Eupatorium

This disease-resistant, summer-flowering genus is great for adding colour and structure, says Graham Clarke.

Eupatorium maculatum (Atropupureum Group) 'Orchard Dene' AGM - photo: RHS Trials/Wendy Wesley
Eupatorium maculatum (Atropupureum Group) 'Orchard Dene' AGM - photo: RHS Trials/Wendy Wesley

The Eupatorium genus, of the Asteraceae family, comprises perennial plants with long-lasting, broad, purple to white flowerheads. These can emerge from late spring, although the main show starts in late summer. The blooms are held on tall stems and act as a beacon to butterflies and bees.

It is believed that the genus consists of some 1,200 species, but recent botanical research suggests it is easier to manage if split into several smaller genera. Some of the species most often encountered in gardens have now been reclassified - and are sold - as Ageratina, Bartlettina and Conoclinium.

A plant trial of Eupatorium at RHS Garden Wisley in 2005-06 demonstrated the diversity found in these closely related plants. Of the 31 entries in the Wisley trial, five were awarded the coveted Award of Garden Merit (AGM).

The garden centre customer should select this genus with care, as some varieties may become too tall, needing prudent placement to look their best.

Arguably the most impressive is the American species E. maculatum Atropurpureum Group AGM, commonly known as Joe-Pye weed. Established plants can frequently grow taller than 2m in a season and make imposing perennials for the back of a border. One of the tallest is the white-flowered E. fistulosum f. albidum 'Massive White' AGM, reaching some 3m.

Breeders have also worked hard to develop shorter cultivars for use in smaller gardens. Of these, one of the best is Dutch plantsman Piet Oudolf's E. maculatum (Atropurpureum Group) 'Purple Bush' AGM, at around 1.5m in height.

Gardeners wanting native plants might consider the hemp agrimony (E. cannabinum). The "hemp" comes from the plant's leaf shape, which resembles cannabis, rather than any narcotic content. It is as attractive to insects as the American species, but its flowers are a dirty purple shade, and although it is a conveniently smaller species, it can be invasive.

To avoid this, one can grow the cultivar E. cannabinum f. cannabinum 'Flore Pleno', which, instead of flowers, has long-lasting, bright pink bracts. This does not produce true flowers, and therefore does not set seed, but because of this it is unattractive to insects, so its appeal is somewhat negated.

Eupatoriums are easy to grow, requiring little more than fertile, moist soil in a sunny position, although several tolerate partial shade. They are generally pest- and disease-free.

They do sometimes look unsightly when the flowers are past their best, so they are often recommended for a wild garden or the back of a border where they are not so visible once they start to fade.

WHAT THE SPECIALISTS SAY

- Bob Brown, proprietor, Cotswold Garden Flowers, Worcestershire

"My own soil is extreme alkaline clay and eupatoriums do not do well for me. However, there is a slightly acidic, sandy soil at Wisley where the RHS trial took place, and the plants there were gigantic in comparison. This says a lot about their preferred growing conditions. They also do well in moist situations - our native hemp agrimony thrives in ditches, for example.

"The cultivar E. rugosum 'Chocolate' Award of Garden Merit (AGM) is a popular plant, with deep purple leaves and stems, and white flowerheads. Throughout the season you can enjoy its rich foliage, and then in October it is really at its best, with lovely white flowers.

"There are many other tender and tropical forms that have been collected and introduced, but they are not hardy here, so their sales potential will be limited. However, one excellent plant worth mentioning is E. arnottianum 'Salsipuede', which produces Ageratum-like terminal heads of powder-blue flowers, from July to November."

- Marina Christopher, proprietor, Phoenix Perennial Plants, Hampshire

"These perennials have great architectural presence and they are some of the best plants for attracting insects. Hoverflies, bees and butterflies, and even ladybirds are commonplace there, which is important to me, as I tend not to use chemicals. In fact, E. cannabinum 'Album' seems to have more butterflies on it when it's in flower than anything else in the garden.

"Eupatoriums have a long season of interest. Our native forms flower at the end of May and June.And these plants are have great architectural presence, too. They provide height, without being woody. However, they often look a bit grubby when the flowers go over, but you don't want to deadhead them because birds will make use of the winter seeds.

"I like E. capillifolium AGM. It's something a bit different, being a foliage plant, with tall, feathery leaves on 2m-high stems. The fresh green foliage carries on throughout autumn."

