The Best Groundcovers for Tennessee (so far)

 

The 'Grey Owl' cultivar of Juniperus virginiana (eastern redcedar) grows as a woody groundcover

It seems to me that there are two basic ways to approach gardening - you can fight against Nature or you can try to form an alliance with Nature.

When it comes to preventing your garden beds from getting overgrown by weeds, the fighting approach calls for annual (or more frequent) applications of mulch, plus annual (or more frequent) applications of herbicide.

This fighting approach seems not only wasteful from an economic and resource standpoint, but also so much effort! And not one-time effort, but constant and unending work.

The alliance approach calls for using some other plant - a groundcover - that can block and suppress the weeds for you while you sit back on your porch and drink a Mai Tai (or an iced tea, for those of the teetotaler persuasion).

Of course, it's not quite as simple as all that.

Sometimes you think you have an alliance with the groundcover, but the groundcover is actually secretly plotting world domination (or at least the domination of your garden beds).

Sometimes your ally proves too timid and weak to stand up to the onslaught of weeds or the indignities of heat, cold, wind, wet and drought that Nature throws its way.

After nearly 5 years of experiments, here are my thoughts on some of the groundcovers I have trialed in my USDA zone 6/7 clay soil Tennessee garden:

Woody groundcovers

Juniperus virginiana 'Grey Owl' loaded with 'berries' (actually berry-like cones)

Juniperus virginiana, eastern redcedar - Both the straight species and many of the J. virginiana cultivars are upright evergreen trees, but the 'Grey Owl' cultivar forms a lovely blue-grey spreading shrub.

After many years, 'Grey Owl' could grow into a large shrub - say 6 or 8 feet tall - at which point it would no longer fit a groundcover definition, though I'm interested to see whether judicious pruning could prevent or at least delay that transition from groundcover to mounding shrub.

The two 'Grey Owl' shrubs in my garden have been fantastic performers so far. They have lots of berry-like cones in the winter that are both ornamentally appealing and theoretically provide food for the birds.

The species is native throughout Eastern and Central North America and seems extremely adaptable and tough. Its only serious enemy in my garden has been the bagworm, although actually this pest has not yet appeared on the 'Grey Owl' plants, but only on some of the upright eastern redcedars around my property. Hand-picking bagworms off a 15-foot tall upright juniper is impractical, but the task should be easier if the bagworms show up on a low-growing 'Grey Owl.' And since bagworms have only one generation per year, the task is not interminable. In any case, I plan to try to combat the bagworms next year with Bt and asters, since I read someplace that asters attract beneficial insects that prey on the bagworms.

Rhus aromatica foliage, August 2015

Rhus aromatica, fragrant sumac - Again, the straight species of this plant is definitely not a groundcover, but rather a vigorous shrub that reportedly can grow 8 to 10 feet tall with 5 years.

As with the 'Grey Owl' juniper, there is a fragrant sumac cultivar called 'Gro-Low' that is much more amenable to usage as a woody groundcover. Gro-Low tends to grow outward much more than upward, forming a spreading, sprawling shrub that could make a tall base layer underneath trees.

I have mixed feelings about fragrant sumac. It has interesting cone-shaped flowerbuds in winter. Its yellow springtime flowers seem highly attractive to little pollinators. The species is a native shrub. It has red berries that provide food for birds, interesting tripartite foliage and excellent fall color. What's not to like?

Well, for starters, it's a bit rough/messy looking. Nothing wrong with that per se, but let's just say it does not have a refined or formal look to it.

It also grows so vigorously that it can be a little tough to keep in bounds (although this could be a real asset if you have a lot of ground to cover).

I've also had some problems with certain branches dying off. I'm worried that it could be a fungal disease called verticillium wilt. Perhaps the problem is that it is growing on very heavy clay soil? I've read that it prefers sandy or loamy soils and can be short-lived on clay.

Also, since this is a deciduous shrub, you will be looking at bare branches and bare ground beneath those branches for around 5-6 months in Middle Tennessee. Although the shrub has a dense growth habit that does a good job of blocking weeds during the growing season, I have seen some weeds beneath the shrub now that the leaves have fallen and uprooting those weeds without damaging the shrub itself can be a little challenging.

Cephalotaxus harringtonia 'Prostrata', Japanese plum yew

Cephalotaxus harringtonia, Japanese plum yew - This is a non-native alternative to to the 'Grey Owl' juniper. I believe the straight species of Japanese plum yew is a large, bushy plant, but the 'Prostrata' cultivar grows as a low, spreading shrub.

