Monday, July 22, 2013

Groundcover Review - Hardy Blue Plumbago, Ceratostigma plumbaginoides

Hardy Blue Plumbago looking good in a morning sun / afternoon shade setting. This is how it looked on June 15th.

I'm a big fan of groundcovers - at least in theory.

Why are groundcovers Good?

1. Groundcovers block weeds.

2. Groundcovers prevent soil erosion / compaction

3. Groundcovers can themselves be beautiful and have wildlife value

4. Groundcovers knit the landscaping together

5. Groundcovers can be low maintenance and cost-effective. If you mulch your beds, you typically have to buy and spread new mulch every year - or pay someone to do it for you. Some mulches like pine bark may last 2-3 years, but even those mulches will often wash or blow away. Constantly refreshing your mulch is not very environmentally-friendly, it's not very kind on your pocketbook and it can be a big job depending on the size of your landscaped area

Hardy Blue Plumbago blooms earlier in a full sun setting, but the leaves look a little bleached and the plant seems less inclined to spread than the ones with afternoon shade. (Although the tendency to spread may have more to do with the fact that the partially shaded front foundation bed is more heavily amended than the heavy clay soil in this back full sun bed.)

So why do groundcovers get a bad rap sometimes? Groundcovers (as the name implies) need to cover ground, but some of them are too good at their job. They cover so much ground that they swamp everything in their way, smothering perennials, invading lawns (and even worse, sometimes invading natural areas), sometimes even tearing down trees (I'm looking at you English Ivy).

What are some of the other groundcovers that give the category a bad rap? Oh, plants like Yellow Archangel, Vinca, Bishop's Weed, etc.

That said, a few bad apples shouldn't give a whole category a bad rap.

So I've been trialing a number of groundcovers at Garden of Aaron to see which ones are tough enough to survive without much coddling yet not so rampant that they take over the property and kick me to the curb.

My preference (as with other plant categories) is to use native Southeastern U.S. plants - ideally Tennessee natives - but I've also tried some exotics.

So here is the first in a series of posts with my thoughts so far on my various groundcover experiments:

Hardy Blue Plumbago and Sweet Alyssum


Hardy Blue Plumbago, Ceratostigma plumbaginoides

Pros:

1. Tough. Made it through drought and 100+ degree temperatures in the summer of 2012 without a whimper. On the cold side, it's hardy to at least zone 6 (though some Dave's Garden reviewers report their Plumbago surviving into zones 5 and even into zone 4 around Minneapolis!)

2. Beautiful self-cleaning blue flowers from late summer to autumn (no need to deadhead). Attractive semi-glossy foliage.

3. Gorgeous red fall color

4. Not too aggressive or fast-spreading. Seems easy to control. Not listed as invasive as far as I know.

5. Does not seem troubled by any pests or diseases.  Some of the Dave's Garden reviewers report the plant surviving for decades in their yards with little human intervention. That's the kind of garden plant I respect and admire!

6. Seems to grow in sun or shade, but has spread further and looks happier with afternoon shade in Tennessee. Perhaps it appreciates full sun in the northern part of its range (hardy to zone 5)


Cons: 

1. Not a Tennessee native (originally from Western China)

2. Deciduous and Herbaceous. This is probably its biggest draw back. One of the main reasons to plant a groundcover is to deter / block weeds. If the plant disappears during the winter, that seems to leave the ground free for weeds to colonize. In reality - perhaps because of the Plumbago roots underground - I haven't really seen any weeds in the spots where the Plumbago was slumbering, but the herbaceous nature of the plant is definitely a concern. Oh and it wakes up quite late in the spring (late April in 2013), so in a climate like Tennessee, you're probably looking at 4-5 months of bare ground and the bare stems of last year's growth. (Incidentally, Missouri Botanical Garden says the plant overwinters better if those old stems are left uncut until the spring. I did not cut them at all, and the new foliage quickly grew up and covered the old stems. So I'd say it's very low maintenance in that regard.)

