Ground Covers

Ground covers have a tremendous appeal to gardeners. Just think, here's a plant that grows almost anywhere, spreads around, produces flowers, and will grow in your garden. Not only that, gardening folklore says that once a ground cover is established, it will prevent weeds from germinating and growing. Gardeners who don't know any better also say that ground covers will compete with grass and crowd it out.

Myths

The reality is that there are some good features of ground covers, but there are also some myths running around the gardening world as well. Here are a few myths I've heard.

  • Ground covers will eliminate grass from your garden. I wish I had a nickel for every gardener who told me that because they had heard it somewhere. Grass is a strong grower and one of the most competitive plants in the garden. There is no ground cover that you can grow that will smother grass. In fact, the opposite is true; you'll have to kill off the grass to establish the ground cover, and then you'll have to weed out any invading grass on a regular basis. This is particularly true for sunnier areas. Shadier gardens have less trouble with grass-invading ground covers. After all, grass doesn't grow well in the shade; that's why we are using a ground cover in the first place. Having said all that, a well established ground cover will reduce weed-seed germination because it shades the soil, reducing the sunlight needed by many seeds to induce germination.

  • Ground covers are no work. The reality is that a good ground cover is work. It is, however, a different kind of work. There is no weekly mowing, but you do have to weed and prune off dead chunks and replace winter damage and perform other chores associated with growing plants. You can grow one single plant or you can grow a bunch of them together and call them ground covers. The reality is that it is still gardening and will require some labor.

  • Ground covers are fast to establish themselves. Well, maybe. Some such as English ivy are pretty fast, establishing a good mat in a year or two. Others such as Vinca major (periwinkle) will take a few years to establish themselves. In the meantime, you'll have an ugly bare spot to contend with. Using an evergreen shrub as a ground cover means a multiyear investment in patience. That's the reality of establishing plants. They work on their own timetables and none are as quick to establish as turfgrasses.

  • You don't have to water or feed ground covers. Why not? They're in the garden and, if they're under the shade of a tree, they are competing with the tree for available water and nutrients. I guess you don't have to water and feed ground covers, and in return, they don't have to grow very well either.

  • Some ground covers will bloom all summer. I know of only one ground cover that blooms for an extended period during the summer and it is a weedy plant not suited for a decent garden because of its rapacious nature. Crownvetch has a muddy pinkish flower that blooms for a long time in the summer and is suited only for wilder areas or holding down the soil on steep slopes. It self-sows so prolifically that, if allowed into a good garden area, it will quickly take over. It is quite difficult to eradicate once established. Ground covers are normally grown for their foliage and ability to … well, cover the ground.

So, is there a plant that will grow in the sunlight provided by the average closet, and flower all summer with absolutely no care? Sorry, only in our dreams. To make things easier though, here are a few things to think about before you plant your ground covers and then I'll give you a listing of some of the more popular plants.

Selecting a Good Ground Cover

Remember, this is a book about growing lawns so the only reason we're choosing a ground cover is that the grass won't grow in that spot. As the author, I can only say that if grass will grow and you don't want to grow it, then you bought the wrong book.

If drainage is poor and the area is wet, you have the option of either draining the ground or installing a bog garden. Similarly, if pH (acidity) is a problem, then you can modify the soil with either lime (to make it less acidic) or sulfur (to make it more acidic). If grass won't grow because of soil fertility problems, get out the compost and fertilizer and correct the problem.

This leaves shade as the final reason the grass won't grow. So, you can either cut down the trees creating the shade (a poor idea in my opinion) or grow an alternative ground cover in this area. I've listed some of the more common ground covers on the next few pages along with some comments about how to grow them.

Preparing the Bed

This is the easy part to write about; the instructions are quite simple. Prepare the ground cover bed in exactly the same way as you would any other lawn area. Remember these rules:

  • Make sure the ground is level and free of all stones, weeds, and debris.

  • Till organic matter such as compost into the soil if possible. Otherwise, if there are too many tree roots, spread it over the ground where you can incorporate it as you plant.

  • Use a starter fertilizer, something with a high phosphorus (second number) and low nitrogen (first number) ratio to boost root growth.

  • Rake the area to prepare for planting and then install the plants.

The only difficulty you may have is tree roots in the planting area and you don't want to chop them up to cultivate the soil. Neither do you want to bring in soil to raise the soil line above the roots (this cuts off oxygen from the tree roots and damages the tree). The only option here is to work manually around the major roots, hand-digging the spaces between the roots and adding compost to these areas. You will wind up chopping up many of the tree's smaller feeder roots but, although this isn't ideal, the tree will survive.

