Phenology: Be Observant

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Apple trees put forth blossoms  when it’s warm enough for honeybees to visit. Photo by Adobe Stock/Leonid Tit.

How do we gardeners know when the time is right to sow our beans or plant our potatoes? When looking in any gardening book, we find complicated charts that involve counting back the required number of days from our area’s last expected frost date for each type of vegetable. But any chart with suggested planting dates can’t possibly take into account the nuances of my small patch of ground in my hilltop garden. I know from experience that I’m unlikely to get the late frosts that plague plots lying lower in the valley. I also know some of my neighbors will enjoy snowdrops a week ahead of me and will be the first to welcome back the songbirds.

There’s an old proverb that says, “Spring is sooner recognized by plants than by men.” Plants, birds, and insects don’t use charts to know when it’s time to burst their buds or sing for a mate. Swallows know when the insect population will be sufficient to feed a hungry brood, and apple trees know to put forth their blossoms when the days are warm enough to bring out the honeybees.

Observing nature’s responses to the environment is known as phenology. The term derives from the Greek word φα?νω (phain?), “to show, to bring to light, to make appear.” It refers simply to the study of how plants and animals are influenced by natural events, such as changes in the weather and day length. Phenology is nature’s calendar, signaling that it’s time for daffodils to push up through the soil, for robins to start building nests, and for maple leaves to turn red.

A Matter of Time

Generations of farmers and gardeners have kept records of seasonal events and observed their correlations to plant, insect, and animal life. From these records they determined the best time to plant to achieve optimum growth and avoid pests. Over the years, these observations gave rise to traditional sayings that indicate a time for action, such as “Plant corn when the oak leaves are as big as squirrels’ ears,” or “Sow morning glories when maple trees have full-sized leaves.”

There’s growing evidence showing the usefulness of these observations. The USA National Phenology Network’s program, “Nature’s Notebook,” is a nationwide initiative to collect and organize observations of plants and animals taken by amateur and professional naturalists. In a recent study published in the journal Nature Ecology & Evolution, this program’s modern-day records from across North America were compared with observations recorded over 150 years ago by naturalist and author Henry David Thoreau in Concord, Massachusetts, plus four decades of observations collected at the Rocky Mountain Biological Laboratory in Colorado. The result was a broader picture on changing weather patterns and their effect on flowering times. The authors found not only earlier flowering, but an increasing variability in the timing of flowering in recent years.

When modern-day records from across North America were compared with observations recorded over 150 years ago by naturalist and author Henry David Thoreau in Concord, Massachusetts, researchers found that plants are now flowering earlier and more variably. Photo by Flickr/Pablo Sanchez.

Predicting when a plant will burst into leaf or flower is a useful tool in planning a pest management program, but with earlier flowering times, insects are also emerging earlier, and their distribution is changing. Some insects are producing multiple generations per year. Fluctuating flowering dates can also affect plants that require cross-pollination from other flowering plants to produce fruit and seeds. Studies like these help farmers predict crop yields by observing the overlap of flowering times and the effects on pollen movement between plants.

Phenology in My Backyard

When I first started growing vegetables in my Northeastern garden, an old-timer advised me to wait until after the full moon in June to set out tomato plants. Having never experienced a frost in my garden later than May, I felt confident that I could plant sooner. Instead of watching the moon, I now observe the flowers and transplant my tomatoes when lily of the valley (Convallaria majalis) is in bloom.

Forsythia (Forsythia spp.) is often cited in phenological studies. One of the most recognizable harbingers of spring, forsythia grows reliably in Zones 4 to 9, bursting forth each spring into an explosion of yellow. In my garden, Eastern tent caterpillars always hatch at the same time forsythia flowers are starting to open. So, before the forsythia buds show any tinge of yellow, I remind myself to check my trees and remove any webs I see. A welcome relief from the monochrome scene of winter, the yellow blossoms also indicate that it’s time to get planting. It’s often said that peas should be planted as soon as the soil warms up. That’s a vague instruction that involves guesswork and sticking your fingers into cold soil. I prefer to watch for the forsythia bushes. When they’re in full flower, and before I cut an armful of the flowering branches to bring indoors, I plant my peas. Rather than rotting in the cold damp soil, my peas germinate quickly and later provide baskets of fresh produce.

Plant peas when forsythia blooms. Photo by Adobe Stock.

After a long winter, honeybees are attracted to — and need — the pollen provided by dandelions (Taraxacum officinale). Their flowers are also a good indicator of warmer days. I allow them to bloom, and when I hear the bees move in, I know the days are warm enough to plant potatoes.

Lilac (Syringa vulgaris) is another useful phenological guide. I plant brassicas when my lilacs begin to leaf out, drop bean seeds in the soil when they’re in flower, and plant tender cucumber and squash seeds when the blossoms fade.

Plant bean, cucumber, and squash seeds when lilac is in full bloom. Photo by Getty Images/Kathy Brant.

I encourage you to make wider observations in your garden. Make note of the day the first crocus opens its petals, jot down when you first spot robins hopping around in the yard, and listen for the first spring peepers. By following nature’s calendar, we all become more aware of our environment. Perhaps it’s time to heed the advice of those old farmers who knew better than to ignore the signs of nature.

Try These in Your Garden

Planting indicators:

• Plant peas when forsythia is in bloom.

• 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 is in full bloom.

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

Insect management observations:

• Eastern tent caterpillars hatch when forsythia flowers start to open.

• Be on the lookout for squash vine borers when the first flowers of chicory open.

• Apple maggot adults are abundant when Canada thistle blooms.

Compiled by the University of Wisconsin-Extension

Jane Fowler gardens, farms, and crafts with her family on a 60-acre homestead in Nova Scotia. Follow her at

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