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  • Eco-friendly, Landscape Design, Plants

    Working hard on the ground

    By vsudol

    March 05, 2008, 2:33PM
    Cascading groundcovers clothe a stone wall. In the foreground is a spreading ornamental thyme, Thymus citriodorus 'Archer's Gold.'

    Groundcovers suffer from an image problem, poor things.

    While we dote on our roses, welcome our spring bulbs, applaud our prolific annuals and sigh over the sheer splendor of our flowering trees and shrubs, we yawn at our groundcovers. The ones we know too well -- ivy, pachysandra and periwinkle -- are pedestrian plants that rarely merit a second glance. They're overused and maybe just a tad boring.

    It may be time to take a second look at a class of plants that can be more than just the utility infielders of the landscape. There is a rapidly expanding array of species on the market that can be put to all sorts of clever uses, and new research to help homeowners choose the best plants for the job.

    Groundcovers are, generically, low-growing perennial plants that, once established in massed groupings, can crowd out weeds, protect soil from erosion and carpet the soil under shrubs and trees. But that's not all. They can stabilize slopes too steep to mow, dress up retaining walls, fill in between stepping stones, direct foot traffic and complement container plants.

    Best of all, they can reduce maintenance and the need for chemical controls, two goals that should warm the heart of any homeowner.

    We owe some of the renewed interest in hardy groundcovers to public road departments that are looking to cut the cost of maintaining medians and rights-of-way. Specifically, we can thank the New York State Department of Transportation, which has funded research at Cornell University over the past several years aimed at identifying species that can thrive without regular mowing, watering and dosing with fertilizers, pesticides and weed-killers.

    Three researchers -- Elizabeth Lamb, Brian Eshenaur and Leslie Weston (who has since left the university) -- have led the charge with groundcover trials in upstate New York and on Long Island. Tending test plots of familiar and underused species, they have generated new information of tremendous value to property owners in the Northeast who want low-impact landscapes.

    Stepping stones are surrounded by the tiny flowers of blue star creeper, Isotoma fluviatilis, in this colorful passageway.

    "What works for road departments can work in home gardens, too," says Lamb. "What homeowner doesn't have a spot needing a low-maintenance groundcover that can stand up to drought, road salts, deer predation, weeds and insect pests -- and look good, too?"

    When we use appropriate groundcovers, we beautify without the burden of care that expansive lawns and conventional perennial borders require. You have to start out right and pay attention as newly planted groundcovers get up and growing, but with a little time and care, the tantalizing long-term goal of a reduced workload is within your grasp.

    Instead of growling over weeds springing up between patio paving, plant a "grout" of creeping thyme that releases a refreshing scent underfoot. Dress a bare retaining wall with cascades of moss phlox, stonecrops and leadworts for bursts of seasonal blooms. Surround an established tree with a bed of lady's mantle and stop reaching for the string trimmer every time you mow. Plant a bank with creeping asters, milkweed and catmint, and enjoy a bonus of flowers and butterflies.

    Bottom line: Groundcovers are workhorses with thoroughbred features, and it pays to make their acquaintance.

    "The trick is to get gardeners to use them -- and plant suppliers to grow them," says Weston. "We put in a demonstration plot of mixed groundcovers at one garden center to show people that these plants can have real pizzazz. It's a new way to think about low-maintenance design."

    Frances Hopkins, owner of the Oregon-based Under A Foot Plant Company, needs no persuasion to hop aboard the groundcover trend. She markets a line of Stepables, plants that can take some foot traffic.

    "I've been traveling all over the world trying to find new colors and textures in groundcover plants," says Hopkins. "People are looking for plants that solve problems in those troubled areas of their yards. Most of these plants are tough as nails and most people can successfully grow them coast to coast."

    Groundwork

    A variegated ground ivy, Glechoma hederacae, carpets the ground under tulips.

    The key to a good groundcover experience is picking the right plant for your site.

    Spend some time assessing the location: Is it sunny or shady? Is the soil rich or nutrient-poor? Do you need evergreen cover or, as in the case of a summer home, is winter attractiveness less of an issue? Do you want flowers or is handsome foliage sufficient? How tall do you want your groundcover to grow?

    No groundcover takes as much routine abuse as lawn grass. But if your groundcover must tolerate some foot traffic, Hopkins' company has a search function on its site, stepables.com. Check off your requirements, exposure, soil conditions, height and growth rate and the site will generate suggestions from the catalogue of 150 plants.

    More helpful websites are listed at the end of this story.

    The plant you choose must be hardy in our climate (New Jersey falls into Zone 5 in the northwestern Highlands, Zone 7 in southern counties and Zone 6 everywhere else). And it must suit, aesthetically, the location you have chosen. In general, select fine-textured plants for small areas and more coarse ones for larger swaths.

    You can investigate plants on the list of top performers that have proven in trials to be good at suppressing weeds (see "Some of the best"), a key characteristic for most gardeners. Pore over more comprehensive results of Cornell's groundcover studies at Cornell Allstar Groundcovers. Plants are sorted by the light they require and by their ability to out-compete weeds.

