Though early March featured unusual warmth across the eastern United States, growers and gardeners beware: The threat of a killing frost looms at the end of the month.
Depending on the section of the country, the last frost averages from March 15 to April 1 in the south-central U.S. to May 15 in New England.
The safest time to plant depends on the type of flowers being grown, according to Richard Weidman of the Rutgers Cooperative Extension of Middlesex County, New Jersey.
"Cool-season annuals like pansy, primula, calendula and snapdragon can withstand the cooler early-spring weather," Weidman said.
However, there could be a late-season frost in parts of the eastern U.S. that may hurt any early plantings, AccuWeather Lead Long-Range Meteorologist Paul Pastelok said.
A cooldown with possible frost from March 19 to 23 could impact the most sensitive plants in the Ohio Valley, northern mid-Atlantic and the interior Northeast.
There may be above-average temperatures for most areas in April, although clouds and possible drizzle may hold temperatures down at times east of the Appalachian Mountains, Pastelok said.
The potentially cool conditions may threaten warm-season annuals that growers look to plant later in the season.
"To plant warm-season tender annuals like zinnias, tomatoes, marigolds and others, the rule is to wait two weeks after the last predicted frost date," Lisa Ziegler, owner of The Gardener's Workshop in Newport News, Virginia, said.
All warm-season annuals will sustain some type of damage if exposed to cooler weather, she said.
"The stress will affect the performance overall like disease resistance and bloom or fruit production throughout the season," Ziegler said.
While an early warmup may entice some growers to get a head start on planting, the risk of losing the plants is still high. Ziegler suggested focusing on the cool-season hardy annuals until overnight temperatures stay above the 60-degree Fahrenheit mark.
If there is a danger of frost after planting, regardless of date, gardeners should cover sensitive plants with a large container such as a frost cap, bucket or garbage can to protect them from the cold, the Rutgers Cooperative Extension said.
Row covers available at many garden centers can be draped over larger plantings to protect them, but these provide only a few degrees of protection. Place row covers on small stakes or wire hoops so they do not touch the tops of sensitive plants.
Plants that are in containers can be brought inside or placed in sheds or garages for protection.