Dear Avant Gardener

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Planting to Beat the Heat

Heather Evans

Trees and shrubs are also an easy solution to reducing your lawn

Flowering raspberry at Mt. Cuba Center (Photo: ​Tom Potterfield​)

Dear Avant Gardener, We finally pulled it together to buy and plant six trees in our backyard surrounding our vegetable garden — three river birch, two red maple, and one redbud that has pink flowers in spring and purple leaves. We can see all the trees from our screened-in back porch which makes us very happy. So far, we have not done anything to turn parts of our lawn over to meadows as you suggested. As I said in my original question, I don't want to retain any more beds that need to be maintained and weeded. How do I create a meadow-like area that is maintenance free? From what I have seen a lot of meadows and wildflower beds turn into weeds. — Too Much Lawn in Cape May, NJ

Phew, is it hot! Last week, we experienced the ​four hottest days​ on earth in recorded history. So thank you for planting all those six native trees. Trees create shade, which cools the surface of the earth. Along with shrubs, trees generally host more caterpillars than meadow plants. In addition, as woody plants, they store carbon in their structure, as well as in their roots.

Before you embark on a low-maintenance meadow, consider leaning into the success you’ve had with trees. Where can you plant trees to feel more of that joy you experience looking at them from your porch? Can you improve the view from some of your windows by centering trees or groups of trees in their sight-lines?

Avoiding the weedy look

When you say meadows and wildflower beds turn into weeds, I can’t tell if you’re talking about poorly installed meadows or whether you — like many others — consider the meadow aesthetic weedy. Because “weed” is a subjective term.

Whereas an executive order defines ​“invasive species,”​ weeds are by definition subjective: ​“a plant out of place.”​ This means a plant noxious to cattle is a weed in a pasture. A plant that overshadows or diverts irrigation from crops is a weed in a vegetable field. And any plant you don’t like can be a weed in your garden.

Sometimes a weed is not a weed. Sometimes a weed is the best tool for the job. – ​USDA​

The subjective nature of weeds hit home when, after visiting Brooklyn Bridge Park, I discovered many plants I’m cultivating in my yard on their weed list. For example, the black cherry tree (Prunus serotina) — a keystone native — is treated as a weed at this 85-acre model of urban biodiversity.

I asked ​Rebecca McMackin​, a self-described “ecologically obsessed horticulturalist” who spent a decade as the park’s director of horticulture, how the park defined “weed,” as well as how to avoid the “weedy” look. She told me:

The first rule at Brooklyn Bridge Park was "you can't pull what you can't ID." You had to know who it was you were killing. That stopped us from weeding out the endangered Symphyotrichum subulatum, or salt marsh aster.

Bella Ciabatoni, who was a gardener and is now Deputy Director of Horticulture identified Pennsylvania pellitory or Parietaria pensylvanica. It's a host plant for the red admiral butterfly and is weeded out of most gardens. It's native but just completely unassuming. But lives in dense shade and doesn't take over. A totally fine plant to leave around.

But in general, weeds were "plants in the wrong place." We pulled many native plants that ran around as well. As far as meadows looking weedy, I think keeping plants very low is important. And honestly keeping them 75% grasses. Then they read as a meadow.

Meadow vs. lawn maintenance

Compared to a lawn, a meadow requires more work up front for the first two to three years, then less once established. For example, meadow installer Meadowshop estimates maintenance will cost $400 to $900 per month during the first two years after installation, “and then drop off substantially.” In comparison, twice monthly lawn mowing costs $90 to $200 per month, according to Homeguide.com.

Throughout the first two years following installation, the horticultural team will make regular visits to monitor plant health, manage invasives, and tidy plant beds. Our design approach minimizes maintenance by using native plants that thrive in the garden. We plant densely and over-seed with annuals to suppress weeds. — ​Meadowshop​

Using cost as a rough proxy for the work involved, meadow care requires more than four times the work as lawn mowing in the first two years. (Personally, I’m spending much less time maintaining my new meadow, but you would probably find it unacceptably scruffy looking.) The upside of meadows is that, once established, the only maintenance many require is an annual mowing in spring, a fraction of the time and/or expense of twice-monthly lawn mowing.

Shrubs as ground cover

If establishing a meadow seems like too much work or expense or you just don't love the look, consider planting islands of native shrubs around your trees instead. Underplanting with species you don’t mow — whether shrubs or herbaceous plants — will maximize the trees’ ecological value by, among other things, creating a soft landing for caterpillars the trees host, what entomologist Doug Tallamy calls pupation stations.

