Globe thistle is a contemporary-looking flower with old-world qualities: its spherical blue blooms are arresting in the summer border, but unlike some eye-catching flowers, globe thistle plants are drought tolerant, deer resistant, and an important source of nectar for butterflies and bees. While repellant to nibbling rabbits and deer, the spiny, spiky foliage also serves an important niche in the wildlife garden by providing food as a host plant for painted lady butterflies. In the fall, when the blooming time is over, globe thistles continue to provide textural interest with their attractive seed heads.
|Common Name||Globe thistle|
|Mature Size||Two to five feet|
|Sun Exposure||Full sun|
|Soil Type||Dry, shallow, or rocky|
|Soil pH||Acidic; 5.1 to 6.5|
|Flower Color||Blue, purple, and white|
|Hardiness Zones||USDA growing zones 3-9|
|Native Area||Central Asia; Southeastern and South central Europe|
Caring for Globe Thistle Plants
Globe thistle plants make a statement in a low maintenance garden. They are good middle or back of the border candidates for a xeriscape, as they need little or no supplemental irrigation. The low maintenance plants generally shrug off plants, and need no tending except for some deadheading to prevent reseeding, if desired. When globe thistle plants fail, it's usually due to heavy clay soil, or wet conditions. Plants growing in dry, sandy soils may thrive to the point of invasiveness.
Globe thistle plants need a full day of sun to remain compact and bloom well. If used as part of a foundation border, plant them on the east or south side of the home.
Good drainage is important for healthy globe thistle plants. The plants tolerate dry and rocky soils. In areas with heavy clay, use raised beds to improve drainage, or plant in containers.
The long tap root of globe thistle enables them to survive drought conditions. Water newly planted globe thistle weekly in the first month to help them get established, then only water if plants show signs of drought stress, like browning of foliage.
Temperature and Humidity
Globe thistle plants thrive in hot, dry areas. In areas of high humidity, provide proper plant spacing and a position in full sun to prevent mildew.
No fertilizer is necessary for the globe thistle. Plants may flop in conditions of too much fertility, and require staking.
Potting and Repotting
The long tap root of the globe thistle presents some potting and repotting challenges. It's best to choose a large pot for a permanent home for new globe thistle transplants. Choose a sandy loam or cactus potting soil for the plants. Repot only if plants need to be divided.
Propagating Globe Thistle
Although it's easy to make new globe thistle plants from seed, you can also propagate them by division. Wait until plants are at least three years old, at which time you should be able to see some new plantlets at the base of the mature plant. In the spring, separate these new plants with a sharp spade and replant. Wear gloves to protect your hands from the spines.
Varieties of Globe Thistle
The white blooms of 'Arctic Glow' contrast with reddish stems and silver leaves in late summer. 'Vetch's Blue' is common in the trade, with dark blue flowers that contrast with grayish-green leaves. 'Taplow Blue' produces five-foot-tall stalks for the back of the border.
While the flowers and foliage of globe thistle is nontoxic to people and pets, its spiny foliage can cause a rash or skin irritation if handled without protection.
Cut away the spent flowers to globe thistle to add to dried flower arrangements, to prevent reseeding, and to encourage re-bloom. Cut dead plants down to ground level in late winter.
Being Grown in Containers
Growing in containers is one way to control globe thistle plants when they exhibit invasive qualities. Plant them in combination with other xeriscape plants that add contrasting color and softer textures, like yarrow or lavender cotton. If you wish plants to return the next growing season, they should be at least two zones hardier than your area, to withstand the deep freezing that occurs in containers.
Growing from Seeds
Collect seed heads from globe thistle plants in the fall, and remove seeds. You can sow outdoors, where germination will occur naturally in the spring. If starting indoors, stratify seeds in the refrigerator for best germination. Use large cell packs to accommodate the long tap roots that begin to form after germination.
Insect pests take advantage of poor growing conditions for globe thistle like wet spring weather. In this case, you may observe aphids or the four lined plant bug. Cosmetic damage is minimal, and decreases as the weather warms, so spraying is not needed.
Globe Thistle vs. Ornamental Onion
If you've fallen in love with the blue and purple spheres of globe thistle, extend the look into earlier spring season by adding some Allium spp. plants. Although the ornamental flowering onion produces similar-looking purple flower heads to globe thistle, the foliage is quite different, being smooth and grassy or strap-like. Alliums grow from fall-planted bulbs, and the vigorous plants increase over time in a non-invasive way.