What hardiness zones is Cosmos atrosanguineus suitable for?

Cosmos atrosanguineus is suitable for USDA hardiness zones 8, 9, 10, 11.

What are the sun requirements for Cosmos atrosanguineus?

Cosmos atrosanguineus requires Full Sun.

How much water does Cosmos atrosanguineus need?

Cosmos atrosanguineus has Low water needs.

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Perennials

Cosmos atrosanguineus

chocolate cosmos

Asteraceae

Mexico (central and western; near-extinct in wild)

At a Glance

TypePerennial

HabitUpright

FoliageDeciduous

Height18–24 inches (45–60 cm)

Width12–18 inches (30–45 cm)

Maturity1 years

Growing Zones

USDA Hardiness Zones

8 - 11

Zone 8

Zone 9

Zone 10

Zone 11

These zones indicate the coldest temperatures this plant can typically survive.

Frost Tolerancetender

Key Features

Attracts Pollinators

Attracts Butterflies

Fragrant (moderate)

Container Friendly

Native to North America

Maintenancemoderate

Overview

A tender tuberous perennial reaching 18–24 inches (45–60 cm) tall and 12–18 inches (30–45 cm) wide, grown as an annual in most of North America. Stems slender, branched, green to reddish, arising from fleshy tuberous roots. Leaves pinnately divided, 3–5 inches (7.5–13 cm) long, with narrow lance-shaped segments, medium green. Flower heads solitary on long stalks, 1–1.5 inches (2.5–4 cm) across, with 8 velvety dark maroon-red to near-black ray florets surrounding a small dark maroon disc; individual florets are silky-textured. A distinctive warm chocolate scent, most pronounced during warm afternoons, arises from the flowers. Does not set viable seed in most garden conditions; propagated by division of tubers or cuttings. In zones 9–11, persists year-round as a tuberous perennial.

Native Range

Cosmos atrosanguineus is native to Mexico, where it is considered extinct or nearly so in the wild. The species survives primarily through cultivation, having been maintained in gardens since its introduction to European horticulture in the late 19th century. Wild populations are documented from oak and pine-oak woodland margins in central and western Mexico.

Suggested Uses

Planted in mixed borders and cutting gardens at 12–15 inch (30–38 cm) spacing where the chocolate-scented flowers can be appreciated at close range. Used in containers of 3–5 gallons (11–19 L); the dark flower color provides strong contrast against lighter companions. Cut flower stems last 5–7 days in water; cut when 1–2 florets on a stem are open.

How to Identify

Identified by slender stems bearing solitary flower heads 1–1.5 inches (2.5–4 cm) across with 8 velvety, very dark maroon-red to near-black ray florets and a small dark disc, combined with a distinctive warm chocolate scent noticeable in warm weather. Leaves pinnately divided with narrow segments, similar to Cosmos bipinnatus but with fewer, broader divisions. Stems arise from fleshy tuberous roots, distinguishing it from annual Cosmos species.

Appearance

Size & Dimensions

Height1'6" - 2'

Width/Spread1' - 1'6"

Reaches mature size in approximately 1 years

Colors

Flower Colors

maroon

burgundy

black

Foliage Colors

green

Fall Foliage Colors

no change

Bloom Information

Bloom Period

~18 weeks

SummerFall

Blooms from June through first frost in annual culture, typically July through October in zones 5–8. Peak bloom July through September. In zones 9–11, may bloom nearly year-round with reduced output December through February. Individual flower heads last 5–7 days; plants produce new buds continuously at branch tips. Chocolate scent strongest on warm afternoons above 70°F (21°C).

Detailed Descriptions

Flower Description

Very dark maroon-red to near-black; velvety textured

Foliage Description

Medium green, pinnately divided with narrow lance-shaped segments

Growing Conditions

Sun Requirements

Full Sun

Requires 6-12 hours of direct sunlight daily

• Full Sun: 6+ hours of direct sunlight

• Partial Shade: 3-6 hours of direct sunlight

• Full Shade: Less than 3 hours of direct sunlight

Soil Requirements

pH Range6.0 - 7.0(Neutral)

357912

Soil Types

loamsand

Drainage

well drained

Water & Climate

Water Needs

Low

Frost Tolerance

tender

Time to Maturity

1 year

Care & Maintenance

Care Guide

In zones 7 and colder, start tubers indoors 4–6 weeks before last frost or purchase transplants; plant outdoors after last frost when soil has warmed to at least 60°F (16°C). In zones 8–9, tubers may overwinter in the ground with a protective mulch layer of 3–4 inches (7.5–10 cm). Space plants 12–15 inches (30–38 cm) apart in full sun. Water when the top 2 inches (5 cm) of soil are dry; overwatering causes tuberous root rot — well-drained soil is essential. Apply balanced fertilizer (10-10-10) monthly during the growing season. Lift tubers after the first frost in zones 7 and colder; store in barely damp peat or vermiculite at 40–50°F (4–10°C) over winter. No serious pest or disease problems; aphids occasionally cluster on growing tips.

Pruning

Deadhead spent flower heads by cutting back to the next lateral bud to keep the plant producing new buds and extend the bloom season by 2–3 weeks. Cut back by one-third in midsummer if plants become leggy. After the first frost in zones 7 and colder, cut stems back to 2–3 inches (5–7.5 cm) above the tuber before lifting and storing for winter.

Pruning Schedule

summerfall

Maintenance Level

moderate

Container Growing

✓ Suitable for container growing

Minimum container size: 3 gallons

⚠️ Toxicity Warning

Non-toxic