Should I Cover My Plants At 39 Degrees?

When It Makes Sense

• Good fit: You have frost‑sensitive seedlings, annuals, or container plants that are not winter‑hardy and nighttime lows are forecasted to reach 39°F for a few consecutive nights.
• Good fit: Your garden beds are exposed to wind and clear skies, which accelerate heat loss, and you have lightweight row‑covers, blankets, or cloches readily available.

When You Should Avoid It

• Warning sign: The plants you are growing are classified as hardy perennials or shrubs that can tolerate temperatures well below 39°F; covering them may trap excess moisture and promote fungal disease.
• Warning sign: A prolonged cold snap is expected, lasting several days, and you lack a way to ventilate the coverings; the risk of overheating and condensation outweighs the modest frost protection.

Pros and Cons

Pros

• Provides a simple, low‑cost method to raise the micro‑temperature around plants by a few degrees, often enough to prevent light frost damage.
• Extends the usable growing season for tender vegetables, herbs, and ornamental annuals, allowing earlier planting or later harvest.

Cons

• Improper ventilation can create high humidity, encouraging mold, mildew, or rot, especially on dense foliage.
• Covering large beds is labor‑intensive and may require frequent adjustment as temperatures fluctuate, leading to added time and effort.

Decision Checklist

• What is the forecasted low temperature and how many nights will it stay around 39°F?
• Are the plants you are protecting frost‑sensitive, or are they rated for colder temperatures?
• Do you have appropriate covering material and a plan for ventilation to avoid moisture buildup?

Alternatives to Consider

Instead of static covers, you might use cold frames, low tunnels, or portable hothouses that offer better temperature regulation. Mulching can insulate root zones for in‑ground plants, while moving potted plants to a sheltered location (garage, porch) reduces exposure without added materials. Row covers designed with breathable fabric can provide frost protection while allowing air flow.

Final Recommendation

If your garden contains tender or young plants and the forecast predicts a few nights at or just below 39°F, using a breathable cover is a sensible precaution. For hardy perennials, well‑mulched beds, or when cold periods are extended, opt for alternative frost‑mitigation methods or simply allow the plants to acclimate. Always monitor moisture levels under any cover and adjust ventilation as needed. For high‑value crops or uncertain weather patterns, consult a local extension specialist or horticultural professional.