Spreading and colonizing shrubs suppress weeds by forming dense thickets that shade out lower-growing plants. You can install them in groups of one to three species to create islands in your lawn, letting the islands expand — and your lawn reduce — as they grow. In addition to planting these shrub islands around trees, you can create lower all-shrub islands.

​Summersweet Native Plant Nursery​, a large, all-native nursery near Cape May, carries many wonderful shrub species. Among them, lowbush blueberry, flowering raspberry, and Carolina and Virginia Rose are low, spreading shrubs that work well as groundcovers. I suggest asking their advice for shrub and tree combinations. When you plant the shrubs, you can seed around them with white clover or micro-clover, which will provide nitrogen and act as a low-growing (non-weedy:) mulch.

Whatever you choose, the more dense vegetation, the more you’ll help cool our planet. Woohoo!

— The Avant Gardener

Why, How, Wow!

Why?

As the Chinese proverb goes, "The best time to plant a tree was 20 years ago. The second best time is now." As the world continues to rapidly heat up, we need to be planting more trees.

Trees reduce temperatures through both shading and evapotranspiration. As part of the city’s Million Trees Project, UCLA scientists analyzed 30 years worth of satellite thermal and tree cover data from various meteorological stations across the city of Los Angeles. They found “city blocks with more than 30% tree cover can be about 5 degrees cooler than areas with less than 1% trees.” Furthermore,

Percentage of shaded tree cover in city blocks explains more than 60 percent of LST [land surface temperature] variations. Other factors such as distance to the coast and topography explains the rest of variations.

The ratio of impervious surfaces to trees was the main determinant of heat distribution over the city regardless of vicinity to the coast or across elevation.

Irrigated grass (lawn) had almost no impact on reducing the surface temperature, suggesting tree shade as a major source of cooling compared to surface evapotranspiration from irrigated grass or trees. — ​Million Trees LA​

Source: ​Million Trees LA​

How

Where you plant trees makes a difference. Here’s advice on where and what to plant from the EPA's ​Reducing Urban Heat Islands: Compendium of Strategies​:

To reduce temperatures and cooling energy needs, trees planted for summer shade should shelter western and eastern windows and walls and have branches high enough to maintain views or breezes around the windows. Trees in these locations block the sun when it is at its lowest angle: in the morning and afternoon. … A building with deciduous trees for summer shade will also allow for winter heat gain to the building. …

It might also be beneficial to shade air conditioner condenser units and other building cooling equipment with trees, vines, or shrubbery, as these units work less efficiently when hot. It is important to follow manufacturer guidelines for ensuring adequate space to allow for proper air flow around the equipment.

For overall energy efficiency, some communities might promote the use of evergreens to block winter winds and reduce heating needs. A row of evergreens might be planted perpendicular to the main wind direction, usually to the north or northwest of a home.

Trees and large shrubbery also can shade pavements to reduce their surface temperatures. Planting trees at regular intervals of 20 to 40 feet (6 to 12 meters) along both sides of a street (see Figure 10), as well as along medians is a common way to provide valuable shading.

Source: ​Reducing Urban Heat Islands: Compendium of Strategies, EPA​

Wow!

The designers of Brooklyn Bridge Park kept things cool — while providing lots of open areas for lounging and casual sports — by surrounding lawn with narrow bands of densely planted areas of trees, shrubs, and herbaceous plants.

The woodland gardens throughout Brooklyn Bridge Park provide shade, windbreaks, and a place to explore. These tree-dominated plantings echo a forest edge ecosystem once common in this region. The dense shrubbery, abundant berries, and herbaceous flowering plants provide habitat and forage for migratory birds that fly along the ​East Atlantic Flyway ​and through Brooklyn Bridge Park in the spring and fall. These “dense hedgerows” also slow the ocean winds and filter the summer sun, making them a perfect hang out for butterflies and Park patrons alike.

Over time, small trees in these gardens will grow large, shading out the sun-loving plants beneath them and creating the permanent canopy. Gardeners manage this succession by planting shade-adapted plants under the growing Oak, Sweetgum, and Locust trees. — ​brooklynbridgepark.org​

One shade-loving shrub I discovered at the park is Gro-Low ​fragrant sumac (Rhus aromatica 'Gro-Low').​ It quickly spreads to form a dense groundcover with gorgeous spring color.

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