Install the ground cover plants in the worked areas. If you adequately water and feed them, they will colonize over the top of the roots.

The only caution is that if you are planting ground cover plants this far under a large tree, make sure the ground cover plant is one that will tolerate such deep shade conditions. Remember that grass won't grow under there; you have to plan for a deep shade-loving perennial. We had the perennial plant Hosta growing under a very large wild apple tree, and the difference between the plants on the outer edges of the tree's shade and those right up against the trunk was quite pronounced. Those on the edges were bigger and healthier than their more deeply shaded neighbors. Given that watering and feeding was the same for the entire bed, the only difference was in the amount of sunlight the plants received. Even shade-tolerating plants will not grow as quickly or bloom as heavily in the deep shade close to tree trunks. This problem is worse under evergreen trees than under their deciduous cousins.

SLOPED GROUND

One other reason why you might want to consider a form of ground cover is the slope of the ground. If it is too sloped to easily mow, ground cover can be a good alternative.

Planting

The closer you plant your ground cover, the faster they will fill in. That seems pretty obvious. The problem is that the closer you plant your ground cover, the more plants you require, and the more expensive it becomes to create that perfect garden carpet. Hey, nobody said gardening was cheap. If you need a large number of plants, you can usually make a deal with your local garden center (particularly if you order them several months ahead of time) to obtain landscaper plants at almost-landscaper pricing.

Landscaper plants differ from retail plants in that the retail plants are almost always sold in small pots with a tag or in four- or six-packs with a tag. These are more expensive. Landscaper plants come in a large flat with no pots, no tags, and minimum amounts of soil on the roots. They are much cheaper, but they take a bit more care to plant (due to the lack of soil on the roots) and more care (more watering) after planting to compensate for the reduced root surface. So, if you have a lot of plants to install—at least several hundred (each landscaper flat will have 50 to 100 plants, depending on the variety)—then you might try seeing if your local garden center can help you out. Plan ahead. Do not go in on the day you want to plant and ask for landscaper pricing—it isn't going to happen.

DIAMOND GRID PATTERN

The actual installation of each plant is as simple as digging a hole and burying the roots. The key is to make sure that the soil is damp when the plants are installed and to water the area well immediately after planting. When I say well watered, I mean turn the area into a mud pie. The plants will look horrid—half-wilted, mud-spattered, with leaves half covered in mud—but they will quickly recover and start growing. Without the mud bath, the plants will struggle. Water daily for the first two weeks and then slowly cut back. Make sure the ground stays moist for at least the first two months if you want to get those ground cover plants to grow and compete with the trees.

When the pros install ground covers, they plant them in a diamond grid pattern. Each plant goes in the middle of the diamond so if you look at the planting from a distance, each plant lines up in several different directions with other plants. This ensures a quick fill-in time with a consistent root area given to each plant.

Hardiness and Winter Protection

For the most part, ground covers should not require winter protection. If you have to work to protect your ground cover with mulches or covering insulation, get a new ground cover. Ground covers are about reducing work. They are not a garden feature that should demand extra work.

Do not feed ground cover in the fall. Feeding only stimulates tender growth and this tender growth is more easily winterkilled.

A local garden center will be able to give you hardiness ratings for most of the ground covers in your gardening zone.

Watering

Ground covers require water, particularly if they are in the shade and rootzone of large trees. You can plan on applying at least 1 to 1½ inches of water a week to the ground cover to keep it growing. Slightly heavier watering, at 2 inches a week, will be necessary during the heat of the summer when the trees are at their greediest, sucking up all available moisture.

As with lawns, the best thing to do is establish a sprinkler system or in this case a drip irrigation system to automatically do the watering for you. Not only does the sprinkler ensure you have equal water applied to all plants, it also cleans the plant leaves since it doesn't create the same water flow as a hose.

HOW MANY PLANTS TO FILL YOUR BED?

Use this table to find the number of plants you require to fill your bed. The left-hand column is the spacing you've chosen. Divide the square footage of your bed by the number in the right-hand column. The resulting number is the number of plants you need to fill your bed at your chosen spacing.

