A recent trip to western Massachusetts included a hot July morning at the Berkshire Botanical Garden in Stockbridge, MA, a public garden that never fails to yield new ideas for next year’s garden. Some of these discoveries are discussed below, including two perennial mulleins, a yellow meadow rue, and a dark purple iris suitable for mass plantings in wet areas.
Yarrow and Catmint
The botanical garden’s full sun island beds were planted with truly heat-tolerant herbaceous perennials, exuberant plants that intermingled in informal drifts. One classic example was the bright lemon yellow flower clusters of Achillea x ‘Moonshine’, a hybrid yarrow, growing into a spreading clump of Nepeta x faassenii ‘Dropmore’, a hybrid catmint with lavender blue blossoms. The yarrow’s deeply dissected, fern-like, aromatic, gray-green foliage was the perfect foil for the catmint’s blue flowers. Throughout July in Marjorie’s Garden, we watch bumblebees forage on catmint in this classic combination of blue and yellow.
Nearby, another catmint, N. racemosa ‘Walker’s Low’, grew in a three-foot-wide swath at the front of the border. Its lavender-blue flowers, topping out at about two feet, were arranged in loose whorls atop aromatic gray-green foliage. With proper shearing of spent flower spikes, this catmint will continue to bloom into fall, much to the delight of the garden cat.
I was pleased to see one of our favorite garden volunteers, common mullein (Verbascum thapsus), purposefully planted as an ornamental at Berkshire Botanical Garden. This is a self-sown biennial found growing in disturbed areas throughout New England, one that we allow to grow along the bed edges in Marjorie’s Garden and often attempt to transplant in its first year if we find it growing in the middle of a bed.
A mullein plant in its second summer is a 6-foot-tall woolly mammoth. Densely hairy frost-green leaves clasp the lower two-thirds of the main stem, those nearest the ground more than a foot in length. The upper third of the stem develops into a stout spike of one-inch yellow flowers that open sequentially, bottom to top, over a period of several days. The flowers bring native bees and other pollinators to the garden; their late-summer seed heads bring goldfinches. On many summer mornings the edges of the large felted leaves are adorned with sparkling jewels, products of overnight guttation.
A highlight of my visit, a discovery that made the sweltering heat worth enduring, was Verbascum chaixii, the nettle-leaved mullein. Interplanted in several areas throughout the Garden’s perennial beds, this native of south-central Europe and central Russia is a winter hardy perennial in USDA Zones 5 to 8. It grows to three feet in height, much smaller than common mullein and therefore much more at home in the smaller full-sun garden. It is also drought tolerant and thus well adapted to cottage gardens and rock gardens.
Nettle-leaved mullein produces flower spikes from May through July, each small yellow flower with a central cluster of purple stamens. A white-flowered cultivar, ‘Album’, also has purple stamens. Like its biennial cousin, it is a pollinator magnet, attracting all forms of native bees.
The perennial bed in Marjorie’s Garden currently grows two species of meadow rue, Thalictrum aquilegifolium, the columbine meadow rue, with basal clumps of lacy, blue-green, maidenhair-like leaves and dense panicles of lilac purple flowers, and T. rochebruneanum, the lavender mist meadow rue, a late-summer-flowering species with lavender-purple petals and yellow stamens, its flowers borne in loose, airy clusters atop sturdy stems that rise to six feet tall. The later species is spectacular when massed.
My visit to Berkshire Botanical Garden offered my first look at T. flavum subsp. glaucum, the yellow meadow rue. The species, T. flavum, is a rhizomatous clump-forming plant the grows to three feet in height. Its flowers, lacking petals, have pale yellow sepals that drop early, leaving a showy tuft of bright yellow stamens. The subspecies glaucum is noted for its attractive silvery blue-green leaves.
Yellow meadow rue should be grown in full sun to part shade. It prefers moist, organic soils with consistent moisture, particularly in full sun. The taller stems will need staking.
Japanese Water Iris
After seeing the mass planting of Iris ensata ‘Black Knight’ at the entrance to Berkshire Botanical Garden, I’ve added it to my short list of iris species worth growing. An entire bed along the entrance walk was devoted to this iris, a testament to its enduring sword-shaped linear leaves, each with a prominent midrib, as much as to the dark purple flowers, each with a splash of yellow in the fall. The flowers have a distinct flattened appearance. I. ensata (synonymous with I. kaempferi) blooms early to mid-summer, later than the Siberian iris (I. sibirica), another popular group of garden-worthy irises with excellent foliage.
While thriving in standing water during the growing season, I. ensata can also be grown in organically rich, medium to wet soils in full sun to part shade. Wet in the spring, constantly moist during the summer are the requirements for success with this group. This makes them ideal candidates for rain gardens that are flooded in spring and remain moist during the rest of the growing season.
We were away from Marjorie’s Garden for six days of true summer weather and were amazed on our return at the amount of growth during that short time in both the vegetable garden and perennial beds. The garden had a good rain the day before we left and, in our absence, seedlings of Shirley poppies grew two feet in those six days. No rain had fallen while we were away, but during the night of our return and through the following day, the remnants of tropical storm Arthur dumped two inches of rain on the garden as it passed by the Maine coast. Not having to drag the hose for a while felt like an extended vacation!