Dear Miss Floribunda,
Some years ago, you warned not to plant peas or prune roses till forsythia blooms. Well, I saw it blooming on Route 1 near College Park last month, but knew better than to plant or prune anything yet. You also said that when the geese return from Canada, you know it’s spring. Just last week I saw them at Lake Artemisia, but I think they’ve made a mistake — they never left and have been there all winter! You advised gardeners to plant vegetable seedlings outside once lilacs are in bloom. I have no idea when that might happen this year, but I wouldn’t trust it either. My hellebores got blasted when they bloomed way too early, though they seem to be alive. The weather is just weirder every year, and I’m not sure how to garden anymore.
Leary on Longfellow Street
As I recall, that column came out after the U.S. Department of Agriculture (USDA) changed our plant hardiness zone from 7a to the warmer 7b in 2012. Since that time, a 2019 study co-authored by a USDA Forest Service scientist suggests that because of the continued warming trend, our zone will change to 8a or even 8b in a few years. Because generally accepted dates for pruning and planting were inaccurate even without considering these changes, I suggested a little study of phenology, which is the science of annual and seasonal changes. Long before almanacs existed, farmers knew when to plant by observing such biological phenomena as bird migration, swelling of nodes on stems, the appearance of shoots from bulbs, and the reappearance of insects and animals coming out of hibernation. Even the Groundhog Day fanfare involving Punxsutawney Phil has its origins in phenology, however mistakenly.
For the record, I have seen the winter jasmine blooming along Route 1, and while it does look a lot like forsythia, it blooms much earlier. You were wise not to plant or prune when you saw it. While the geese will get along fine, climate change is contributing to the projected extinction of many other birds, among them the American goldfinch. The Baltimore checkerspot, Maryland’s state butterfly, is among the pollinating insects that are threatened.
As for when to set out seedlings, that really does depend on their variety: Lettuce seedlings, for example, can be planted outdoors much earlier than tomato seedlings. Since you have many good questions, I suggest you attend the Hyattsville Horticultural Society seedling workshop March 21 at the home of Dr. Julie Wolf, 4008 Hamilton Street. Dr. Wolf is a plant physiologist at the USDA and studies how plants respond to climate change. There will be a brief meeting at 10 a.m., and then the workshop will continue till noon. In addition to answering questions as she demonstrates, Dr. Wolf will distribute written material you can take home.
I decided not to wait till the workshop, and went straight to the horticulturist’s mouth to give you a preview. Here are the results of my cursory interview with Dr. Wolf:
Q: Dr. Wolf, although I think hellebores can bounce back easily, are there plants that can be seriously harmed by blooming too early?
A: Yes, Miss Floribunda! Many perennial plants may be stimulated to grow earlier than usual, and many gardeners might be tricked into transplanting their seedlings to the garden too early. For many perennial plants and trees, including most orchard fruits, this can be a problem. Leaf and flower buds survive winter in a dormant state, after undergoing physical and chemical changes that prevent destructive ice formation inside their cells. To break dormancy and start bud growth, low temperature followed by a longer period of warmth is often required; this mechanism has evolved over millennia to allow plants to survive winter in their location. After dormancy breaks, leaf and flower buds are no longer resistant to cell damage from ice formation. A freeze occurring after spring warmth has broken dormancy is called a “false spring” event, and these have always occurred. But because recent climate change has caused warmer, more variable and more unpredictable temperatures, false springs have become more common and more of a risk to our plants, to returning birds, and to the mutualism between plants and pollinating insects.
Q: What can you tell us about climate change in our area? Are we going to become semi-tropical soon?
A: “Soon” is relative, and I don’t think the USDA will reclassify the plant hardiness zone of Hyattsville to 8b in fewer than 50 years, but the climate is warming as fast or faster than the most extreme scenarios of our climate models. Hardiness zones are based on the single coldest day likely to be reached in a year, since that sets a clear boundary on plant survival. Therefore, the hardiness zones aren’t really a reflection of the whole year’s climate, even though summers are getting hotter, but a reflection of what’s called “extreme minimum temperature.” A subtropical climate zone is characterized by hot and humid summers and mild winters. You might say we already have those conditions now!
Climate change is expected to warm average winter temperatures more than the coldest winter temperatures and has already begun to do so. In Maryland, our average daily high temperatures during the hottest months of the year have been 84-86 F, but by 2070 they are expected to be 98-100 F. The authors of the U.S. Forest Service paper you allude to state that “by the end of the century, more than 85% of the country may be in a different plant hardiness zone, resulting in mismatches with climate that may be challenging for current plant species, particularly cultivated crops in horticulture and agriculture, including orchards.”
Q: What can the average gardener do?
A: I encourage Hyattsville home gardeners and landscapers to do the following:
- Either resist the urge to plant out your seedlings early or watch the weather carefully and be prepared to cover your transplants if you can’t wait.
- Select plants that will be able to tolerate the changes predicted for our region by the U.S. National Climate Assessment (https://nca2018.globalchange.gov/).
- Expect and be prepared for warmer and wetter winters and springs, hotter summers and autumns, and potential periods of summer and fall drought.
Thank you very much, Dr. Wolf. One more thing: if anyone has already made the mistake of setting seedlings out too soon and has lost them, you have a second chance. The Hyattsville Horticultural Society will sell seeds at the Community Forklift 11th Annual Garden Party on Saturday, April 4 from 10 a.m. to 3 p.m. Some seeds can be planted directly outdoors at that date, while others can still be started indoors. Community Forklift is located at 4671 Tanglewood Drive in nearby Edmonston.