Two apt gardening terms epitomize my sometimes self destructive plant selection process. I clearly practice the art of  “zonal denial,” which I define as a compulsive attraction to plants at the edge of my zone, best described as Sunset Zone 5 ½ or  USDA Zone 8b.  (At 370 feet above sea level and enjoying close proximity to the moderating influence of Puget Sound,  I figure I’m actually somewhere in between Sunset Zone 5 and Oregon’s Willamette Valley Sunset Zone 6, hence the one-half.)

Thunbergia gregorii, Orange Clock Vine, Hardy to USDA Zone 9

The Sunset Magazine zones, unlike the USDA hardiness scheme, take account summer high temperatures, regional micro climates and other factors, rather than simply winter maximum recorded low temps.

As much as I would like to take credit, I can’t claim to have coined the term zonal denial.  I first saw the term on a sign at Cistus Nursery in Portland Oregon.

The time of year was late fall, and in a section of the nursery defined by a lath house-like structure, a small sign announced that the plants inside were of the “zonal denial” persuasion!  I bought three or four of these beauties in gallon pots.

— Zonal denial mitigation tool —

I should have paid closer attention to the outdoor propane heater atop a typical 20 gallon LP tank pointed directly at my purchases.

Many of my zonal denial plants — in anticipation of a major winter cold snap — are consigned to pots, moved into a crammed 8 X 14 foot greenhouse or covered in mulch, burlap and other sundry protective improvisations. Many others are long since gone.

When not buying plants or seeds better suited to San Diego or the Willamette Valley in Oregon, I’m guilty of planting aggressive spreaders that refuse to coexist with other less robust plants.  These thuggish plants are often attractive, but are lousy at harmonizing with their more well mannered neighbors.

Actually that’s not entirely accurate.  Initially many aggressive spreaders seem okay, filling in nicely and covering empty space. Later on when they exceed their boundaries, overwhelm neighboring plants, and require endless trimming, yanking and composting do the consequences become clear.

Here’s just one example of one of my endless struggles,  a variegated ground elder vs. Japanese Fountain Grass (Hakonechloa macra ‘Aureola’).

Japanese Fountain Grass and Aegopodium podagraria ‘variegatum’

The above photo gives the impression these two handsome plants are getting along side by side. What you might not guess is the aggressive ground elder when left to its own devices will completely overwhelm the slow growing Japanese fountain grass.

Aegopodium podagraria ‘variegatum’ aka ground elder, bishop’s weed

In fact, I’ve spent more time than I care to admit picking this tenacious spreader out of the Japanese Fountain Grass, not to mention from the wood chip path.

Ground Elder proudly displaying its metastatic root

The Royal Horticultural Society doesn’t mince words:   “Aegopodium podagraria, ground elder  is a fast-growing, invasive, perennial weed that can spread quickly to form a carpet of foliage that will crowd out less-vigorous plants in beds and borders…It spreads via rhizomes (underground stems), which can regenerate from a just a tiny fragment left in the ground.”

So how did a savvy plant guy like me introduce this thug to my garden?

Four years ago I bought a few four inch pots along with some annuals to decorate the base of our son’s “Chuppa” or marriage canopy.  It was an October wedding, annuals were not in great supply and the variegation was attractive.

After the wedding, not wanting to waste this lovely variegated ground cover, I planted the leftovers without bothering to investigate the plant’s questionable habit. I soon learned this “energetic” perennial would keep my as busy as the Euphorbia amygdaloides var. robbiae (Mrs. Robb’s bonnet) that likewise has a rhizomatus root system that can only be described as invasive.

— Mrs. Robb’s Bonnet beside (and invading) a wood chip path —

In the Euphorbia’s case, what I wanted was to quickly establish a large single swath of plants in a defined border.  The euphorbia obliged and I’ve been an energetic weeder ever since.