SPACING IN INCHES DIVIDE YOUR BED FOOTAGE BY:
4 0.11
6 0.25
8 0.44
10 0.7
12 1.0
15 1.56
18 2.25
24 4.0
30 6.25
36 9.0
48 16.0
60 25.0

As with all garden plants, do not water after August to allow the plant to begin to harden off. This bit of water stress at this time (remember that the fall rains will be ample to sustain life) will help the plant get a bit tougher in preparation for winter.

Controlling Weeds

Controlling weeds in ground covers is mostly done using the Armstrong method. (Arm strong—strong arm—get it?) Yes, that's right-by pulling the weeds out by hand.

Hand weeding is a time-honored tradition in ground cover patches. Give it a thorough going-over first thing in the spring as soon as the weeds start to germinate.

Corn gluten will stop weed seeds from germinating in any ground cover plant and does not harm established plants. Unfortunately, the same cannot be said about other pre-emergent herbicides used on ornamental crops. Read the label before you use any chemical as a pre-emergent weed control in ground covers.

Mulches work well for many ground covers. Lay down the mulch in the fall and the perennial ground cover will grow up through the mulch while the annual germinating seeds either will be prevented from germinating or can easily be weeded as they try to establish themselves. This, of course, implies that you have removed all perennial weeds (or stopped them from establishing themselves), because they will grow through a mulch as well. It is difficult to mulch evergreen ground covers or ground-hugging plants such as Ajuga; the mulch is difficult to apply in the first case, and smothers out the plant in the second.

Do not use lawn herbicides on ground covers. These will, for the most part, treat the ground covers as weeds and either kill them or stunt their growth. Also, when applying chemicals to the lawn, take care that they do not drift over onto the ground cover area.

MORE IS BETTER

A general rule of thumb when planting perennials for wildflower establishment is to use one plant per square foot in your design. This will fill in the garden area fairly quickly at a reasonable price. If you plant fewer than this, say one plant per 2 square feet, you'll have open spaces for weeds to colonize. If you plant more than this density, it will fill in more quickly. As with much of gardening with plants—more is better.

Feeding

A ground cover is much like any other garden plant, it will only thrive if it is fed properly. The easiest way is to spread compost over the plants in the very early spring before they start to grow. The compost will settle down to the soil and the new growth will come up through it. With evergreen ground covers, still apply compost first thing in the spring, but then water the area heavily to wash the compost off the leaves down to the ground.

Do not apply lawn fertilizer to ground covers. Lawn formulations are too high in nitrogen and will burn the ground cover. Take care to avoid sprinkling lawn fertilizer onto ground covers that are on the edges of lawn areas. This is particularly true of weed-and-feed formulas, not only will the fertilizer burn the ground cover but the herbicide will damage it as well—a double dose of problems.

General plant food with a balanced formula, (e.g., 10-10-10) can be substituted for compost when applied at the correct amount (see the label for directions). However, compost is still the undisputed champion at feeding ground covers.

Pruning

GROUND COVERS: SNEAKY AND AGGRESSIVE

The reality of ground covers is that, by and large, they are aggressive spreading plants. They will not be content to stay where you plant them. For example, a common name for Ajuga is bugleweed. Once this plant escapes from the ground cover area into the lawn, we now call it buglelawn. The Ajuga is too low to be hurt by the lawnmower and it will take herbicide or determined hand-weeding to eliminate it.

We made the mistake of allowing a Lamiastrum cultivar out into the garden, because I thought I wanted its silvery foliage to mix with the plants that were already there. It took three years of steady digging to remove it from the bed after I decided it was no longer welcome. Most ground covers have a place in the plan of the garden, but that place is usually not in a flower bed.

A sharp shovel is necessary to limit the expansion of most ground covers once they are happily established where you want them. Use this shovel every spring and immediately on seeing an escapee.

Generally, if you want a ground cover for low maintenance, you just don't prune it. However, depending on the plant, you may find that some pruning is a good idea at certain times of the year. For example, some gardeners like to cut the dead flower stalks off spring-flowering plants with a string trimmer to neaten up the look of the ground cover. With plants that get too tall and leggy such as Aegopodium, a string trimmer or lawn mower set to 4 to 6 inches does a good job of bringing the plant back to size and thickens up the stand. Instead of a leggy mess, you'll have an attractive planting after the week or two it takes the plant to regrow. With either of these situations, do not scalp the plant too close to the ground or you'll weaken it. With some flowering plants such as geraniums, a heavy pruning after the first bloom flush will bring on a second bloom later in the season.

