How to use trees and shrubs for noise reduction — good news about planting solutions

Dear Avant Gardener, I recently moved to a house with a loud, busy road about 50 feet from the property line. I'm hoping to put a 300 to 400 foot hedge around my half acre property boundary for privacy, aesthetics, and sound reduction. The previous owner removed all the trees, but directly behind me is 64 acres of longleaf, slash, loblolly, and hardwoods (not in a burn rotation). My soils are well-drained and acidic (see test). Are there any native privacy or sound reduction hedge or tree choices? A wishful bonus would be a bird attractant option! — Bird-loving Becky, Macclenny, FL

Phew! I’ve just emerged from a rabbit hole of journals like Sound Acoustics and Journal of Sound and Vibration. As you expect, trees and shrubs do reduce road noise. Foliage absorbs sound and stems and branches scatter it. A moderate density, 16 feet-deep tree belt will reduce sound by about 10 decibels, with sound continuing to decrease with greater density and depth. Soft ground also reduces sound, whether covered in leaf litter or turf. For example, after passing through a hedge, 50 feet of lawn on the inside of a hedge dampened sound an additional 10 decibels, according to a 1970s study. (See more in “Why, How, Wow!” below.)

Of course, it takes several years for even the fastest-growing trees to grow in. That’s one reason a recent Canadian study of the sound-reducing effect of various types of vegetation adds so much to decades of prior research on tree belts and lawn. Surprisingly — and to my delight, tallgrass prairie and agricultural vegetation, which can grow to full height in one season, reduced sound by about as much as forest, more or less depending on the frequency and the season.

This means replacing some turf with mid-height plantings (more than three feet) such as tall native meadow plants and grasses will further decrease the noise level on your property. Also, you can expect the hedge to mitigate road noise long before it reaches its full height. With a combination of hedge and meadow plantings, you should be able to reduce substantially the noise on your property. In the meantime, adding a bubbling water feature for the birds will reduce your awareness of road noise.

Soil tests: What’s important, what to ignore

The portion of organic material in your soil — 0.96 percent — is on the low end even for Florida’s typically sandy soil. You’re going to want to build that organic material over time by letting leaves and other vegetation decompose naturally near where it falls. In the meantime, you might consider adding a top-dressing of compost or natural wood chips before planting your hedge. But please do not buy the almost 30 bags of fertilizer your test results recommend; not only are these amendments not needed for native plants, they will likely favor weeds more than your natives. As for your soil’s acidic pH of 5.9, that’s pretty typical of natural landscapes in the East and most native plants will thrive in it.

Creating a hedgerow — or woodland edge

Google “hedge” and you’ll see mostly monocultures, often of non-native spruces, box, or — egad — Chinese privet, a Category 1 invasive in Florida. Instead, I  recommend you plant a mix of native species, variously called a tapestry hedge, a wildlife hedge, or — in the United Kingdom — a hedgerow.

When I asked him for suggestions for your hedge, David Chiappini of Chiappini Farm Native Nursery said, “The more species the better.” I agree — to a point. Repeating shrub and smaller tree species will make your hedge more cohesive. If you repeat every 50 feet for 300 feet, you’ll need six of each species on average. If you plant in a zig-zag pattern every four feet, you’ll need 75 plants of 12 species — give or take a few. (For a sample privacy hedge planting plan, see Creating a Privacy Hedge.)

Fortunately, there are many broad-leaved evergreens in north Florida that provide year-round screening and noise absorption. Thicket-forming shrubs like cherry laurel and swamp myrtle, which gardeners often avoid, will add density that will help with soundproofing. You may even be able to get saplings of such shrubs free from friends and neighbors who have more than they can handle. You’ll be able to keep them in check by mowing along each side of your hedge once every one to three years. In general, you can save money by buying smaller-sized plants, which catch up quickly once established in the ground.

To attract birds, include lots of berry-producing shrubs in your hedge, including some with winter berries like dahoon holly and wax myrtle. The pines and oaks trees in the forest next door host host hundreds species of moths and caterpillars, providing plenty of food for nestlings. However, without periodic prescribed burns, these woods may be low in shrubs that produce seeds and berries for birds and other wildlife.

Identifying trees and shrubs for your hedge

You can use Florida Native Plant Society’s Plant Finder to identify plants native to your county. Because you have little sun, I searched in your county for plants that thrive in sun and attract birds, sorting them by plant form to identify shrubs and trees. Except those that evolved in swampier conditions and require regularly moist soil, most of the resulting 20 or so shrubs and 40 trees would be great in your hedge. I suggest you generate your own list on the FNPS site and be ready to pick alternatives when you go to the nursery. 

