by Joyce Tomaselli, Community Horticulture Educator, Cornell Cooperative Extension Dutchess County

A version of this article appeared in the Times Herald-Record in May of 2017.

Many sources provide great advice on sowing seeds.  Key factors include sufficient light and temperature for growing the seeds.  But the question inevitably arises when to transplant them outdoors. Don’t rush.  Cold soil and air temperatures can stress plants.  Wait at least a week or two after the last frost.

The latest map from the National  Oceanic and Atmospheric Administration (NOAA) shows the last spring frost date for the Hudson Valley to be between New York, which indicates April 16^th and May 15^th. But of course these dates are only guides.  In 2020 we had a hard frost on May 15^th that severely damaged many plants.  There is no guarantee that frost will not occur after the last date shown.  This map does not tell you the best time to plant tomatoes or other frost-sensitive crops. That date is usually several weeks after the last frost, after the soil has warmed and nighttime temperatures are reliably well above freezing.

• Nighttime temperatures should be consistently above 45°F, and the soil should be warm, about 70°F.  You can use black plastic mulch to warm soil and/or row covers, hot caps or other protection to keep plants warm early in the season. Remove covers whenever temperatures exceed 85°F.

• Harden off plants before transplanting by reducing water and fertilizer, not by exposing to cold temperatures, which can stress them and stunt growth.

• Remember that microclimates have a tremendous effect. Some valleys and low-lying areas can be plagued by late frosts on clear, cold nights when there is radiational cooling.  Large bodies of water (such as the Hudson) may moderate air temperatures at some sites, reducing the chances of frost.

Since plant development is temperature-dependent, rather than depending on specific calendar dates we can make decisions based on Growing Degree Days, a measure of accumulated heat or by observing phenological events of plants.

Phenology is a branch of science that studies the relationships such as the life cycles of plants and animals and environmental changes. Seasonal changes such as weather or temperature can be correlated with natural events such as bird migration, plant budding, flowering or fruiting and insect activities.  For example, in the Hudson Valley migrating hummingbirds usually arrive when forsythia bloom. Centuries ago, Native Americans began planting corn when oak leaves are the size of a Snowdrop (Galanthus nivalis).

University of Wisconsin Extension has provided some examples of phenological correlations:

• Plant peas when forsythia blooms.

• Plant potatoes when the first dandelion blooms.

• Plant beets, carrots, cole crops, lettuce and spinach when lilac is in first leaf.

• Plant corn when oak leaves are the size of a squirrel’s ear.

• Plant bean, cucumber, and squash seeds when lilac is in full bloom.

• Plant tomatoes when lily-of-the-valley plants are in full bloom.

• Transplant eggplant, melons, and peppers when irises bloom.

So watch the calendar, but also watch for other clues from nature when you’re making decisions about planting and transplanting outdoors.