I've had some issues with Japanese plum yew. I had an upright ('Fastigiata') cultivar that totally failed to thrive and was soon removed (something similar seemed to happen to a 'Fastigiata' specimen growing at our local botanic garden), but the two 'Prostrata' specimens in my garden have performed fairly well in a partial shade setting. Groundcovers for the South says, "Plum yews are slow to establish and require careful attention for about two years. Once established, however, they are relatively drought tolerant and carefree."

Unlike junipers, plum yews reportedly respond well to shearing, and I've heard they can even spring back from rejuvenation pruning. Japanese plum yew is a dioecious species (meaning that plants are either male or female).

 I don't know if 'Prostrata' is a male or female cultivar, but since it's the only cultivar I have at the moment, I don't anticipate getting fruit. The sources I have seen disagree on whether Japanese plum yew fruits are poisonous. (The seeds, branches and leaves of true yews - Taxus spp. - reportedly are very poisonous.)



Groundcover vines

I've just begun to experiment with using vines as groundcovers.

Virginia creeper foliage in August 2015

Virginia creeper foliage in November 2015

In several places where native Virginia creeper (Parthenocissus quinquefolia) has sprung up in the garden beds, I'm letting it grow to see if I can manage it as a deciduous groundcover. These are early days (I used to pull the Virginia creeper seedlings until this past year) so none of these vines have covered much ground yet, but I should have more information by next year. Although it is deciduous, the vine seems to catch blowing leaves in autumn, which gives me hope that these leaves would provide enough of a groundcover to prevent germination/growth of some weeds.

Lonicera sempervirens (coral honeysuckle), summer 2013

I also plan to experiment next year with growing our native coral honeysuckle (Lonicera sempervirens) as a shrubby groundcover. I currently grow coral honeysuckle on one of my porch railings, which it forms a semi-evergreen vine that flowers for many months at a time and attracts hummingbirds in the summer. It seems incredibly resilient in terms of bouncing back from frequent hard prunings, so I have high hopes that it would make a good groundcover. (Of course, I'd have to keep a close watch on it to make sure that it did not overwhelm other plants in the bed with its vining/climbing tendencies!)

I haven't tried it yet, but I'm also considering experimenting with growing Decumaria barbara (climbing hydrangea, wood vamp) as a vining deciduous groundcover...


Herbaceous groundcovers

This is probably the biggest category, although it can be subdivided further in several ways - there are groundcovers that creep or run by herbaceous means (stolons or rhizomes) and there are those that cover ground by reseeding. I suppose one could theoretically imagine an annual herbecous groundcover - I've experimented this winter with sowing the native annual plains coreopsis (Coreopsis tinctoria) in the hopes it will fill in gaps throughout the garden, but I tend to believe the best herbaceous groundcovers will be perennials.

What characteristics should a good perennial groundcover have?

- It should have a spreading rather than an upright character. This is fairly obvious. Even though I like plants like Hibiscus moscheutos (swamp rose mallow) and Platycodon grandiflorus (balloon flower), they have a clumping upright growth habit that does not say 'groundcover' to me.

- It should multiply and cover ground in some way! Amsonia tabernaemontana (eastern blue star) has proven to be a tough perennial with a spreading growth habit. It thus meets the first criteria mentioned above, but it dies back to the ground every winter, leaving no obvious presence and must start anew in the spring. As it reaches its mature size (reportedly 3 feet wide), it might cover a good amount of ground from late spring to early fall, but I don't think it fits the definition of an ideal groundcover. In other words, I think it leaves plenty of bare spaces for weeds to get established.

- It should not be too rampant. This is a tricky one and the acceptable threshold will differ for every gardener. I approach this on a case by case basis, but I do have a few guidelines that help me determine whether something is too rampant or aggressive for my garden. I'm willing to cut the natives more slack with the understanding that they were here first, they probably have some beneficial impact on the local ecosystem and in a worst case scenario, if they get wildly out of control, I probably won't be doing any irreparable damage to the environment. It may sound silly and grandiose to think in these terms, but I don't want to be the gardener who unleashes the next kudzu on Middle Tennessee, so I exercise an abundance of caution. I'm particularly wary of groundcovers that spread secretively underground. So plants like blue star creeper, creeping raspberry and more recently hardy blue plumbago all got the boot.