Below are my choices. They are large shrubs and small to mid-sized trees, appropriate for your mid-size lot, with showy flowers or berries (F), evergreen foliage (E), thicket formers (T), and/or autumn foliage color (A).

1. American beautyberry (Callicarpa americana) F

2. Cherry laurel (Prunus caroliniana) F, E, T

3. Chickasaw plum (Prunus angustifolia) F, T 

4. Dahoon holly (Ilex cassine) F, E, T

5. Eastern red cedar (Juniperus virginiana) E

6. Inkberry (Ilex glabra) E, T

7. Swamp bayberry(Morella caroliniensis) T

8. Southern arrowwood (Viburnum dentatum) F, T

9. Southern magnolia (Magnolia grandiflora) F, E

10. White fringe tree (Chionanthus virginicus) F

11. Wax myrtle (Morella cerifera) E

12. Winged sumac (Rhus copallinum) F, T, A

Happy planting!

— The Avant Gardener

Why, How, Wow!

Why?

Tens of millions of Americans live with highway noise. According to the Federal Highway Administration,

Levels of highway traffic noise typically range from 70 to 80 dB(A) [decibels] at a distance of 15 meters (50 feet) from the highway. These levels affect a majority of people, interrupting concentration, increasing heart rates, or limiting the ability to carry on a conversation. The noise generated by a conversation between two people standing 1 meter (3 feet) apart is usually in the range of 60-65 dB(A). Most people prefer the noise levels in their homes to be in the 40-45 dB(A) range, similar to the levels found in a small office. A reduction of sound from 65 to 55 dB(A) reduces the loudness of the sound by one half, while a reduction of sound from 65 to 45 dB(A) results in a loudness reduction of one quarter. – Federal Highway Administration

How

Evidence-based ways to decrease the experience of traffic noise in and around your home include planting a hedge of trees and shrubs and replacing hardscaping and turf with tall meadow/prairie plants.

While forest, trees, and shrubs continue to be effective for managing noise pollution, our results suggest that other types of land cover can also be useful (e.g., tallgrass prairie). . . . Both the forest and tallgrass prairie sites had substantial year-round litter layers compared to the relatively exposed soils of the agricultural site, which likely contributed to their overall superior sound attenuation of low frequencies and lack of seasonal variation (Van Renterghem et al. 2012). — Urban Ecosystsems

Beyond its sound dampening effect of a hedge, screening a busy road from view makes it less annoying, according to a recent Belgian study. Only 8 percent of respondents who looked out their windows at vegetation on the street-facing side were annoyed by the loud road noise (73 dbA on average) whereas 34 percent of those who looked out at the road were at least moderately annoyed.

A 5 m [16 feet] depth of vegetation barrier was found to be an ideal depth for traffic noise reduction. Without the vegetative barrier, the observed mean noise levels were 78 dB. On average, vegetative barriers (moderate to dense) were able to reduce traffic noise by 9–11 dB. Trunk size was found to be linearly related to traffic noise abatement and synthetic barriers were found to be inferior to tree belts both psychologically and in absolute values of noise. – Applied Acoustics

Wow!

Although new to North America, mixed species hedgerows have been a defining feature of the English and Irish countryside for hundreds of years. Devon Hedge Group, Green Restoration Ireland, and others are publicizing their benefits for wildlife and climate and establishing maintenance best practices.

Like green veins that run throughout the British countryside, hedgerows can act as corridors teeming with native wildlife. In a study made in 2011–2013, the Devon Hedge Group’s Rob Wolton identified an astonishing 2,070 species, mainly insects, in one 85-metre stretch of hedge at a nearby Hatherleigh farm. Small woodland birds and dormice nest among the nooks and crannies, and badgers, foxes and rabbits navigate through these sheltered highways to reach nearby woodlands. Bats feed on the insects living among these linear nature reserves, barn owls hunt along the high banks for small mammals and the berries and fruit growing on the shrubs provide food for other creatures throughout the seasons. – BBC Countryfile

Related Resources

• Curious about sound levels at your house? Try your Apple Watch or a decibel reader like this app for iPhone.

• Want to know the noise level at a property? Most listings on Realtor.com include noise level.

• Want to find plants that support birds? Check out Audubon’s Native Plant Database.

• Ready to make a bubbler for birds (or noise mitigation)? Here’s a how-to video.