How to create a lovely lush, evergreen city garden to withstand climate change; plus, removing lead from soil

Dear Avant Gardener, The land under my house was flooded during Hurricane Katrina and is probably contaminated. I have some planting areas on each side, mostly shaded. I have let "weeds" grow and have left leaves and vegetation on the ground to add organic matter to the soil. The plants I have bought are mostly edibles in pots; the former owners or birds and squirrels planted the rest. I am going to get a rain barrel ASAP to water the garden, because we have had an historic drought here and salt water is creeping up the Mississippi River towards New Orleans. I have a lot to learn about sustainable gardening and what native plants are best for my place. – Shirley, New Orleans

F@#king flooding! A recent study suggests 18 million American homes are now at substantial risk for flooding – twice the prior federal estimates. And that hit home for me – literally – when a seven foot surge swamped our Florida property last month.

Get tested

Many New Orleans (and other historic city) yards have elevated lead levels. I recommend that you have your soil tested by LSU’s College of Agriculture; kits are available at numerous New Orleans locations. For $5 per sample, you can test lead and arsenic levels and, separately, salt levels. If your tests show elevated lead or arsenic (a carcinogen), see precautions and phytoremediation options in “How” below.

In New Orleans, it has been discovered that two third of homes and yards have an elevated concentration of Pb. …The age of a home is the most common indicator of Pb concentration levels. ... Many homes in historical New Orleans were built before the ban of Pb paint in 1978. The climate is hot and humid which means there is a lot of moisture. This moisture can cause the Pb paint to chip off and contaminate soil. After Hurricane Katrina, Pb contamination from older buildings polluted surrounding areas after being remodeled. ... Pb commonly accumulates on the soil surface, where it can remain for up to 2000 years. – Journal of Geoscience and Environment Protection​

You might also find the routine and organic material test results interesting. You should see high organic material levels thanks to all you’ve added over the years. Just remember that the optimal organic matter level is less than 10 percent; many well-meaning organic gardeners overdo it. Also, please ignore soil amendment recommendations in the test results, which are based on traditional agriculture and horticulture and not generally appropriate for native plants.

Soil salinity an issue nationwide

I admire your far-thinking concern about soil salinity, a growing problem from the Carolinas to California. In the parched West, one cause is using recycled water for landscaping. In northern states, road salt can cause soil salinity issues. In the Southeast, inundation (like here in Cedar Key) and groundwater salinity as a result of saltwater intrusion (as in New Orleans) are threatening vegetation.

Elevated salinity levels cause crop yield declines, coastal forest loss, salt-tolerant invasive species takeover, eutrophication and marsh migration. Few crops can grow in areas with salinity levels that are constantly greater than 2 parts per thousand, which is a lower concentration than you’d typically find in a salt water intruded field. – USDA​

In addition to watering your plants with rainwater, you can plant salt-tolerant shrubs and trees as the backbone of your ornamental plantings. Although it’s unlikely that New Orleans’ extreme drought – 25 inches less rain than the 30-year average of 65 – will continue, unusual weather events are increasingly common.

Fortunately, some of the best native trees and shrubs for a characteristic New Orleans shade garden are salt tolerant. And they will not generally require watering once established.

Classic Southern city garden style

Beyond salt and possible contamination, the shady conditions of city gardens like yours are unsuited to trendy native prairie wildflowers, which love sun. But fear not; you have other options. And, like most native plant solutions, this one is especially authentic to your New Orleans home.

I’m talking about the lush, evergreen gardens of historic coastal cities of the South, like Charleston and Savannah, as well as New Orleans. These were inspired by Parisian private gardens and are characterized by symmetry, enclosure, lush evergreen foliage, and few flowers.

In New Orleans – with its warm weather, rich sandy loam soil, and (usually) plentiful rain – a blowsy, overgrown look is typical, with draping foliage and ferns growing from every crack. Charlestonians, meanwhile, adapted the style for narrow, shady side yards like yours by planting long, low hedges and climbing vines, as you can see in the "Wow!" images below.

Lush evergreen city gardens

1. Style cues: Symmetry, enclosure, lush foliage, few flowers

2. Activities: Passing through, admiring from inside

3. Furniture: Metal cafe chairs and tables

4. Hardscaping: Trellis; iron gates; brick walls; gravel, brick, or limestone paving; iron and stone planters; stone statuary

5. Plants: Interesting foliage, especially big-leaved evergreens like magnolias, rhododendrons, hollies; clipped hedges; vines scaling walls; few flowers in limited white or cool color palette

6. Size: Small and enclosed; long, narrow side yards

7. Best locations: Historic cities and pre-1940s suburbs across the country; small enclosed gardens anywhere

8. Work required: Low, especially if small shrubs are chosen to avoid clipping; no mowing

Salt tolerant structure plants

The backbone of these Southern gardens has always included salt-tolerant evergreen native plants with interesting foliage, notably palmettos, sweetbay magnolias, and live oaks (with native Spanish moss, natch). In place of exotic and ubiquitous box, you can use dwarf varieties of yaupon holly and wax myrtle, both fantastic native, evergreen, salt-tolerant hedge plants. And they are not threatened by blight, as box is. Alternatively, if you prefer a more natural, tropical look, you can use groups or hedges of Adam’s needle, another salt-tolerant native that thrives in part shade.

