Annual
Cosmos
Cosmos bipinnatus · Asteraceae
Also called: Mexican Aster, Garden Cosmos
Cosmos (Cosmos bipinnatus) is a low-water annual well suited to Tucson and the low desert. It grows to 2-4 ft tall (some to 5-6 ft), 1-3 ft wide in full sun, with a fast growth rate.
Cosmos at a glance
- Water use
- Low (established)
- Sun
- Full sun (afternoon shade in peak summer heat is beneficial in Tucson)
- Mature size
- 2-4 ft tall (some to 5-6 ft), 1-3 ft wide
- Growth rate
- Fast
- Bloom
- Pink, rose, white, crimson, lavender (daisy-like with yellow centers), Spring through fall; in Tucson best flushes are spring (Mar-May) and again after monsoon rains into fall. Reseeds readily.
- Cold hardiness
- Frost-sensitive (USDA 9a-9b); killed by hard frost. Tender annual completing its life cycle in one season.
- Soil
- Tolerates poor, well-drained soils; not picky about native desert soil. Avoid rich, overly fertile beds. Neutral to slightly alkaline pH is fine.
- Native range
- Mexico (high-elevation regions); naturalized widely
- Best used as
- Color beds, Pollinator/butterfly gardens, Cut flowers, Cottage-style plantings, Reseeding wildflower mixes
- Wildlife
- Excellent for bees, butterflies, and other pollinators; seed heads attract birds (especially finches).
- Toxicity
- Non-toxic to humans, dogs, and cats (ASPCA lists Cosmos as non-toxic).
How to grow Cosmos in Tucson & the low desert
Watering
Water deeply 2-3 times per week to establish, then 1-2 times weekly; drought-tolerant once established and prone to flopping/excess foliage if overwatered or over-fertilized. Sow as a cool-to-warm season annual in Tucson: plant Feb-Apr for spring bloom or after monsoon onset in late summer; avoid the brutal June pre-monsoon heat for new seedlings.
Fertilizer & nutrients
Low feeder; lean soil produces more flowers. Skip or use only a light application of balanced fertilizer at planting. High nitrogen yields lush foliage at the expense of blooms.
Pruning & care
Deadhead spent flowers regularly to prolong bloom; pinch young plants once for bushier growth. Cut back leggy plants by a third mid-season to rejuvenate.
Notes
An easy, fast warm-season annual for Tucson color. Direct-sow seed for best results. Tends to reseed itself. Protect tender seedlings from late frost and shade them through the worst June heat until monsoon humidity arrives.
Sources: University of Arizona Cooperative Extension (Pima County Master Gardeners); AMWUA Landscape Plants for the Arizona Desert; ASPCA Toxic/Non-Toxic Plant List