Greenhouse Fennel Plants
Fennel is a member of the carrot and parsley family and is related to dill, caraway, and cumin. It produces aromatic fruits that are incorrectly referred to as seeds. While the fennel seeds are a delicious addition to many foods, this perennial is more commonly grown for its bulb. The fennel bulb does not grow underground but above the soil line. As it grows, soil is piled up around it (blanching) to keep the bulb from becoming green and to retain its sweetness. Fennel can become quite a large plant and has a very deep root system, so when growing fennel in a greenhouse, a large container must be used with plenty of room for the roots. Grow greenhouse fennel plants in a container that is at least a foot (30.5 cm.) deep, or an even better option is a 5-gallon (19 L.) tub.
How to Grow Fennel in a Greenhouse
Fennel seeds are slow to germinate. Sow seeds in the early spring. Plant more than you need and thin them out as soon as they have two sets of true leaves, leaving behind the strongest seedlings to grow. The soil should be around 60-70 F. (16-21 C.) for germination to occur. It should be well-draining and moderately fertile. Fennel tolerates a wide pH range but thrives between 7.0 and 8.0. If you are growing multiple fennel plants in the same container, be aware that their close proximity will likely not result in bulbing, although it will still provide you with plenty of leaves and seeds. Space multiple plants 10 inches (25.5 cm.) apart when thinning.
Greenhouse Fennel Care
When seedlings are 4 inches (10 cm.) tall, transplant into a container that is filled with light soil and pebbles at the bottom to ensure good drainage. As the bulb begins to grow, hill up around it with soil to keep it sweet and white. Keep the plants moist but not soggy. Avoid putting fennel close to dill or coriander, which will cross-pollinate and result in some unpleasant flavors. Fennel is fairly unbothered by pests but aphids or whiteflies may attack the plants. Apply a pyrethrin-based insecticidal detergent to rid the plant of pests.
