How to grow eryngiums and best varieties

Sometimes your borders need a plant that is so different to those around it that is shines out.  Thistle-like eryngiums, sometimes known as sea holly, can be just the ticket and the intriguing colour combinations and attraction for pollinators makes them a welcome addition to any sunny border.

To find the best eryngiums to grow, Which? Gardening magazine grew a range of varieties to see which were problem free and would give us the best display of flowers.  

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Key facts

PLANT TYPE Hardy perennial

POSITION Full sun

SOIL Well-drained, moist or dry by variety

How to grow eryngium: month by month

Best eryngium varieties

Which? members can log in now to see the full results and which are our Best Buy varieties. If you’re not a member, join Which? to get instant access.

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Height x spread: 60 x 60cm

Despite the name, this variety isn’t a traditional alpine which need very sharp drainage. Instead it’s native to alpine meadows and forest clearing when the soil in both deeper and wetter than on scree beds. In the garden it does best on a good, deep soil that is moist but well-drained. In our trials it was a dense, healthy, upright plant that had beautifully pale flowers and followed by intense blue stems.

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Height x spread: 110 x 70cm

This species grows naturally on rich soil on riverbanks, so it doesn’t mind a moist soil. There are countless varieties bred from the straight species and it’s easy to see why it would be a nurseryman’s dream. The flower stems branch repeatedly, creating a mass of little purply-blue flowers. The coloured stems kept coming, so it looked great for weeks. It does need staking, so put supports in early to catch the stems before they topple over.

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Height x spread: 90 x 50cm

Bred from another Best Buy eryngium, this variety stood out from the crowd with its variegated leaves. These gave a long period of interest, even when the plants weren’t in flower and had attractive foliage. The mass of flowers open white before turning pale purply-blue and they complimented the leaves perfectly. They were a magnet for insects of all kinds. We lost one plant at both our southern trial site and our northern trial site, but the remaining plants were strong and healthy. 

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Height x spread: 75 x 50cm

There are a number of x zabelii varieties and these hand on the best attributes of its two parents. The exuberant ruff around the flowers come from E. alpinum while the attractive leaves come from E. bourgatii. It prefers a soil that’s on the dry side and is quite poor. We loved the strong colour in the flowers and this spread down the stems as the flowers matured, lasting long after the flowers had turned silver.

Full testing results for eryngiums

How we test eryngiums

We planted three plants of each variety at the Which? Gardening magazine trial garden at Capel Manor College in north London and at a northern trial site outside of Edinburgh. We planted the trial in spring and assessed our plants weekly until November, looking for how well they were flowering and any problems with pests and diseases. After flowering had finished and the heads had turned brown we removed spent stems to allow new ones to come through. We also removed old leaves that had turned brown throughout the season and in the spring before new growth emerged. We grew the plants for at least two year. We had a few problems, notably blackfly at Capel Manor, fungal diseases and the plants getting too wet in very rainy weather causing a few to die. 

Caring for your plants

Planting 

Plant in a sunny spot. The soil should be well-drained, but most of the varieties enjoy some moisture so long as they are not sitting in wet soil.

Jobs to do

Water your plants while they are establishing if the soil is too dry. 

Many varieties will need to be staked, so put in supports before the stems start to fall over.  

Many varieties self-seed prolifically. Weed out most of the seedlings, but leave a few to replace any plants that may die after a few years. 

Propagating

Eryngiums self-seed and so it’s easy to grow more plants from the seedlings. You can also grow many of the varieties we have tested from packets of seed. Collect your own once the seeds are ripe and sow into seed-sowing compost. Place in a cold frame to germinate and grow. 

Root cuttings are easy to take. Dig up the plant in late winter. Choose a root that is around pencil-thick and cut it at the top, nearest to the plant. Cut the bottom of the root at a slant to show which is the top and bottom. Cut into sections around 5-7cm long and place around the sides of a 9cm pot, filled with free-draining compost mixed with grit. The top of the cutting should be just at the surface of the compost. Place in a greenhouse and give it some bottom heat if possible. Pot on when new shoots grow. 

You can divide large plants, but the divisions often take some time to settle in. 

Problems

Blackfly 

Keep a lookout for early signs of infestation and squish small colonies with gloved hands. Encourage ladybirds to visit. Larger outbreaks can be sprayed with a Best Buy aphid control. 

Read more about blackfly.

Fungal diseases

These can cause the whole plant to rot. Regularly check any affected plants and remove then, including the roots, should it take hold.