Short answer
Ornamental grasses usually flop because they are growing too fast or too soft, or because they are not getting enough light. The most common culprits are too much fertilizer (especially nitrogen), rich or overly moist soil, too much shade, and an old clump that has died out in the middle. Wind, heavy rain and wet snow can finish off a tall grass that was already weak.
Common causes
Too much nitrogen
Extension and nursery sources agree on this one. Excess fertility, especially in the form of nitrogen, encourages lush growth that cannot support itself. If you fertilize grasses with lawn feed or you plant them next to a fertilized lawn or bed, this may be the cause. See whether to fertilize ornamental grasses.
Not enough sun
Most ornamental grasses want full sun, around six or more hours a day. In shade, stems stretch toward the light and become thin and weak. Some shade-tolerant grasses, such as northern sea oats, can produce taller, floppier plants in shadier conditions.
Rich, moist soil
Grasses that evolved on lean prairie soils often flop in very rich or constantly wet ground. Heavy amendments at planting can contribute. Good drainage and average soil usually give sturdier growth. Little bluestem is a classic example; upright cultivars such as ‘Standing Ovation’ are covered in little bluestem varieties.
An aging or overcrowded clump
Clump-forming grasses tend to die in the center as they age. The outer ring of stems then falls open and outward like a donut. Missouri Extension recommends dividing regularly to keep clumps young, often around every third year, depending on the grass. See how to divide ornamental grasses. Overcrowding can have a similar effect; see spacing for ornamental grasses.
Tall cultivars in exposed sites
Some varieties are naturally taller or looser than others. Planted where wind funnels through, or where heavy rain and snow load the plumes, they may lean or fall.
Quick fixes this season
- Support the clump. Loosely tie the stems together with soft twine, or use a grow-through ring or stakes placed early in the season. Nature Hills mentions that tall grasses may need staking or bundling during storms or heavy snow.
- Stop feeding. Do not add more fertilizer to a floppy grass.
- Water sensibly. Avoid overwatering an established plant.
- Do not cut a flopped plant to the ground in midsummer unless it is badly damaged, since you will lose the season’s display.
Fixes for next year
- Cut back and divide. In spring, cut the clump back (see cutting back ornamental grasses), and divide it if the center is dead or the clump is large.
- Move it to more sun. A grass in too much shade will not do well, however you support it.
- Cut back feeding. Skip fertilizer for the season and let the plant toughen up.
- Improve the siting. Plant tall grasses where they have shelter from the strongest wind, with room to spread.
Choose sturdy grasses
Some grasses hold themselves up better than others. Upright forms such as ‘Karl Foerster’ feather reed grass and many switchgrass cultivars are often chosen for stiff stems, while arching types like some Miscanthus and fountain grasses are meant to arch and spill. Match the form to the spot. For a list of options, see common names for grass plants.
When flopping is normal
Snow and ice can flatten grasses in winter, and many bounce back or can simply be cut back in spring. For the year-round routine, see our ornamental grass care guide.