Phenology: Mother Nature's calendar

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Oak leaves emerging.                                                                    Eli Sagor/Flickr

As the days grow longer and warmer, gardeners anxious to get their gardens going play a dicey little game with the weather.

A couple of warm days and you’re convinced it’s okay to start your engines. Garden centers soon will be filling up with seedling plants, waving them in front of your plant-deprived nose. And then there’s all those seed packets burning a hole in your pocket — can’t you get some of them going outside as soon as things start greening up?

Hold on there! There’s a big difference between the first day of spring and the first spring day. Until the end of April in most of New Jersey, and later than that in higher, northwestern elevations, frost is always a possibility.

Spring moves north about 15 miles a day, which can seem a little too slow for most of us. You can check the date of the last frost for your area (call your county office of the Rutgers Cooperative Extension Service), but you have to remember that these dates are an average.

Air temperatures tell only half the story. You need the soil to warm, stoking all that underground activity that makes the plant world go ’round. Set out in cold, wet soil, tender plants will sulk, at best. One night of frost and they’re history.

This doesn’t apply to bulbs and perennials, which are up and sprouting now. These cold-hardy plants are acclimated to their place in life and pace their growth to temperatures and length of day. The odd frost might burn some of their foliage but won’t do them in, which is why your daffodils will bloom in spite of getting snowed on.

Farmers of old, and savvy gardeners of the modern age, take their springtime cues from the state of indicator plants. Checking Mother Nature’s own calendar, they wait until the forsythia or lilac is blooming, or the native trees are leafing out, before they reach for their trowels. These phenomena are correlated to soil temperatures and the accumulation of heat in the environment from the strengthening sun of spring.

Phrenology, a study of bumps on your head.      Emmet Connolly/Flickr

The science of observing the annual cycles of plants, insects and migratory animals is known as phenology - and it's far more credible than the sound-alike pseudo-sci­ence of phrenology, which has to do with predicting character from the bumps on your head. Unlike the unreliable “forecasts” of groundhogs and wooly bear caterpillars, phenology relies on long-term collection of hard data.

In ancient history, it served peo­ple well to know when salmon would be running or when wild rice might be ready for harvest. Many of the most common bits of phenological wisdom have made their way into aphorisms or rhymed cou­plets, the better to remember them. Carl Linnaeus, the father of plant taxonomy, and Aldo Leopold, the father of the modern envi­ronmental ethic, were both studious phenological record keepers.

The whole business fell out of favor but has been experiencing a revival since the 1960s. Today, there are zillions of fans, groups, so­cieties and data banks – for starters, check out the website of the National Phenology Network, which tracks signs of spring as the season moves north. There's a good discussion of phenology, too, in the helpful book, "The Gardener's Weather Bible" by Sally Roth (Rodale Press).

You can keep your own phenology datebook, recording the first appearance of critters like spring peepers (those cheepy little frogs), robins, and orioles, and the date of first bloom for forsythia, lilac and dandelions. Keeping records can help you fine-tune your planting schedule and alert you to the emergence of insect pests, correlated to the leafing out of host plants.

Meanwhile, I've gathered up from various sources a collection of natural cues:

  • When the forsythia (or daffodils) blooms, plant peas and apply pre-emergent crabgrass suppresser.
  • When the red maple blooms, plant onions.
  • When the first dandelions flow­er, plant potatoes.
  • When the lilac is in first leaf, plant beets, lettuce, spinach and cole crops like cabbage and broccoli.
  • When the lilac is in full bloom, plant beans, cucumbers and squashes.
  • When oak leaves are the size of squirrels' ears, plant corn (squirrels' ears are about ½ inch in diameter). Alternately, plant corn when apple blossoms fade and fall.
  • When mulberry trees set fruit, plant out tender annuals like impa­tiens and geraniums.
  • When the dandelions are in bloom, plant carrots and strawberries.
  • When the lily of the valley flow­ers, plant tomatoes.
  • When the irises (the familiar German bearded kind) bloom, set out transplants of eggplant, melons and peppers.
  • When the orioles return, plant gladiolus and canna.