How to Grow Pyrethrum

Procure Pyrethrums

• Although pyrethrums can be grown from seed, the seedlings won’t bloom their first year, may produce deformed flowers and are often less vigorous than plants propagated from root divisions or basal cuttings. Also, the seeds of double types don’t always come true and may produce single flowers instead. For the best results, procure plants from a local nursery rather than bare-root mail order specimens, which don’t necessarily travel well.

Provide for Pyrethrums

• To reach their full potential, pyrethrums should grow in very well-drained, sandy and mildy acidic soil – with a pH between 5.8 and 6.8 -- in full sun. Because their foliage often remains shorter than their flower stems, they need to be near the front of flower beds and borders and spaced at least 1 foot away from other plants, to ensure that they receive enough light and air. Scatter an organic fertilizer such as 5-5-5 around the pyrethrums in early spring and midsummer, applying 1 cup to every 20 square feet of ground, before scratching and watering it into the soil. Once established, the plants shouldn’t require supplemental water except during drought conditions. During establishment, water weekly to keep the soil moist for approximately three weeks or until the plant has established itself into the garden. Do not overwater or have soggy conditions. Due to their strong-scented foliage, they usually aren’t bothered by insects.

Prune Pyrethrums

• Pyrethrums can be floppy, so pinch back their tips when they are about 6 to 8 inches tall to encourage more stocky growth. If the plants threaten to keel over later anyway, support them with grow-through hoops or twiggy branches. Since only one flower develops on each pyrethrum stem, you can cut back individual stems as their flowers wither. Once the plants have stopped blooming in midsummer, clean your pruning shears by swabbing them with rubbing alcohol and prune all of the pyrethrums back to the rosettes of foliage at their bases. They may bloom again less prolifically in autumn.

Protect Pyrethrums

• As pyrethrums can be killed by alternate freezing and thawing heaving their roots out of the ground during winter, give them at least 2 inches of mulch to ensure that the ground beneath it stays frozen. To keep the plants from becoming too crowded, divide them once every two years in early spring. To do so, dig up the pyrethrums while their foliage is still below 6 inches in height, hose off their roots and divide those roots into smaller sections by easing them apart with your gloved hands. You can then replant those sections, 1 foot apart, at the same level they grew before.