Can Ageratum Handle Frost?

Frost-Tolerance

• Flossflowers are frost intolerant. The cold kills the flowers, progressively browns and kills leaves from the top and outer reaches of the plant downward. A light frost may nip flowers and leaves, while the lower stem and roots remain alive. If warm temperatures return and soil is moist, new sprouts may grow from dormant buds lower on the stems. Usually, gardeners simply pull up frost-damaged flossflowers and replace them.

Growing Time Frame

• Across temperate parts of the United States where frosts occur from fall to spring, flossflowers are only placed outdoors after the frost threat passes in the spring. Depending on climate and latitude, that's anytime from April to early June. The best growth and flowering occurs when temperatures remain in the 50 to 80 degrees F range. Excessive summer heat usually renders flossflowers stressed and leggy, not looking as attractive. In the subtropical areas of the country, such as U.S. Department of Agriculture plant hardiness zones 9b and warmer, plant flossflowers from fall to spring when frosts aren't expected and temperatures are comfortable.

Protection

• If flossflowers are planted outside early in the spring or you wish to extend their longevity in autumn when a frosty night looms, temporary protection can save them. The day before the expected frost, allow sunlight to bask the planting area warming the soil. Watering the soil early that day ensures the foliage dries by sunset and the plants are not dehydrated. By sunset, cover the flossflowers with an old bed sheet, thin blanket or manufactured frost cloth. The covering traps ground heat and prevents frost from forming on the plants overnight. Remove the sheets the next morning so plants receive light and before air temperatures rise above 40 F.

Caveats

• Covering flossflowers with fabric sheets may not always or fully protect plants from overnight subfreezing temperatures. Temperatures down to 30 F may not harm the flossflowers with a thin bed sheet over them, but if the temperature remains below freezing for more than four hours, cold can penetrate and kill some flowers and foliage. If a hard freeze is anticipated with temperatures as low as 26 F, a thicker blanket may buffer the cold. Thick, lightweight blankets with lots of airspace from batting layers hold more ground heat and prevent outside cold from readily penetrating to reach the flossflowers. Stakes or other support items prevent a heavy blanket from crushing and breaking stems of flossflowers.