Never prune a plant in the late summer or early fall before it has been knocked back by a hard frost. Pruning in late summer will cause the plant to throw new shoots and these may not harden off enough by winter to survive. The mess of the dead foliage the following spring and the weakened plant (it spent all its energy producing a shoot that didn't survive to replenish the root) are not the kinds of situations you want in your garden.

We've discussed the most commonly used perennial ground covers, but you should also understand that any plant—if installed closely enough together—can act as a ground cover. A perfect example of this is Nepeta or catmint. If planted 18 inches apart, Nepeta will grow together to form a wonderful mass of violet blooms and fragrant foliage. There are several ornamental forms that are a delight in the perennial garden and would be even more delightful planted in a mass of ground cover.

If you have a favorite plant and it grows well enough in the area you want to cover (instead of grass), then experiment with it as a ground cover by planting it close enough to its neighbor. Don't be sidetracked by a list of common plants if you have, or desire, an uncommonly delightful garden.

MAP OF NORTH AMERICAN TEMPERATURE ZONES

GROUND COVER PLANTING CHART

NAME APPROX. ZONE HARDINESS SPACING (INCHES) COMMENTS
Aegopodium podagraria (Goutweed) 3 12 Vigorous grower, will smother less aggressive plants. Hardy and aggressive, 18 inches tall. Mow when it gets straggly. Easy to grow, hard to eliminate.
Ajugaspecies and cultivars (Bugleweed) 4 12–18 Vigorous, semi-evergreen. Dense mats, 4 to 6 inches tall. Full sun to heavy shade. Prefers moist but well drained soils. Shallow rooted, so dry soils are a problem. Protect from winter winds.
Asarum canadense (Wild ginger) 3 12–18 Deciduous, fast growing, 6 inches tall, hardier than the European form. Wonderful for shade to dense shade. Fast establishing.
Asarumeuropaeum (Wild ginger) 4 12–18 Evergreen, 5 to 6 inches tall, slightly more tender than native form. Likes organic soils in shade, slow to establish. Without snow cover, protect from harsh winter winds.
Brunnera macrophylla (Hardy forget-me-not) 18–24 Deciduous, forget-me-not type blue flowers in spring, rough foliage, 18 to 24 inches tall. Grows almost anywhere but does best in part shade with good organic soils. Plants grown in full sunlight will show scorched leaves in midsummer.
Campanula poscharskyana (Bluebells) 3 18 Self sows with abandon, and will pop up almost everywhere it can find a protected spot to germinate. Grows 12 to 18 inches tall. Long bloom time, excellent blue flowers. Grows in sun to part shade in almost any soil except heavy clay and hot and dry conditions.
Cerastium tomentosum (Snow on the mountain, Snow-in-summer) 3 24 Vigorous grower with silvery foliage, 18 to 24 inches tall. Inconsequential white flowers produced on this rampant grower in midsummer. Wants full sun. Well drained, poor soil is best. Feed it and it grows open, floppy, ugly stems.
Convallaria majalis (Lily of the valley) 2 12–18 Deciduous, shade lover that grows 8 inches tall. Very fragrant spring flowers. Will slowly expand to cover a wide area. Will grow in almost any soil. Too much sun burns the leaves. If grown under trees, feed every spring to help it compete.
Coronilla varia (Crownvetch) 3 18 Deciduous, sprawling, invasive grower, 18 to 24 inches tall. Good for banks in rough places, (e.g. steep banks beside highways). Slow to establish, hard to eradicate. Muddy pink flowers in June to July. Does best in full sun, tolerates shade. Mow to keep compact.
Epimediumspecies (Barrenwort) 3–4 12 Deciduous in North, semi-evergreen in warmer areas, 8 to 12 inches tall. Flowers in May, colors depend on species. Slow growing but nice plant for a garden setting. Best in part to full shade in rich soils. No dry soils. Does well under trees if kept damp. Excellent plant.
Galium odorata (Sweet woodruff) 4 12–18 Deciduous, (evergreen in warmer climates) delicate looking, does best in medium to deep shade in moist soil with adequate organic matter. Combine with English ivy.
Geranium species (Cranesbill) 3 18–24 Easy to grow, thrives in organic soils in sun to part shade, 6 to 18 inches tall. Do not fertilize or will grow sparsely. Prune heavily after blooming to rebloom. May need thinning or rejuvenation every 4 to 5 years.
