How to prune two popular grass-like plants

Several weeks ago, I wrote about how to prune ornamental grasses. This article covers a similar topic: how to care for two widely used and loved perennial plants that resemble oversized grasses.

New Zealand flax (Phormium species)

There are only two species of Phormium, but there are lots of named cultivars and hybrids with different sizes and leaf colors. In fact, these plants (which are not true flaxes) are grown primarily for their foliage. Their evergreen leaves are fairly narrow, stiff, and sword-like, with parallel veins and sharply pointed ends; they can be either upright, arching, or in between. Each plant forms only one to a few tall flower stalks, which emerge in late spring to early summer and rise above the foliage. They bear many nectar-rich, tubular flowers that attract hummingbirds. For more information on Phormiums, see this article by Sonoma County Master Gardeners.

African iris or “fortnight lily” (Dietes species)

These evergreen plants are in the iris family; they aren’t lilies as one of their common names suggests. Their leaves are narrow, dark green and strap-like, and they grow in large, compact masses. The two species most often grown in our area are Dietes iridioides, which has six-petaled white flowers with yellow and purple accents, and Dietes bicolor, which has cream colored flowers with maroon accents. They’re grown both for their attractive leaves and pretty flowers.

Pruning advice

Like ornamental grasses, the leaves of these plants emerge from central, ground level “crowns”; there are no branches. Each leaf grows from a small clump, and the plants are formed from many tightly packed clumps.

Both of these plants are dramatic in appearance, but if the leaves are cut partway off their beautiful form will be severely disfigured. Phormium and Dietes plants aren’t meant to be trimmed like hedges or balls, because that ruins their naturally elegant, fountain-like look. If the tapered ends of the leaves are sheared off, they won’t grow back, resulting in a plant full of persistently ugly, straight-cut leaves. Knowledgeable plant people consider such treatment a “crime against horticulture.”

Once Phormium and Dietes species are improperly pruned, the only options for regaining natural-looking, attractive plants are to: (1) replace the plants entirely or (2) undertake several years’ worth of corrective pruning. If attempting to save such injured plants, remove mangled leaves little by little over a period of months or years, depending on the extent of damage.

You might remember that most ornamental grass species benefit from a seasonally appropriate and severe pruning to just a few inches above ground level every one to two years. DO NOT prune the above-listed plants in that fashion; it will severely damage them or even kill them.

The correct way to prune Phormium and Dietes plants is by hand, cutting the leaves off at the base, as close as possible to the ground without damaging other parts of the plant. When pruning, be sure to use very sharp pruners for clean cuts, since the leaves are tough and fibrous. Also, wear gloves and eye protection to guard against injury from the sharp terminal points of the leaves. Dead foliage can sometimes be pulled out rather than cut off, but be careful that you don’t pull out an entire living clump along with the dead leaves.

For some excellent advice and nice visual guidance on pruning Phormiums, watch this YouTube video. The same techniques apply to Dietes plants.

Incorrectly pruned African iris (Dietes iridioides) at Micke Grove Regional Park, compared to the same plant in full bloom and natural form (with a Phormium in the background). (Photos © Kathy Ikeda)















The flowering stalks of these plants, when left to grow naturally, add a lot of visual interest. However, incorrect hedge-type trimming destroys the blooming parts of these plants and eliminates much of their seasonal beauty and wildlife value. The flower stalks of Phormium and Dietes plants are very different, and they require different pruning methods.

The tall flower spikes of Phormium plants only last for a year, and they can be pruned out at the base of the plant when the blooms are spent and the stalk is dry. As with the leaves, prune them out as close to the base as possible, or carefully pull them out if they are dried and break free from the plant easily.

The flowering stalks of Dietes plants — unlike those of Phormium — persist from year to year, and they often re-bloom. They should be left on the plant, and should only be removed if they die or are in an unwanted position. Once the flowers bloom, large, ovoid green seedpods form, each filled with numerous small black seeds. If the seed pods are left on the plant to dry, they pop open and disperse seeds to nearby areas. To prevent vigorous self-seeding, periodically pluck off the seedpods as they form.

I’d like to use this opportunity to promote a valuable resource here in our county: the Green Gardener program. Sponsored by the San Joaquin County Department of Public Works, Solid Waste Division, this training program educates landscapers in correct pruning practices, environmentally safe landscape management principles, water conservation, green waste reduction, and much more. It’s worthwhile for local public agencies and businesses to encourage their landscaping staff to attend this program, because properly trained individuals know how best to care for landscapes and avoid costly maintenance-related mistakes. It’s much cheaper in the long run to hire a knowledgeable professional than to replace plants or trees damaged or killed by improper care or outright butchering.

Far too many planted areas have been irreparably harmed by untrained landscapers who know how to operate mowers, blowers, and hedge trimmers, but know nothing about the acceptable care of plants. Landscape management should be treated with the respect accorded any other profession, both from the standpoint of those hiring the landscapers and from the perspective of those doing the job. I sometimes ask the question, “Would you hire an untrained person to do repair work on your home or to care for your child?”  The same caution and screening principles should apply to the gardening/landscaping world. Even homeowners should take care to hire well-trained gardeners.

My hope is that people will enjoy learning about their plants and give them the careful, loving treatment they deserve. It makes a world of difference, and results in more beauty in our lives!

A reminder: All past San Joaquin Master Gardener articles are available on The Record’s blog site “What’s Growing On.”

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