Early in the fall, we New Englanders hear predictions about the upcoming foliage season. Either it’s going to be spectacular, or not so great. Maybe there’s been a drought, or excessive rain. Or an early frost is expected. Or a late spell of hot weather is on its way.
Do any of these factors matter? Why do deciduous trees and shrubs change color in the fall? What role does the weather play? I consulted the website of the U.S. National Arboretum to find out.
To understand how leaves change color in the fall, it’s helpful to understand the tree’s annual cycle. Deciduous trees and shrubs in the northeast have a relatively short growing cycle. It starts in the spring, when warm weather and longer days cause the plants to break dormancy. At this time, overwintering buds break out and send forth new stems and leaves.
By late June or early July, most trees have finished their growth cycle and have set buds for the following year. For these buds to flower the following spring, they need to go through a cold period in which they are dormant. Winter usually takes care of that. But if we have unseasonably warm weather at some point during the winter, it may cause buds to think it’s spring and to break dormancy. In that case, flowering trees and shrubs will burst into flower and be done for the season. (In my experience, PJM rhododendrons seem particularly vulnerable to this trick of nature.)
After the tree sets its buds for the following growth season, it gets to work producing carbohydrates to support the next growth season. It stores these nutrients in its branches, roots and buds. As some of us may recall from high school biology class, plants produce carbohydrates through the process of photosynthesis. Basically, the plant’s leaves contain chlorophyll, a green pigment that draws energy from the sun to combine with carbon dioxide and water to form carbohydrates, such as sugars and starches. Sunlight causes chlorophyll to fade, and it is constantly being replaced so it can do its job efficiently throughout the warm season.
Leaves contain not only chlorophyll, but other pigments in a range of colors. Xanthophylls are yellow pigments; carotenoids are orange pigments. They assist chlorophyll in harvesting light energy and are present in the leaf throughout the food production period. Anthocyanins are red and purple pigments that are produced in the fall by sugars trapped in the leaves. But as long as leaves are producing chlorophyll, the green color dominates.
In the fall, as the days grow shorter and it’s dark for longer periods, nights reach a “threshold value” and big changes start happening. A corky layer of cells called the abscission layer forms at the junction of leave and stem. This gradually cuts off the supply of chlorophyll to the leaves, and the other pigments that have been overshadowed by the green chlorophyll have their moment in the sun, so to speak. All these pigments break down and lose their color eventually through sun exposure or freezing temperatures. Eventually, the abscission layer weakens, causing the tree to shed its leaves.
Weather conditions affect leaf color and duration. When fall weather is unusually dry, as it has been this year, the abscission layer tends to form earlier, causing leaves to fall before they’ve changed color. Sunny days with low temperatures cause chlorophyll to be destroyed more rapidly, giving way to yellows and oranges. The same conditions also promote the production of anthocyanins, which means brighter reds and purples. Of course, rain and wind can bring the curtain down early on the show.
Regardless of the weather and the sometimes dire foliage predictions, fall in New England always looks pretty darn terrific to me, no matter how long it lasts.
Nasami Farm Nursery at 128 North St. in Whately is holding a class Nov. 18, 1 to 3 p.m. on using plant materials gathered outdoors to create beautiful arrangements for the home or, specifically, the Thanksgiving table. Instructor Kate Stafford will take the class outside to learn best practices for collecting plant materials. Then the group will return to the classroom to create unique display pieces. Bring pruners and a container or vase for your arrangement. Cost: $30 for members; $36 for nonmembers. Space limited to 15. To register, go to: www.newenglandwild.org/learn/our-programs.
The Hilltown Seed Saving Network will hold its sixth annual Fall Seed Exchange Nov. 18 from 1 to 4 p.m. at the Community House in Cummington. It’s a great opportunity to learn to save your own garden seed, get answers to your seed-saving questions and exchange seed that you’ve saved for other locally saved seed.
Please note that you don’t have to bring seed. People at all levels of experience are welcome, including those who have never saved seed before.
The event will begin with a talk by Amy Pulley of Wing and A Prayer Pollinator Nursery in Cummington. Pulley is actively working to restore pollinator habitats throughout the area and educate gardeners, children and families about how to support pollinators by planting more pollinator plants (and saving their seeds).
The seed exchange will begin at 1:30 p.m. There will be plenty of time to browse and help yourself to the dozens of homegrown varieties of tomatoes, beans, kale, wildflowers and many other crops. You can drop off seed to share at the Community House as early as noon.
Everyone who is bringing seed to the event is encouraged to list it online using the Hilltown Seed Saving Network’s Virtual Seed Bank at http://seednetwork.herokuapp.com. Once you have registered, you will be able to browse varieties that other people in the Hilltowns are saving and bringing to the Seed Exchange.
The suggested donation for this event is a sliding scale of $5 to 10. For more information, visit hilltownseeds.wordpress.com; or contact Sadie at 413-475-2692.
Back by popular demand, on Nov. 16 from 3 to 5:30 p.m., is the Berkshire Botanical Garden’s workshop on propagating woody plants. Woody plant specialist Adam Wheeler of Broken Arrow Nursery in Hamden, Connecticut, will teach participants how to make more plants through hardwood and evergreen propagation.
Set at the best time of the year for collecting, the workshop will teach techniques for collection, insuring successful rooting, cultivation requirements and timing and care of easily propagated varieties. Participants will take home a selection of unusual deciduous and evergreen plant material in a simple propagator and will be able to collect an interesting selection of woody plant material on the grounds of the Berkshire Botanical Garden.
The workshop is open to all levels, but space is limited to 20, so register early. Walk-ins are welcome as space allows. Cost: $30 for members, $40 for nonmembers, plus a materials fee of $10. For more information and to register, go to: berkshirebotanical.org.
Mickey Rathbun can be reached at foxglover8@gmail.com.