Calendula

Sow seed a quarter inch deep and cover it well, because light inhibits germination. Sow once soil temperatures reach 60F, and expect germination in 1 to 2 weeks. For earlier bloom, start seed indoors about 6 to 8 weeks before your last frost date. Sow a second and third batch a few weeks apart to keep flowers coming. In the mildest winter zones (roughly 9b through 11) the calendar flips and calendula is grown as a fall-sown winter and spring crop instead. Calendula is a cool-season annual that germinates best around 60F and prefers temperatures under 85F, so seed sown into cold soil rots and seed sown late spends its best weeks in heat it cannot flower through.

Move indoor-started seedlings or nursery transplants into the garden once frost danger has passed, into a well-drained spot in full sun. Once the young plants are several inches tall, pinch out the vegetative growing tips to force branching. Calendula survives down to about 25F but young transplants have no hardening behind them, and pinching early turns a single leggy stem into a bushy plant that carries many more flower heads.

After the seedlings emerge, thin them to stand 8 to 12 inches apart. Snip the extras at the soil line rather than pulling, so you do not disturb the roots of the keepers. Crowded calendula holds damp air in the canopy, and that stagnant air is exactly the condition powdery mildew needs. Spacing is the cheapest mildew control you will ever apply.

Pinch or snip spent flower heads off at the base of the stem, and keep at it every few days for as long as the plant is blooming. Plants push a fresh flush roughly two weeks after picking, and will run almost continuously until the first heavy frost. Once a flower head sets seed the plant reads its job as done and slows flowering. Removing heads before seed matures keeps it in vegetative bloom, and it also stops the volunteer seedlings calendula scatters when heads are left to ripen.

Supply 1 to 1.5 inches of water per week through midsummer, counting rainfall. Water at the base rather than over the top of the plants, and do not overwater, since established calendula tolerates fairly dry conditions. If you do water overhead, do it mid-morning so the foliage dries quickly. Soggy soil is a bigger risk to calendula than dryness is, and dense damp canopies hold the high humidity and shade that powdery mildew wants. Watering at the base keeps the soil supplied without keeping foliage wet for long stretches, which is what invites other leaf diseases. Plants may languish in prolonged heat regardless of water, then recover on their own when cooler weather returns.

Check the growing tips and the undersides of leaves for aphids and whiteflies. A stiff spray of plain water dislodges them and is the first control to reach for. Insecticidal soap is an option on heavy infestations, but only on seedlings before the plant is in flower. Calendula flowers almost continuously from late spring to frost and is a heavy pollinator draw, so once it opens there is essentially no safe spray window. Aphids also vector cucumber mosaic virus, which is why you knock the colonies down early rather than waiting.

Look for dusty white patches on the leaves as summer humidity builds. Manage it by improving air circulation, thinning or removing the worst-affected plants, and keeping irrigation off the foliage rather than by spraying. Powdery mildew is calendula's main disease and it establishes in still, humid air inside a crowded canopy. Cultural control works here because the flowers are harvested for eating and drying, which rules out most fungicides.

Harvest whole flower tops, or just the petals, when the flowers are fully expanded. Pick after the dew has dried, and dry the heads in a well-ventilated place out of direct sunlight. Plants re-bloom in about two weeks, so harvest is a repeating cut-and-come-again task, not a one-time event. Petals are picked at full expansion because that is when the head is at its fullest, and drying out of sunlight preserves the orange pigment that direct sun bleaches out. Wet petals mold rather than dry.