Short answer
Usually not, or only lightly. Most ornamental grasses do best in average soil without much feeding. Too much fertilizer, nitrogen in particular, tends to produce lush, weak growth that flops over, and it can mute foliage color. If your grass is growing well, you probably do not need to fertilize it.
Why grasses prefer lean soil
Many ornamental grasses are selections of prairie or woodland-edge plants that grow in soils where the main nutrients come from their own decomposing leaves. Proven Winners points out that mulching with shredded leaves in fall mimics that natural cycle and that feeding often makes grasses stretch and flop. Gardening Know How compares it to a lawn: fertilize grass and it grows fast, which in a clump of tall ornamental grass leads to weak stems.
When feeding can help
A little fertilizer can be reasonable in these cases:
- Poor soil. Very sandy or depleted soil may leave plants small or pale.
- Pale or weak growth. If an established plant looks faded and unusually small, a light feed may perk it up.
- Container-grown grasses. Nutrients in potting mix get used up or washed out. See growing ornamental grasses in pots.
- Miscanthus. Gardening Know How notes that Miscanthus tends to respond better than most to extra fertilizer and water.
Before feeding, check other causes of poor growth: too much shade, dry soil, crowding, or an old clump with a dead center that needs dividing.
When to fertilize
If you do feed, do it once in spring, just as new growth begins. Avoid feeding in summer heat, and do not fertilize in fall. Late feeding can encourage soft growth that is damaged by cold, and one extension source notes that late fertilizing may reduce the winter hardiness of warm-season grasses.
For newly planted grasses, most advice is to hold off and let the plant establish first. Planting in soil with some compost in it is usually enough. See how to plant ornamental grasses.
What to use and how much
- Compost or well-rotted organic matter. A top dressing around the base each spring is the gentlest option, and also improves the soil.
- A balanced, slow-release granular fertilizer. If you use a general-purpose fertilizer, choose a balanced formula, apply at a light rate, and follow the package directions. Sprinkle it on the soil around the root zone rather than on the foliage, work it in lightly, and water.
- Avoid high-nitrogen lawn fertilizers and products formulated to promote flowers or fast leaf growth.
Rates quoted by different sources vary. One rule of thumb is about a quarter cup per plant for a general-purpose fertilizer. When in doubt, use less than the label suggests and watch how the plant responds the following season.
Containers
Potted grasses can be fed through the growing season with a slow-release fertilizer or a diluted liquid feed. Stop in late summer or early fall so the plant can slow down for winter. See overwintering ornamental grasses.
Mistakes to avoid
- Applying fertilizer to wet or dry foliage, which can burn it.
- Feeding late in the season.
- Treating a floppy plant with more fertilizer. If yours falls over, see why ornamental grasses flop over.
- Over-applying “to be safe”. Less is better.
For the full picture of how to look after grasses through the year, see our ornamental grass care guide.