    In one of her articles, Weston describes the profile of plants most likely to effectively smother weeds over time. The species should rapidly establish a dense canopy and prevent light from reaching the soil where weed seeds lurk -- generally, plants at least 10 inches tall will do the job better than low-growing ones. Good choices include lady's mantle, catmint and coral bells.

    One fascinating aspect of the Cornell research is the discovery that some plants suppress weeds through alleopathy, or the release of chemicals that inhibit the growth of nearby plants. Creeping phlox and coneflowers are known to share this trait; catmint and goldenrod are two more chemical warriors identified by Cornell's studies.

    "Some of these plants might not be suitable for planting under shrubs or trees," warns Lamb. "And while you want something that fills in quickly, you don't want it to spread everywhere. That's the chief characteristic of invasive plants."

    Gardeners need to be aware of what plants have made pests of themselves with rampant habits. Invasive species, many of them imported, can make you rue the day you brought them home. Familiarize yourself with the worst offenders, some of which are listed in the story "Gardener beware."

    More tips from the Cornell team: Succulent plants like sedum, thyme, lamb's ear and creeping phlox are best at tolerating hot, dry sites. Goldenrod is especially resistant to damage from road salts. Catmint, creeping phlox and lady's mantle are undisturbed by deer.

    When it comes to design, Weston believes "the more, the merrier" and is an advocate of mixing groundcovers in any given bed. Silvery lamb's ears, deep-hued heucheras, grass-like liriope and blooming species like goldenrod, catmint, sedum and asters can provide a constantly shifting array of flower color against a constant background of foliage color and texture.

    Woolly Thyme, Thymus praecox, stabilizes a slope with creeping phlox blooming above it.

    Planting and maintenance
    Once you've pondered the prime issue -- location, location, location -- reconcile yourself to this fact: The key to success with groundcovers is preparation, preparation, preparation. If you think you can pop plants into rough ground and forget them, you are almost sure to fail.

    To get your bed off to a good start, remove all grass either by using a nonselective herbicide like Roundup (follow directions closely) or by smothering sod with layers of newspaper, cardboard or plastic sheeting. Unless your soil is in tip-top shape, now is the time to add amendments, fertilizer, lime if needed and grit or sand to improve drainage. Few groundcover plants will tolerate permanently damp or waterlogged soil.

    If you are planting on a slope, you will need to dig individual pockets for each plant to stabilize the soil after planting with mulch or netting to prevent plants from becoming dislodged by rain. It is especially important on slopes to plant in a staggered pattern rather than in rows that allow water to run off in straight lines.

    Scotch moss, Sagina subulata 'Aurea,' softens an informal stone stairway.

    How many plants will you need? Most people underestimate the number required. Faster growing plants can be spaced at a greater distance than those slow to spread, and gardeners on a budget can increase spacing if they are willing to keep the bed weeded until groundcovers establish themselves.

    Consider a fairly small area of 100 square feet (10 feet by 10 feet). You'll need 178 plants spaced 9 inches apart, 225 plants spaced 8 inches apart or 400 plants spaced 6 inches apart. The Stepables site has a calculator that can help you estimate your needs.

    Watering is important in the first few seasons as groundcovers develop the root system that will support their future growth. Mulching the soil before or after planting will help retain moisture, and also is your first line of defense against the inevitable weeds.

    Once groundcovers are planted, you won't be able to use contact herbicides to attack weedy invaders. There are pre-emergent herbicides that inhibit the germination of annual weed seeds, but Hopkins doesn't recommend them since she feels they stall the growth of desirable plants, too. Regular hand-weeding will be required until groundcovers fill in.

    "Weeds happen," she says. "Realistically, you have to exercise due diligence, as in a weekly weeding for the first season or two."

    An annual feeding will help groundcovers thrive. While many need only light pruning, some benefit from a yearly mowing, down to about six inches, to stimulate vigorous new growth. They will look terrible at first, but quickly produce a new flush of growth.

    Perhaps the most prudent approach to incorporating groundcovers into the landscape is to start small.

    Take one hard-to-maintain spot -- a narrow passage through a side yard, a circle of ground beneath a tree or a weedy area around a retaining wall -- and experiment with selected groundcovers. As plants fill in, you can expand the zone devoted to them and can propagate your stock to carpet more space.

    When you've solved one problem spot, move on. There's a whole new world of amiable plants ready and able to give you respite from the tedium of repetitive chores like mowing and weeding. Who doesn't want less work and more landscape interest? Groundcovers can put both at your feet.

    More helpful web sites can be found here, here and here.

    For more on selecting a well-behaved groundcover, see "Picking a groundcover" elsewhere on this blog.

    To reach Valerie Sudol, write to her at The Star-Ledger, 1 Star Ledger Plaza, Newark, N.J. 07102-1200 or e-mail her here.

    Previous story: Big Easy flower show

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