Repotting guide

When & how to repot Mexican sunflower (Tithonia rotundifolia)

Also called Mexican sunflower, Tithonia.

More about mexican sunflower

About Mexican sunflower

Tithonia rotundifolia · also called Mexican sunflower, Tithonia · flowering

Mexican sunflower is a vigorous, heat-loving half-hardy annual from Mexico and Central America producing vivid orange-red, daisy-like blooms up to 8 cm across on tall, branching plants from midsummer to first frost. An exceptional pollinator plant and cut flower. Thrives in poor, dry soil in full sun; struggles with rich soil, excess nitrogen, and root disturbance.

Mature size: 120–180 cm tall, 60–90 cm spread

Watch for — Stem lodging and breakage: Tall stems on the species type are hollow and brittle. Stake plants in exposed positions once 60 cm tall. Avoid excessive nitrogen feeding, which makes stems even weaker. Grow compact cultivars ('Goldfinger') in windy sites.

How to tell mexican sunflower needs repotting

Repotting on a calendar is less reliable than reading the plant. For mexican sunflower, 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 mexican sunflower 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 mexican sunflower

Pot on seedlings as they grow; not a perennial repot. Mexican sunfloweris grown for one season, so the question is really “how often to pot on” — keep moving it up before the roots circle. Tall, erect, branching half-hardy annual; produces a thick central stem with lateral branches each terminating in a flower.

What size pot to step mexican sunflower up to

Pot mexican sunflower 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 mexican sunflower

Pot mexican sunflower 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 mexican sunflower

Pot on before it is root-bound. Check mexican sunflower 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 poor to average, well-drained loam or sandy loam; ph 6.0–7.5 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 mexican sunflower 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 mexican sunflower

Mexican sunflower wants poor to average, well-drained loam or sandy loam; ph 6.0–7.5. Deliberately thrives in poor, low-fertility soil. Rich, fertile soil or heavy nitrogen feeding produces excessive leafy growth, weak, hollow stems prone to lodging, and fewer flowers. Avoid heavy clay and improve drainage with grit where needed. Always use fresh mix when you repot — reusing old, broken-down soil reintroduces the compaction and poor drainage you are repotting to fix.

Repotting mexican sunflower — frequently asked questions

How often should you repot mexican sunflower?

Pot on seedlings as they grow; not a perennial repot for mexican sunflower. Mexican sunflower is a seasonal crop, so you pot it on as a growing plant rather than repotting a perennial. Step seedlings up gradually into poor to average, well-drained loam or sandy loam; ph 6.0–7.5 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 mexican sunflower need?

When is the best time of year to repot mexican sunflower?

Can you put mexican sunflower straight into a much bigger pot?

No. Even a fast-growing mexican sunflower 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 mexican sunflower after repotting?

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