3. Does not seem to have much wildlife benefit. I don't recall seeing any pollinators at the flowers, nor did I see birds going after any seeds. (Some sources actually say that Hardy Blue Plumbago can do a good job of attracting bees and butterflies...maybe I just need a larger patch of the flowers to get the pollinators' attention? I'll keep a close eye on them this year and report back if I spot any beneficial insects.)

4. Spreads slowly. This is an attribute from a control standpoint (you don't have to worry about it taking over the whole flower bed when your back is turned), but means you'll need a lot of plants and/or a lot of time if you hope to cover a large amount of ground. I have not tried dividing the plant yet to accelerate its spread. This year, the plants in the front of the house (in partial sun and looser soil) seem to have expanded their coverage, but the plant in full sun and more compacted soil looks pretty much the same size as it did last year.

Conclusion:

Recommended, but with reservations.

UPDATE 12/13/15 - I would no longer recommend hardy blue plumbago. In fact, I've done my best to rip my patch Ceratostigma plumbaginoides out of the front foundation bed this autumn. 

From an aesthetic standpoint, I did not care for the foliage, the growth habit or the fact that its deciduous nature meant that it did a poor job as a groundcover for 5-6 months of the year. The deciduous aspect would be less of a concern in a climate with dependable snow cover, but here in Tennessee, where snow cover is usually bare or non-existent, a deciduous groundcover equals bare ground.

And then there was the aggressive/pushy/invasive nature of the plant. It's not a rapid spreader - at least in my heavy clay-based soil. I didn't find it traveling 10 feet underground in a single season. But slowly, inexorably, it was pushing outward and over-running other small perennials in its path. When I tried to pull up pieces of the groundcover, I found an extensive and deep network of criss-crossing roots underground. Those roots tended to break easily if pulled, making it quite a chore to uproot a patch of hardy blue plumbago.

As I discuss in my upcoming roundup of recommended groundcovers, I believe that the ability to uproot and easily remove pieces of a groundcover - or an entire patch - plays a major role in determing the garden worthiness of a plant --- especially a non-native / exotic plant.

So yep, hardy blue plumbago joins blue star creeper and creeping raspberry on the Not Recommended list of groundcovers.

Which groundcovers do I recommend as of December 2015? So far, some of my favorite herbaceous groundcovers include the cranesbill geraniums (especially Geranium sanguineum and G. x cantabrigiense), lamb's ear, wall germander, rose petty, Geneva bugleweed, Coreopsis verticillata and Epimedium x perralchicum 'Frohnleiten'.

UPDATE APRIL 2017 - I no longer recommend the bloody cranesbill geranium (G. sanguineum) or lamb's ear or wall germander or any of the bugleweeds (Ajuga species).

All of those are exotic plants. The bloody cranesbill and the bugleweeds especially are invasive, aggressive and hard to remove. 

Lamb's ear is easy to remove, but not great aesthetically in the garden. It looks awful in winter and it's a pain in the posterior to remove the smelly, decomposing old growth in spring. Lamb's ear cultivars like Helene von Stein rarely flower. The species does have long-lasting fuzzy spikes adorned with purple flowers that attract pollinators (+1 for the lamb's ear), but unfortunately the pollinated flowers lead to quite a bit of self-sowing in the immediate vicinity.

Wall germander has some of the same issues as hardy blue plumbago. It's slowly invasive through underground rhizomes. It's much more evergreen than the plumbago, but the old stems eventually go bare. They don't collapse on their own though, and it's a pain to go in and manually remove the old dead stems. I can't recall, but I think this plant can also get a bit floppy and messy-looking unless trimmed. It does have flowers that attract pollinators (+1)

Basically, I would discourage anyone from planting the bloody cranesbill or the Ajugas outside of their native habitats. Even there, I'd be leery of them unless you like super aggressive plants.

The lamb's ear and the wall germander are a bit more manageable and have certain appealing characteristics. They weren't right for me, but if you have a small garden or don't mind a groundcover that demands a certain amount of work, they could work for you.

Personally, I prefer Geranium x cantabrigiense 'Biokovo', rose petty (Erigeron pulchellus), as well as wild strawberry (Fragaria virginiana), golden groundsel (Packera obovata) and 'Blue Spruce' sedum.