Repotting guide

When & how to repot Purple Fountain Grass (Pennisetum setaceum 'Rubrum')

Also called purple fountain grass, red fountain grass.

More about purple fountain grass

About Purple Fountain Grass

Pennisetum setaceum 'Rubrum' · also called purple fountain grass, red fountain grass · flowering

Purple fountain grass is a tender ornamental grass with burgundy-red foliage and arching, foxtail-like purple plumes from summer to frost. It forms a graceful fountain-shaped mound and is grown as an annual or container specimen in cold climates, overwintered only where frost is absent. Heat-loving and sun-hungry, it adds rich colour and movement.

Mature size: 0.9-1.2 m tall and 0.6-0.9 m wide.

Watch for — Green, dull foliage: Insufficient sun fades the purple colour and reduces flowering; move to the brightest possible spot.

How to tell purple fountain grass needs repotting

Repotting on a calendar is less reliable than reading the plant. For purple fountain grass, watch for these signs:

Roots circling the bottom of the module or pot, or poking out of the drainage holes.
The seedling dries out within a day and growth has visibly stalled.
Roots are white and matted in a tight spiral when you tip the plant out.
It has outgrown its current container for the stage of the season — pot purple fountain grass on before it becomes hard root-bound.

For the underlying biology of a pot-bound root system and why it stalls a plant, see our guide to spotting and fixing a root-bound plant.

How often to repot purple fountain grass

Pot on seedlings as they grow; not a perennial repot. Purple Fountain Grassis grown for one season, so the question is really “how often to pot on” — keep moving it up before the roots circle. Tender, warm-season clump-forming grass with a fountain-like mound of arching purple foliage and nodding flower spikes; frost-sensitive and treated as an annual or overwintered as a dormant container plant in cold regions..

What size pot to step purple fountain grass up to

Pot purple fountain grass on gradually — a seedling jumped straight into a huge pot sits in cold, wet, airless soil and stalls. Step up one or two sizes at a time as the roots fill each container, finishing in a large final pot or the ground. The aim is roots that never circle and never check.

Not sure of the exact diameter? Our pot size calculator takes the current pot and root spread and tells you the right next size — it deliberately recommends a single step up, never a big jump.

The best time of year to repot purple fountain grass

Pot purple fountain grass on through the active growing season, whenever roots fill the current container — there is no single date, just "before it becomes root-bound". Avoid potting on during a cold snap.

Step-by-step: repotting purple fountain grass

Pot on before it is root-bound. Check purple fountain grass regularly; move it up as soon as roots reach the edge of the cell or pot, not after they have circled.
Step up one or two sizes. Choose the next container up — not a giant one. Cold, wet, unused soil around a small root system stalls seedlings.
Knock it out gently. Support the stem, tip the pot, and ease the rootball out without breaking it. A little teasing of circled roots at the base is fine.
Pot into rich mix. Set it into fresh fertile, well-drained soil or quality potting mix at the same depth (tomatoes are the exception — they can go deeper to root along the stem).
Water in and grow on. Water well, keep it in good light, and resume feeding once it is established and growing again.

Aftercare

Water purple fountain grass in well and keep it in bright light; a freshly potted-on seedling can wilt for a day while roots settle, so do not overcompensate by drowning it. Do not fertilise for about 1 week — fresh mix already carries nutrients and feeding freshly disturbed roots scorches them.

The right soil mix for purple fountain grass

Purple Fountain Grass wants fertile, well-drained soil or quality potting mix. Grows in most well-drained soils and thrives in rich container compost. Sharp drainage is important, as cool wet soil quickly rots this heat-loving grass. Always use fresh mix when you repot — reusing old, broken-down soil reintroduces the compaction and poor drainage you are repotting to fix.

Repotting purple fountain grass — frequently asked questions

How often should you repot purple fountain grass?

Pot on seedlings as they grow; not a perennial repot for purple fountain grass. Purple Fountain Grass is a seasonal crop, so you pot it on as a growing plant rather than repotting a perennial. Step seedlings up gradually into fertile, well-drained soil or quality potting mix so the roots never circle the cell, ending in a large final container. A root-bound transplant stalls and never fully recovers.

What size pot does purple fountain grass need?

When is the best time of year to repot purple fountain grass?

Can you put purple fountain grass straight into a much bigger pot?

No. Even a fast-growing purple fountain grass should only go up one pot size at a time. A vastly oversized pot holds a reservoir of wet soil the roots cannot reach, which stays cold and soggy and rots the roots — the opposite of what you wanted.

Should you fertilise purple fountain grass after repotting?

Not immediately. Wait about 1 week after repotting purple fountain grass. Fresh mix already contains nutrients, and feeding freshly cut or disturbed roots burns them. Resume your normal feeding routine once you see new growth.