Hemerocallis (Daylily) 2–3 18–24 Most are deciduous, some southern varieties are evergreen, 12 to 36 inches tall. Flowers heavily in July (although newer forms have longer bloom times). Grows best in sun to light shade, easy to grow. Fills in quickly but not an aggressive spreader. Not bothered by pests or disease.
Hostaspecies (Funkia, plantain lily) 2–3 18–24 Deciduous, part shade to shade lovers, 6 to 35 inches tall. Blooms in Jury with lilylike stems. Will grow in sun if kept moist, but otherwise does poorly in sun. Many different cultivars to mix and match for a wonderful garden design.
Lamiastrum galeobdolon “variegata” (Yellow archangel) 4 24–30 Deciduous, trailing, vinelike, aggressive spreader, mounding 18 to 24 inches tall. Yellow flowers in May to June. Grows almost anywhere except deep shade and dry soils. Give it water and it will grow anywhere! Easy to grow, hard to kill.
Lathyrus latifolius (Perennial sweet pea) 3 24–30 Wandering vine that mounds or scrambles up over banks. Self-sows but is not dense, plants can grow up through it. Flowers in midsummer. Grows best in full sun but will tolerate part shade. Once established, it survives neglect easily in rough places.
Lysimachia nummularia (Moneywort) 3 18 All of the Lysimachia family are rampant spreaders, from the shortest like L. nummularia (2 inches) to the taller like L. clethroides (3 feet). Grows in sun or shade and has bright yellow flowers in midsummer. Prefers damp soils. Will invade the lawn and is difficult to eradicate because of its low-growing nature.
Pachysandra terminalis (Japanese spurge) 4 12 Evergreen plant, slow to establish. Pest free, 12 inches tall. Grows best in damp shade with rich soil but will tolerate dry part shade once established. Will burn in the harsh sunlight.
Phlox subulata 3 18 Evergreen to semi-evergreen depending on cultivar, but may burn a bit over the winter, 4 to 6 inches tall. Sun to light shade. Blooms in a variety of colors in early spring. Grows best in well-drained soils; clay soils will kill it. Does not require heavy feeding.
Polygonum cuspidatum var. compactum (Fleeceflower) Decidous. vigorous grower, 12 to 24 inches tall. Spreads by underground rhizomes—fast! Quite attractive foliage—reddish shades depending on season. Grows best in full sun, in almost any soil except heavy clay. Shear first thing in spring before new growth starts for best performance. Do not let loose in good garden.
Potentilla tabernaemontani 4 18–24 Deciduous (may be evergreen in warm climates). 3 to 6 inches tall. Spreads by aboveground rooting stems and is fast/invasive. Bright yellow flowers. Grows in well drained but not fertile soils. Best in full sun but tolerates some shade (flowers reduced in shade). Easy to grow, hard to contain.
Sedum 3 12–18 Large family of sun-loving, spreading plants. Normally 2 to 6 inches tall. Grows best in sunshine and well drained soils but will tolerate some shade. In shade, becomes thin and sparse. Damp soils and heavy clay will usually stunt them. Some species, (e.g. S. acre) classed as weeds in some jurisdictions. Easy to grow, long-lived. Most often used in rocky areas such as rock gardens.
Teucrium chamaedrys (Germander) 4 18 Deciduous in cold climates and evergreen in warm, 8 to 18 inches tall. Spreads 12 to 18 inches wide with wonderful purple-rose flowers in June to July, sporadically afterward. Mulch the first season or two until well established. Prune in early spring to remove winterkilled branches. Excellent, noninvasive plant for sun or very light shade.
veronicaspecies (Creeping speedwell) 4–5 12–18 Deciduous plant, 8 to 18 inches tall. Some species are classed as lawn weeds in some jurisdictions. Mostly spreads by seed or aboveground rooting stems. Bluish flowers in May and June. Easy to grow, hard to eradicate from lawn if it colonizes.
Vinca minor (Periwinkle) 4 12–18 Deciduous to evergreen depending on climate, 8 to 18 inches tall. Starts slowly but once established grows quickly. Forms a mat of stems and leaves with bluish flowers. Does well in most soils except heavy clay or waterlogged. Prefers shade and part shade garden areas. Shear in spring for thicker growth.
Waldsteinia ternata (Wild strawberry) 4 18–24 Evergreen to semi-evergreen, 4 to 6 inches tall. Does best in full sun but will tolerate shade. In shade, sparser growing. Grows well in almost any soil except heavy clay. Loves organic matter in the soil.
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