Perennials for that lush, leafy look

To lean into the lush, leafy look, I recommend adding a vine or two and lots of ferns to your structure. And you’re in luck because you’ll find excellent options at the upcoming City Park Conservancy’s Pelican Greenhouse plant sale: sensitive fern, royal fern, coral honeysuckle, and trumpet creeper. The first three tolerate shade and trumpet creeper part shade, as does American wisteria, another great option. These ferns and vines are not salt tolerant, but they are faster growing and cheaper to replace than the shrubs and trees. Salt tolerant, flowering options for shade include sweetscent (Pluchia odorata) and standing cypress (Ipomopsis rubra), but these are harder to find.

Installing all these locally native plants will qualify your yard as a Louisiana Certified Habitat. Consider applying, so your neighbors will see they can make a beautiful, traditional Southern garden with native plants.

Adapting your Yardenality™

Are you disappointed that my solution doesn’t include more flowering plants? In my Yardenality™ poll, most readers preferred flower-filled garden styles. But the first dictum of ecological landscaping is “right plant, right place.” If salinity is not a serious issue, you’ll find plenty of woodland flowers for your shady yard. And if you have sunny areas, meadow plants will thrive. That said, I hope you will lean into the limitations of your site. As Charles Eames said, “Design depends largely on the sum of all constraints.”

— The Avant Gardener

Why, How, Wow!

Why?

As floods in old industrial sites in urban areas become more frequent, so will contamination with toxic materials – disproportionately impacting lower income residents and people of color. When the waters receded from New Orleans after Katrina, the soil salinity was lower than expected, but researchers found serious heavy metal contamination, mostly lead and cancer-causing arsenic.

Climate change is raising the probability that urban flooding will remobilize and spread hazardous wastes contaminating the soils of potentially thousands of former industrial sites, increasing exposure risks for urban residents generally—a finding suggestive of a new kind of climate injustice for more socially vulnerable communities. – Environmental Research Letters

How

Traditional remediation of contaminated soil involves digging it up and replacing it. As an alternative, phytoremediation – using plants to remediate soil – is a hot research topic.

Phytoremediation is a nontraditional approach to remediate contaminated soil involving the use of green plants. Phytostabilization involves the use of plants to stabilize contaminants to reduce human exposure. Phytoextraction involves the use of plants to accumulate contaminants in aboveground shoots, which can be harvested to recycle or discard. – Applied Spectroscopy Reviews​

This method is often preferred over ex-situ [off-site] because of low cost, reduced impact on the ecosystem, environmentally friendly, provides easy monitoring of plants and the possibility of uncomplicated recovery and re-use of valuable metals. – Microchemical Journal​

In Louisiana, scientists tested various ornamental garden plants – all tropical exotics – to provide an inexpensive and easy way to remediate pervasive low-levels of lead around homes in New Orleans, a serious threat to human health. All the tested plants removed lead from the soil to some extent. Shade-loving brake fern (Pteris vittata), however, accumulated far more lead than any other species, especially at high soil concentrations.

Leaves of brake fern accumulated nearly 450 ppm in dried leaf tissue. This species has historically been documented as a hyperaccumulator in previous research. ... The importance of this is that it gives homeowners a shade loving plant that can be planted in deep shade near a tree where grass does not receive enough light to thrive. – Journal of Geoscience and Environment Protection​

Homeowners with high but not hazardous levels of lead in their soil (see chart below) might experiment with phytoremediation, growing plants and then removing them, disposing of them properly (contact local hazardous waste removal for guidance), then retesting. For those with the right conditions, brake fern is available online.

In the meantime, homeowners with heavy-metal contaminated soil should take precautions to avoid inhaling or ingesting contaminants, according to the USDA:

• Grow only ornamental plants, not fruits and vegetables.

• Wear gloves while working in these soils.

• Use raised beds filled with uncontaminated soil for growing food.

• Keep dust at a minimum with dense planting.

• Wash vegetables carefully to remove soil and dust deposits.

Wow!

Note how symmetry, varied foliage textures, and creative use of vertical space enlarge and enrich these classic Southern side yards. Imagine similar designs with the above native plants. That's wow!

Related Resources

• Rethinking your urban yard? See The #1 Design Solution for an Urban Backyard.​