Can You Cut a Silphium Perfoliatum Back to Control Height?

Cup Plant Culture

• Cup plant thrives in full sun wherever soil is moist but drains well. It is a U.S. native often found in prairies and damp areas and is recommended for rain gardens. The plant grows well in neutral to alkaline loam soil, though it adapts to clay as well. Cup plant flowers from mid to late summer. As a herbaceous perennial, cup plant dies back to the ground every year. Prune off old growth in late winter or early spring.

Controlling Size

• Most perennials, including cup flower, that bloom late in the season can be cut back to make them bloom at a shorter height. The technique is useful for creating a denser plant with more flowering tips and for delaying bloom. Cutting a cup plant back by one-half its height in early summer causes the plant to bloom several feet shorter and about two weeks later than unpruned plants. You can cut the whole plant back, or just parts of the plant to stagger bloom. Sterilize pruning tools before using them and after cutting each plant by wiping with a diluted bleach solution or household antiseptic cleaning spray to prevent the spread of disease.

Technique

• The way you make the cuts depends on your gardening style and the time you have available. When pruning cut plant growing in the middle of other perennials, you might want to prune carefully, cutting just above leaf clusters to avoid leaving ugly stubs. In a more wide open space, however, neatness doesn't count. In "Fine Gardening" magazine's blog The Dirt, landscaper Amanda Thomsen reports that running her cup plants over with a tractor in late spring each year does the trick just as well. When you do prune by hand, watch out for sticky sap -- the plant goes by the alternate common name rosin weed -- cup plant releases when you break or cut its stems.

Controlling Spread

• Trimming cup flower back by one-third, or down to the next leaf below a flower cluster, after the first flush of bloom fades decreases height in late summer as well as forces more bloom. It also serves the important purpose of controlling self seeding. This vigorous plant has an "unstoppable urge to reproduce," according to the Lady Bird Johnson Wildflower Center's Native Plant Information Network, and will self seed volunteers everywhere in your garden if you don't deadhead religiously.

For the Birds

• Consider letting your cup plant go a little wild, however, if you want to attract birds. The "cups" for which the plant is named are formed by the leaves that join in pairs around the plant stem and hold water. Birds like to drink from them. The prolific flowers also produce seed that attract goldfinches and other songbirds. When the plant is allowed to form a dense patch, birds also use it for daytime cover.