How to Grow Crocus

Crocus are small, cheap and among the very first flowers of the year, often opening in February or March and sometimes through snow. They belong to the iris family and grow from corms, flattened underground stems that are replaced by a new corm every year. The flowers open in morning sun and close at night and on cloudy days.

Which crocus to grow

Most garden centers sell three groups:

• Dutch crocus (C. vernus) are the large hybrids, with flowers in white, yellow, purple and stripes. They are the showiest but, according to Clemson, don’t naturalize as well as the species.
• Snow crocus (C. chrysanthus) bloom earlier, with smaller flowers in many colors.
• C. tommasinianus (“tommies”) have slender lavender flowers. University of Maryland Extension calls them one of the best crocus for lawns and compacted soil under trees, and says they spread even where rodents are a problem. NC State, which also calls this species snow crocus, says it can bloom with snow on the ground and is reportedly resistant to squirrels.

For South Carolina, Clemson recommends cloth of gold crocus (C. angustifolius), snow crocus and tommies, along with their cultivars, over Dutch crocus.

Fall-blooming crocus, including saffron crocus (C. sativus), are planted at a different time of year. More than 4,000 dried stigmas go into one ounce of saffron, UC Riverside notes. Don’t confuse any of these with colchicums, the highly poisonous “autumn crocus.” Colchicums have six stamens; crocus have three.

Where to plant crocus

Choose full sun or light shade; full sun gives better flowers. Because crocus finish before trees leaf out, they work well under deciduous trees, along paths, in rock gardens, and in lawns.

Soil is the key. NC State recommends gritty, well-drained soil, ideally a sandy loam with compost, and says to avoid heavy clay. Crocus like moisture in fall and spring while growing, but a dry, warm spot in summer while dormant.

How to plant crocus corms

Plant in fall once the soil has cooled below about 60°F. In the North that is mid-September through October; Clemson plants in November in South Carolina. The dates for your zone above show the window.

1. Depth. Set corms about 3 to 4 inches deep (Clemson, Rutgers, Illinois). NC State uses about 2½ inches and Arkansas about 2 inches.
2. Spacing. Two to 4 inches apart. NC State fits 35 to 70 corms in a square foot for a carpet effect.
3. Grouping. Plant in clusters, not lines. Arkansas plants 6 to 12 corms together, Illinois at least 25, and South Dakota State suggests groups of 20 to 30.
4. Feeding. UC Riverside recommends a small amount of fertilizer at planting and warns that too much produces leaves instead of flowers.
5. Mulch. Rutgers mulches small early bulbs only lightly so the mulch doesn’t get in the way of the shoots.

Arkansas Extension tucks corms under creeping phlox or dianthus, so the same patch of ground flowers twice.

Care after bloom

Leave the grassy leaves for about six weeks after flowering (NC State) so the new corm can store food. In a lawn, that means holding off on mowing those areas until the crocus leaves turn yellow.

Well-sited clumps persist and spread for years. Some species also self-sow. When flowering drops off because of crowding, dig the clump after the leaves wither, separate the corms and replant. NC State suggests dividing about every four years.

Squirrels and other pests

Wildlife is the biggest problem. Clemson and Rutgers both list crocus among the bulbs animals dig up and eat (Rutgers names voles, squirrels, rabbits and deer), and NC State calls squirrels particularly good at finding fresh corms. South Dakota State is the exception, reporting that crocus are among the last bulbs animals bother in its area. Where diggers are a problem:

• Lay chicken wire or hardware cloth over new plantings, anchored with staples or bricks, and remove it before spring growth or let shoots grow through.
• Plant among daffodils, which animals avoid.
• Choose C. tommasinianus, which rodents bother less.

The other common problem is corm rot in wet or heavy clay soil. Choose a gritty, well-drained spot or a raised bed.

Crocus in warm-winter areas

Crocus are rated for zones 3 to 8 by NC State, but they grow in mild California too. UC Riverside reports they do well in urban California in zones 7 through 9 and plants corms in late October or November. Sources differ on chilling: UC Marin Master Gardeners, in zones 9 and 10, recommend pre-chilling corms in the refrigerator at 35 to 45°F for one to three months before planting, or buying pre-chilled corms, while Napa County Master Gardeners, citing a Northern California trial, consider chilling optional in the Bay Area. Expect short-lived flowers during warm spells.

Crocus in pots

Crocus force easily. Rutgers fits about 15 corms in a 6-inch pot. Hold them at 35 to 48°F for at least 12 to 13 weeks, then bring them into a bright, cool room to bloom three to four weeks later.