Southampton - Spring is right around the corner. We gardeners are ordering seeds and planning what we're going to plant. We also have to figure out when to put our seeds and plants into the ground, and that depends on the weather. An important marker is the occurrence of the last spring frost. Some plants can go out before the last frost and others must wait until the weather is warmer. The USDA has tables listing the average dates of the last frost in different locations, but those dates are averages. Here on the East End, you can expect the last spring frost from mid to late April, depending how far east and how close to the water you live. (The Cornell Cooperative Extension office in Riverhead has a map of Long Island frost dates on their website, www.ccesuffolk.org). But dates aren't always reliable. Some years we experience a frost near the end of May.

Instead of relying on the calendar to gauge planting times, we can observe what the local flora and fauna are doing and watch for natural indicators that can tell us more surely when winter is gone and spring has truly come. There is a science of using natural indicators to monitor weather conditions from one year to the next, called phenology. Understanding a bit about phenology can come in very handy for a gardener. You'll lose far fewer plants to late cold snaps and other vagaries of the weather.

Lilacs in bloom. (Courtesy Photo: yougrowgirl.com)

Phenology is the study of events that occur in regular cycles during the lives of plants and animals. The phenology of perennial plants in the local landscape is closely related to temperature and daylength, and you can use plants as natural indicators of when conditions are right for planting flowers, herbs and vegetables. Plants usually don't start to grow until the weather is warm enough for them. In a cold, wet spring they will get a later start, and so should your garden plants.

Make notes in your garden journal or a notebook or calendar on when trees, shrubs, and wild plants in your yard start to grow, leaf out, form buds and burst into full bloom. Keep notes on weather conditions, too. After several years your journals can begin to help you choose the best indicator plants in your neighborhood.

You can also use phenology to predict the annual arrival of pest populations in your garden. When a major infestation occurs, look around and make notes of plants in the landscape that are in bloom, at a particular height, or at another readily observable stage in their development. If after several years the plant is always at the same stage when the pests arrive, you may be able to use it to predict when they will come. Then you can take measures to protect the plants they're likely to attack.

Farmers and gardeners have been using phenology for thousands of years. It was familiar both in China and ancient Rome. Hesiod wrote that it was time to plant when the cranes migrated. Here in America, one Native American tradition was to plant corn when the oak leaves were as big as a mouse's ear (the advice is still valid today).

All plants are sensitive to climate and weather conditions, but indicator plants are the most reliable and predictable in their performance. Indicator plants are different in different places. On Long Island, for example, cabbage root maggots tend to arrive when the forsythia are in bloom, but upstate they come when a common weed, yellow rocket, flowers. Forsythia in bloom also let you know it's time to spread corn gluten meal on the lawn to kill weeds without chemicals.

Probably the most widely used indicator plant is the old-fashioned common lilac (Syringa vulgaris). Lilacs are good indicators because their annual development is very regular and easy to observe, and because they grow in most of the country. Here's how to use them:

 • When lilacs first begin to leaf out, that is, when the widest part of the first emerging leaves goes beyond the bud scales that had enclosed the leaf buds, it is safe to plant hardy, cool-weather plants like peas, lettuce, spinach, sweet alyssum and calendulas.

 • When lilacs are in full bloom, that is, when all of the flowers on 95 percent of the plant's flower clusters are fully open, it's safe to plant tender, warm-weather plants such as corn, tomatoes, basil, impatiens and dahlias

I've been watching the lilacs and oak trees in my yard for years to time some of my plantings and they haven't been wrong yet. Why not try finding some indicator plants at your place?