Choose one of these top performers from rigorous plant testing
The Chicago Botanic Garden's evaluation program has looked at nearly 10,000 different plants over the past 30 years, most of which have been included in one of the more than a hundred comparative tests we've ever done, large and small. If you've read my articles over the years, you know that comparative tests are a great way to measure the merits of a plant against other similar plants. But not every plant ends up in comparative testing; Individual plants have become more common in our experiments due to plant introduction programs and generous support from independent nurseries. Sometimes these single plants whet my appetite, leading to a great deal of exploration. This is true for gentians (Gentiana spp., zones 3–9) and mountain hydrangeas (Hydrangea serrata cvs., zones 6–9). Whether they're stand-alone plants or from an unreported comparative trial, consider the following plants my "best of the rest"—a selection of the best perennials we've evaluated in recent years. Until now in the pages of Fine Gardening.
1. ‘Rosie Posie’ hyssop (Agastache ‘Rosie Posie’)
Zones: 5b–8
Size: 22 inches tall and 24 inches wide
Conditions: Full sun; Average to dry, well-drained soil
Flowering period: from early summer to autumn
Flower Color and Size: Purple-pink, 1 inch long
Overwintering hyssop is a challenge for us, which is why we haven't tested them on a large scale. Who wants that heartache? Blue- and purple-flowered varieties are the most reliable, but those that bloom in pink, red, orange, or yellow make me laugh. Imagine my joy when 'Rosy Posey' henna survived one winter, and then another, and then another. The hot pink flowers are beautifully paired with dark magenta calyxes and, like many mint relatives, retain their color long after the flower has fallen. Nonstop from early summer through fall, the vibrant blooms are visited by hummingbirds and pollinators of all stripes. The bright green leaves are deliciously fragrant and disease-free, and the ball-shaped habit stays compact all summer long. 'Rosy Posy' thrives in full sun.
2. 'Millennium' Ornamental Allium (Allium 'Millennium')
Zones: 5–9
Size: 20 inches high and 13 inches wide
Conditions: Full sun; Averagely dry, well-drained soil
Flowering period: from early summer to autumn
Flower Color and Size: Rose-purple, 1¾ inches wide
On a suggestion a few years ago, 'Millennium', an ornamental allium unknown to me at the time, was brought in for comparison with 'Summer Beauty' and 'Summer-Peak-A-Boo'. I did not know that 'Millennium' was already widely appreciated by gardeners; It was eventually honored as the Perennial Plant of the Year in 2018. 'Millennium'—typos aside, is a distinctive perennial with rosy purple midsummer flowers that float like lollipops above clusters of dark green, strappy leaves. Starburst-like seed heads hold up well for a long time and eventually crumble. (Seedlings aren't uncommon, so if that's a concern, consider dropping the head.) Garden companions for stocky 'Millennium' are nearly endless, but it mixed wonderfully with billowy calamint (Calamintha nepeta, zones 5–7) in trials. Both are pollination stations.
3. 'Paint the Town Magenta' Pinks (Dianthus 'Paint the Town Magenta')
Zones: 4–9
Size: 8 inches tall and 14 inches wide
Conditions: Full sun to part shade; Well-drained soil
Flowering period: mid-spring to early summer (blooms again)
Flower Color and Size: Magenta, 1¼ inches wide
Like hyacinths, lilacs are often short-lived in our heavy soils, so growing them is almost a dare, but a win is a win. In my continued search to find a pink that would grow well for us, I was delighted to find 'Paint the Town Magenta' which is beautiful and very reliable. Beginning in mid-spring and for more than a month, vibrant magenta flowers completely envelop the plants. Blooms are over and the blooms are gracefully shed, leaving the attractive blue-green, spotted leaves a backdrop to the decaying flowers that reappear in autumn. A low, mounded habit is always elegant, Does not open to reveal its crown or melt in wet, humid weather. 'Paint the Town Magenta' has held up for five years in our occasionally wet spot, but a well-drained soil is best.
4. 'Azure Rush' Geranium (Geranium 'Azure Rush')
Zones: 5–9
Size: 14 inches high and 39 inches wide
Conditions: Full sun to part shade
Flowering period: late spring to mid-autumn
Flower Color and Size: Pale blue, 1½ inches wide
Rozanne geranium (Geranium 'Gerwat', Zones 5-8) took the gardening world by storm a while ago, but now it's getting some time in the sun.Sports rozanne 'Azure Rush' embodies the best qualities of the long-blooming rozanne, but its pale blue flowers are their ghost. They have a beautiful contrast with the eye. Being sterile, 'Azure Rush' blooms continuously from late spring until frost, and does not require deadheading to induce repeat blooms. Both cultivars have aromatic pinnate leaves and mounding habits, although the short stems in 'Azure Rush' are less rambling and more compact than Roseanne—14 inches tall and 39 inches wide, compared to 20 inches tall and 60 inches wide. 'Azure Rush' is adapted to a variety of light exposures in average soils, so it can easily bridge the transition zone between full sun and light shade in the landscape.
5. Indian Lilac (Spigilia marylandica)
Zones: 5–9
Size: 24 inches high and 22 inches wide
Conditions: Full sun to light shade; Average to moist, fertile, well-drained soil
Flowering period: Early summer to late summer
Flower Color and Size: Red and yellow, 2¼ inches long
It took many attempts to establish the native Indian pink, but it was worth the pain of those losses when we succeeded. Exotic bright red flowers, 2 inches long, bloom in early summer, then bloom again in late summer. From above, the flowers look like brilliant yellow stars, but from the side, their slender trumpet shape is clearly visible. The strange flowers look like little soldiers lined up in lopsided clusters. With dark green leaves up to 24 inches tall and wide and a bushy habit, Indian lilacs provide a fresh look all summer long. In sun or light shade, Indian lilac prefers moist, well-drained soil and will seed once established; When I discovered the seedlings in 2019 I was stunned. Last spring, I added 'Little Redhead' (Spigilia marylandica 'Little Redhead', Zones 6-9), a new vegetatively propagated selection with improved uniformity that already looks promising.
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