How to Protect a Marigold From Frost

Plant the marigolds in the bed in spring after the soil temperature reaches 65 degrees Fahrenheit. Planting earlier risks damage to the roots from a cold snap.

Cover newly set out marigolds when frost is expected. Cut the bottom off a milk jug and place the jug over the plants. Remove the jug once temperatures rise above freezing so the air inside the jug doesn't overheat. Alternatively, cover an entire bed with a floating row cover.

Place a string of incandescent holiday lights underneath the milk jug or frost covering. Position the lights so they don't touch the plant stems. The small amount of heat generated by the lights keeps the marigolds warm down to 28 F.

Mulch the garden bed in summer with a 2-inch layer of bark or similar mulch. The mulch insulates the soil and helps maintain its temperature. Dark-colored mulch absorbs more heat in spring and fall.

Trim off any frost-damaged leaves that turn black or brown after the weather warms. Damaged plant material can attract disease or insects to the marigold.