Thistles have a terrible reputation. Not only are most of them seriously, painfully prickly, but they are often invasive, too.
But not all thistles are bad. Take Berkheya purpurea, the South African daisy relative with serious flower power from mid-June through July.
This unusual perennial daisy from the Drakensberg, Natal and NE Cape Province produces scads of striking lavender or sometimes white daisies for months in the middle of the Oregon summer. Plants reach about 3.5’ tall in the Willamette Valley and a healthy clump can spread about that wide within five years or so.
I chatted with Leonard Foltz, co-owner of Dancing Oaks Nursery to learn how Berkheya do in the Willamette Valley. According to Foltz, the trick to success is to give plants excellent drainage and your sunniest southern or western exposure. "We have it mulched with a quarter-ten crown mulch," he says. "We’ve had our clump for years, with no hardiness problems in winter."
If in doubt about your drainage, start with the best-draining area of your garden and incorporate spacefuls of gravel or pumice in an area 1’ deep and 2’ wide. You can also plant on a slight mound or scrape away the topmost inch of soil and replace the soil with a gravel mulch.
Berkheya purpurea is an uncommon plant in our region and can be hard to find in nurseries. This may be because it is often labeled "Zone 8" – even though it’s quite cold-tolerant and can survive temperatures as low as 5F when given hot sun and great drainage. But in the right spot, plants will increase into hefty clumps, providing a spectacular midsummer display.
Berkheya purpurea grows in moist, rocky, grassy slopes in its native habitat but, once established, is relatively drought-tolerant in the Portland area, perhaps thanks to a substantial taproot. It mixes well with hot pokers (Kniphofia sp.), Yucca sp., Crocosmia and other upright flowering plants with architectural foliage.