Maybe this sounds familiar: Spring starts extra early, in a month that not so many years ago felt like part of winter. And then, just when the calendar says spring is barely over, summer arrives like a dome of heat—and without the much-needed, piercing rain.
Just as April pretended to be May, June embodied August. And all bets were off.
The best advice for gardeners facing unfamiliar weather patterns due to climate change is to expect the unexpected. But how do we do that?
“Things are changing, and so the gardener's focus must change, especially when it comes to caring for woody plants,” says Daniel Weitoish, arborist supervisor at Cornell Botanic Gardens in Ithaca, New York. “Our updated job description will likely require foresight and prioritization, not just scheduled maintenance.”
“Instead of just looking at the calendar and saying, 'It's July 15, time to do X or Y,'” he said, “we need to take care of the plants, watch them and respond to the signs they show.”
Things no longer happen in the order we are used to, so we must tune in to clues and learn about the garden in this new world order.
“Being present, observing, knowing what to look for: it feels more intimate to communicate with the plants and listen to their needs,” said Mr. Weitoish.
He knows, for example, that various dogwood species (Cornus) are indicator species – that is, they indicate stress early, before it becomes apparent in other trees and shrubs.
“Many dogwood species will wilt and their leaves will curl,” he said. “That's the prime example of what you might look for in many of your other plants when you ask yourself, 'Do I need to start triaging?'”
He added: “Even more worrying is the drooping foliage, which does not recover during a cool or wet period.”
In June, the population of Katsura trees (Cercidiphyllum japonicum) in the Cornell Gardens was identified by its own sign.
Normally, in the heat of late summer or early fall, when the katsura leaves begin to decompose, they give off a smell like cotton candy or burnt brown sugar. “We noticed that smell in June this year,” Mr. Weitoish said, recalling how a cool, wet spring suddenly turned 32 degrees. “They were stressed; they were hit hard by the temperature change. The trees that expect such a long summer just didn't have that chance.”
Even the well-known Annabelle hydrangea (Hydrangea arborescens Annabelle) showed its displeasure and stressed red maples (Acer rubrum) began to show their autumnal colors in mid-summer.
What signals has your garden given you? Pay attention to them – and make notes for the future, because not every symptom has to correspond to a current weather event.
That's another part of the equation, Mr. Weitoish explained: Trees have a different time scale. They have “a metabolic memory,” he said, as if they keep “a budget or a ledger of their reserves.”
For this reason, he added, it is “significant to look at what the weather and climate, pests and stressors have been like over several years” to understand what we are seeing now.
For example, if the autumn is very dry, trees may lose their leaves early. By the following spring, we may have forgotten that stressful autumn, but the plants will not. If further stresses follow – for example, an early warm spell that causes the trees to leaf out, followed by a late frost – the effect is cumulative.
“And then summer comes, but it is not so hot, and we wonder why everything suffers,” Mr Weitoish said.
By controlling stress at the moment it occurs, later cumulative effects can be avoided.
Water consciously, create shade
When dealing with stressed plants, there are a few important things to keep in mind: Don't prune them, and don't fertilize them.
If you see yellowing foliage, have it diagnosed with the help of your regional extension service. (Find yours here.) Don't intervene without testing the soil. It's never been more important for gardeners to connect with local experts.
Much of the tactical advice seems obvious, but there is a new urgency – and less room for error.
It's no surprise that a conscientious watering schedule is essential in gardens where erratic rainfall and periods of extreme heat are becoming more common. “We want the rain to do the watering, but if it hasn't rained for a week or so, or it's only drizzling lightly, that's when we start to look more closely,” Mr. Weitoish said. “Dig a finger into the soil and if the top three inches are dry, that's when I would start watering.”
You may have heard this before, but it's worth repeating: Gardeners armed with a hose nozzle suitable for washing cars may be giving plants a bath rather than a proper soak. Water infrequently but thoroughly, and you end up drenching the soil rather than the plants. Constantly wet bark on trees and shrubs can lead to rot; repeated watering of the leaves contributes to fungal diseases.
Drip hoses are a great way to distribute water only where it's needed. And be sure to water at night or in the morning, rather than in the heat of the day.
Mulch is another important tool for both maintaining soil moisture and regulating soil temperature. Cornell gardeners use mulch made from ramial wood chips (chopped small and medium-sized branches) composted with fall leaves.
Better plant selection, smarter planting
Identify the areas in your garden that seem to have suffered the most in recent years. Some plants that used to tolerate full sun, for example, may not adapt to a more intense version of it. Do you transplant them or change their location?
In the Botanical Garden, a slatted structure, a so-called slatted house, provides permanent shade for some plants, but there are also simpler solutions.
Planting a conifer on the southwest side of a garden can mitigate the afternoon sun that puts a lot of stress on perennials, undergrowth and shrubs. “It probably won't provide any shade this year,” Mr. Weitoish said. “But in five years you will have started to solve the problem.”
Until then, opaque shade cloth – woven polypropylene fabric in various densities – could serve as a temporary rescue for small plantings. Tarp-like versions, sold hemmed and with grommets, are easier to set up on supports than unfinished material on rolls.
Although gardeners make many of their management decisions in response to the weather—to specific events at a particular time—it is now time to make some long-term decisions, taking into account the general climatic development in advance.
When a new USDA hardiness zone map was issued in November, gardeners in half the country learned their locations had officially shifted. Many saw their average annual winter extreme minimum temperature (the metric on which zones are based) reported half a zone, or 5 degrees, milder, confirming something they had probably already sensed.
The Cornell Botanic Garden, formerly Zone 5b, is now Zone 6a, presumably expanding its range of hardy plants. This year, staff planted crepe myrtles (Lagerstroemia indica) for the first time, a flowering tree that has long been synonymous with southern gardens. Mr. Weitoish is also enthusiastic about the southern magnolia (Magnolia grandiflora) and the bigleaf magnolia (M. macrophylla).
Regardless of the species, the right planting is crucial for success, especially with all the additional stress factors. The goal: maximum resilience.
“Take the time to do what's best for the roots,” he said. Remove trees or shrubs from their pots and spread out the roots. Position them so that the root contact point – where the first structural roots meet the trunk – is at soil level or slightly higher.
Once the plant is in place and the hole is filled and mulched, use water — not your foot — to plant it in. “No stomping on the planting surface,” he said. Let the water wash the soil particles down, displacing any air pockets.
More pressure and hope
There is so much more to think about.
“There are actually changes in disease patterns,” Mr. Weitoish said, noting that fungal pathogens such as Cytospora and Rhizosphaera are present in spruce. “It's really difficult to grow Picea pungens, the Colorado blue spruce and several other spruce species.”
The increased humidity promotes fungal infections. Heat waves can affect the pollination of various crops (e.g. tomatoes). Some pests have a great chance of expanding their range to new regions and possibly producing more offspring during the extended frost-free season.
At some point, winters in many traditional areas will simply be too warm for the venerable sugar maple, scientists predict.
The Cornell team members are aware of all this, but they are not discouraged. In addition to trying out new tree species, they have also taken on other forward-thinking projects, including the creation of a water-efficient gravel garden.
Hard times have certainly led to updated policies on drought preparedness and the like, but the outcome of recent strategic planning sessions was not just minutes and procedures.
There was another important mission, too. “One of our core values,” said Weitoish, “is to provide hope.”
Margaret Roach is the author of the website and podcast “A Way to Garden” and a book of the same name.
If you have a gardening question, email it to Margaret Roach at [email protected] and she may address it in a future column.