How to Trim Back Blue Perennials

How and when to trim back blue perennials depends on the species.

Trimming or cutting back refers to severely pruning a plant, usually down to a height of about 2 inches. This improves growth and appearance while encouraging additional blooming. The proper method and timing for trimming back blue perennials varies, depending on the species. Another form of trimming back, known as pinching, keeps plants growing in a compact, dense form. This type of pruning benefits several species of blue perennials as well.

Things You'll Need

• Hedge shears

Instructions

1. Cut back spring-flowering blue perennials with hedge shears after flowering ends. Leave one-half the original height when pruning these flowers. Siberian bugloss, columbine and clematis are examples of blue perennials that benefit from this treatment.

2. Trim back summer-flowering blue perennials with hedge shears to a height of 2 inches or less, depending on the species. For perennials that develop a weathered appearance later in the growing season, trim back after flowering. Prune phlox back to half its height in late May or early June, for example. Trim back before flowering if you want to keep the plant shorter than its mature height, suggests the University of Minnesota. This is a good step to take with Russian sage in the spring, for instance. False indigo and phlox are two blue summer-blooming perennials that need this type of pruning for optimal growth. If you see new growth at the bottom of the plant, do not prune this off. In other cases, simply cut the plant down to ground level.

3. Pinch back some blue perennials with your fingernails to remove the main bud and encourage the plant to produce additional blooms. Do this with asters until early July, suggests University of Illinois Extension. Other plants that benefit from pinching back include balloon flower and Russian sage.

Tips & Warnings

• Deadhead phlox and other blue perennials to reduce reseeding, increase blooming time and encourage re-blooming within the same growing season.

• Do not pinch back certain blue perennials, including columbine and others that grow in a rosette.

References

• Iowa State University Extension: Perennials Tolerant of Dry Conditions
• Purdue University Extension: Recommended Perennial Flowers
• University of Vermont Extension: Native Perennials Add Color to the Flower Garden
• North Carolina Cooperative Extension: Perennial Flowers for North Carolina
• University of Illinois Extension: Pinkching and Pruning--A Perennial Primer
• University of Minnesota: Pruning Perennials

Resources

• University of Illinois Extension: To Prune or Not to Prune--Perennials
• Purdue Extension: Cut Back Perennials Now or Later?