Rubenza Cosmos Plants

• Variety: Cosmos bipinnatus 'Rubenza'
• Common name: Cosmos / Cosmea / Mexican Aster
• Type: Half-hardy annual
• Flower form: Single
• Colour: Ruby-red ageing to dusky rose
• Height: 60–75 cm (24–30 in)
• Flowering period: June–October
• Position: Full sun (minimum 6 hours)
• Soil: Well-drained, ordinary to poor fertility
• Spacing: 25–35 cm (10–14 in)
• Good for cutting: Yes – excellent, with a long vase life
• Container suitable: Yes, in a large pot
• RHS AGM: Yes
• Awards: Fleuroselect Approved Novelty
• Delivered as: Jumbo plug seedling. Collection from Castle Cary also available

One of the few cosmos that changes colour as it flowers, and one of the shorter ones, which makes it far easier to place than the tall Sensations.

Rubenza – Ruby Red That Fades to Antique Rose

Cosmos 'Rubenza' is a single half-hardy annual, around 60 to 75 cm tall, carrying deep ruby-red flowers that soften to a smoky dusky rose as they age, from June right through to October. It holds the RHS Award of Garden Merit and a Fleuroselect novelty award. Because the blooms fade at different rates, a single plant shows two tones at once: fresh crimson flowers above older rose-pink ones. The effect is richer than any flat red, and it deepens as the nights cool in September.

It is semi-compact, so it sits comfortably in a mid-border or a large pot without the staking gymnastics the 1.2-metre varieties demand. The feathery foliage stays neat, the stems are sturdy, and it flowers hard for months if you keep cutting or deadheading. Bees and hoverflies work the open centres all day. Like all cosmos, it flowers best on poor soil, so resist the urge to feed it in the ground.

How the Colour Works

The fade is a natural feature of the variety, not a trick of the light. Each flower opens a saturated ruby and loses pigment steadily over several days, ending close to a warm antique pink before the petals drop. Plant a drift of it and you get every stage of that journey side by side. It is one of the reasons Rubenza reads so well in a vase, where cut stems carry on opening and shifting tone over the week.

Great Partners for Rubenza

The ruby tones want a clean foil. Plant it with the pure white of Cosmos Sensation Purity to make the red sing, or with the soft lemon of Cosmos Xanthos for a warmer, sunset combination. For real depth at the back of the bed, run it alongside dark dahlias such as Bishop of Llandaff – cosmos and dahlias share a season and a Mexican heritage, and they flower together until the frosts.

Why Choose Ashridge Cosmos Plugs?

Every one of our cosmos plugs is hand-sown and grown right here at our Somerset nursery. We grow from the best quality seed in peat-free compost (we do not collect cosmos seed, because named varieties like Rubenza do not come true from it), and every plant is hardened off before it leaves us. You get a sturdy jumbo plug with slow-release fertiliser already in the rootball, ready to go straight out.

Plants are dispatched by next-day courier from May onwards, weather permitting, in purpose-made recycled cardboard packaging, and they are ready to plant the moment they arrive. There are real people on the phone in Somerset if you need us. We have been growing and selling plants since 1949, hold the Feefo Platinum Service Award, and were voted a Which? Best Buy plant supplier.

Frequently Asked Questions

How tall does Rubenza cosmos grow?

Rubenza reaches around 60 to 75 cm, shorter and sturdier than the tall Sensation cosmos. That semi-compact habit makes it one of the easier cosmos for a mid-border or a generous pot.

Can I grow Rubenza in a pot?

Yes. Its shorter height makes it a good container cosmos. Use a pot of at least 10 litres, a loam-based compost, and feed fortnightly with half-strength tomato food once buds form, as nutrients leach from pots.

Is Rubenza good for cutting?

Excellent. Cut stems keep opening and shifting from ruby to rose in the vase, often lasting a week. Cut just as the buds colour up, and the more you pick the more the plant produces.

What is the difference between Rubenza and Velouette?

Both are rich crimson cosmos, but Rubenza is a plain single that fades to rose, while Velouette is a striped bicolour, crimson edged with white. Rubenza is also shorter.

Will my cosmos come back next year?

No. Cosmos bipinnatus is a half-hardy annual and will not survive a UK winter. It may self-seed in mild, well-drained spots, but seedlings rarely come true. See our how to grow cosmos guide for end-of-season care.