this chart of soil temperature conditions for vegetable seed germination provided by the Alabama Extension System.

Soil temperature is an important factor in deciding when to sow vegetables. Learn more about how you can monitor soil temperatures in your area.

Learn More!

More great tips on helping garden and landscape plants survive cold weather are covered in this Chatham Gardener post.

For tips on helping plants survive extreme cold, visit this Chatham Gardener post from the record-setting cold event of Feb. 2015.

Learn more about early spring vegetable gardening:

Use Extension Search to find research based information from Cooperative Extension systems across the U.S.

Extension Gardener classes and workshops teach science-based, sustainable gardening principles and practices for central NC. Sign up for upcoming classes.

Help spread science-based, sustainable gardening information and advice in your community by becoming an Extension Master Gardener℠ volunteer! Learn more about the Extension Master Gardener Program in Chatham County

Visit your local N.C. Cooperative Extension center to learn more about gardening and landscape care. Find your county Extension center.

Subscribe to the Chatham Gardener email list to receive timely updates on sustainable lawn, garden, and landscape care for the central NC Piedmont.