Fennel (Foeniculum vulgare) is a hardy perennial herb that typically is grown as an annual. Fennel seeds and young leaves are used in a wide variety of cuisine as a spice. Fennel plants usually are grown from seed. The plants don't handle transplanting very well after they're more than 5 inches tall. Fennel can be grown easily in containers as long as it gets enough light. Growing fennel in pots indoors will allow for the harvest of herbs year-round.
Fill a 12-inch-diameter planter pot that has drainage holes in the bottom with well-draining, moderately fertile potting soil. Place two to three fennel seeds on the soil surface. Cover the seeds lightly with ¼ of an inch of potting soil.
Water the potting soil to evenly moisten it. Set the planter pot in full sunlight in a south-facing window or outdoors if all danger of frost has passed.
Water the potting soil once or twice each week to keep it evenly and moderately moistened at all times. Don't allow the soil to dry out while the fennel seeds are germinating, which usually takes eight to 12 days.
Thin the fennel seedlings to select the strongest one to remain in the pot after the seedlings are about 1 to 2 inches tall. Gradually reduce watering the fennel to only once every two weeks as it becomes established.
Begin to harvest the fennel leaves when the plants are about 6 inches tall, picking off only the top 2 inches of leaves. Harvest the seeds as soon as they turn from green to brown, which usually takes about 65 to 70 days. Remove the fennel plant heads and place them upside-down in a paper bag until they dry.
If you're growing several fennel plants in a larger container or trough planter, space the plants about 10 inches apart. Instead of an all-purpose potting soil, grow fennel in a soilless potting mix.
Ensure that the fennel plants receive enough light, especially if you're growing the plants indoors and not placing them outside in full sun. Place fluorescent grow lamps about 2 to 4 inches above the tops of the fennel plants to provide artificial light in the absence of sunlight.