IN PRACTICE

- Matthew Tanton-Brown, manager, The Place for Plants, Suffolk

"We stock eupatoriums in two different areas: in the A-Z beds, and also in the area given over to moisture-loving and bog plants. Eupatoriums are not often seen recommended for such situations, but they do enjoy a moist soil.

"We sell four different forms. One of them, E. maculatum Atropurpureum Group AGM, can just walk out the door at this time of year when the young foliage is showing purple. Our plants are mainly in three-litre pots. Sometimes we'll have them in smaller pots, but if they don't sell by the end of the season we have to pot them up."

SPECIES AND CULTIVARS

- E. album 'Braunlaub' (now Ageratina altissima 'Braunlaub') produces white flowers from July to October, is shade tolerant and reaches 60cm.

- E. arnottianum carries violet-blue flowerheads on stems 80cm high.

- E. arnottianum 'Salsipuede' produces Ageratum-like terminal heads of powder-blue flowers from July to November and grows to 75cm high.

- E. cannabinum, the hemp agrimony, forms a dense clump of reddish stems, producing purple, pink or white flowerheads from July to September.

- E. cannabinum 'Album' has white flowerheads that are extremely attractive to insects.

- E. cannabinum f. cannabinum 'Flore Pleno' has pink double flowerheads on 1m stems from July to September, which do not attract insects.

- E. capillifolium Award of Garden Merit (AGM) is a foliage plant, producing tall, feathery leaves on 2m-high stems. It is tender, but stands out as a foliage plant during summer and autumn.

- E. capillifolium 'Elegant Feather' grows to 1.5m and has feathery foliage. The leaves turn reddish brown as the season fades. It is usually non-flowering and insignificant when it is in bloom.

- E. coelestinum (now Conoclinium coelestinum) has soft powder-blue flowerheads from August to October, on stems that are just 30cm high.

- E. fistulosum f. albidum 'Massive White' AGM is the hollow-stemmed Joe-Pye weed, with dark yellow-green leaves and white flowers. It may also be found under the name 'Album'.

- E. fortunei 'Variegatum' carries serrated grey leaves with cream edges turning maroon in autumn. Scented, pink flowers emerge from August to November on 1.3m-high stems.

- E. ligustrinum (now Ageratina ligustrina) AGM has bronze-green foliage and scented flowerheads of grey-white from August to November. It is a small shrub, 1m in height.

- E. maculatum Atropurpureum Group AGM produces maroon-pink flowers from August to October, on stems that can grow to 3m high.

- E. maculatum (Atropurpureum Group) 'Album' carries broad heads of off-white flowers between August and October, on stems up to 3m high.

- E. maculatum (Atropurpureum Group) 'Gateway' has purple stems and light purple flowers on stems 1-2m high.

- E. maculatum (Atropurpureum Group) 'Glutball' displays maroon-pink flowers from July to September, on 2m-high stems.

- E. maculatum (Atropurpureum Group) 'Orchard Dene' AGM produces dusky-pink flowers from July to September on 2.4m-high stems.

- E. maculatum (Atropurpureum Group) 'Purple Blush' AGM was raised by Piet Oudolf of Holland. It has dusky-pink flowers from July to September on 1.5m-high stems.

- E. maculatum (Atropurpureum Group) 'Riesenschirm' AGM displays large heads of fluffy wine-red flowers on dark stems between August and October. It grows to 2.5m tall.

- E. occidentale (now Ageratina occidentalis) makes a compact dome of pink-lavender flowerheads followed by fluffy seed heads, on 45cm-high stems.

- E. rotundifolium has wide heads of white flowers from September to November, on compact stems just 50cm tall. The leaves are hairy, ribbed and rounded.

- E. rugosum 'Chocolate' (now Ageratina altissima 'Chocolate') AGM is bushy, with deep purple leaves and stems. White flowerheads are displayed on 80cm-tall stems from September to November; dry flowers can be kept until January.

- E. sordidum is tender, with heads of scented, purple flowers, like Ageratum, and with large, blackish, velvety leaves when stood outside in summer. It can reach 2m high.

- E. variabile 'Variegatum' develops cream-splashed and broadly margined leaves as the season progresses. Scented, white flowerheads are carried during August and September. It grows to 1m high.


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