Bulbing fennel is an annual plant grown for its edible bulb. Common fennel is a perennial grown for its seed. Bulbing fennel is significantly smaller and faster growing than common fennel, which can grow to be four to five feet tall. Bulbing fennel produces a white bulb above the ground, which is harvested for culinary use. Both bulbing fennel and common fennel are susceptible to root rot. Simple crop and water management can go a long way toward preventing root rot.
The first indication of root rot in your fennel will appear as a yellowing of the foliage. This will be followed by foliage wilt. If you have bulbing fennel you may also notice a diminished or lack of growth in the fennel bulb forming above ground. Harvest a plant to confirm root rot. Root rot will turn the plant's root black and mushy. Roots may even disintegrate when touched.
The primary cause of root rot is excessively damp soil and poor soil drainage. Bulbing fennel is an annual crop, which needs regular water to promote quick and steady growth. Perennial common fennel is slower growing, drought tolerant and may be less susceptible to root rot if it is not being watered as aggressively as annual crops. However, if your soil does not drain well, water can build up in the soil, causing roots to rot due to lack of oxygen. Root rot can also be caused by several kinds of fungi, which thrive in moist environments.
Treatment - Drainage
Root rot can be contained if treated right away. The best method of treatment is to improve soil drainage and ensure you are not over watering. Adding a layer of mulch to the soil will assist with water drainage. When adding mulch around already established fennel plants be certain the mulch does not mound around the bulb, increasing moisture at the base of the plant.
Treatment - Roots
In some cases it may be necessary to lift your plants from the soil and treat the infected roots directly.
Use a sharp pair of scissors or clippers to remove damaged roots. Treat the roots with a fungicide before replanting. This method of treatment is best done with perennial common fennel. It is important to note that bulbing fennel does not respond well to tampering with their roots. You should expect some crop loss if you attempt to lift bulbing fennel and replant. When replanted, treated plants will need particular care to try to bring them back to health.
Soils with a heavy clay content drain poorly. When possible, before planting any variety of fennel, break up clay soil to a depth of two to three feet. Add a mix of mulch and sand to the soil. This will allow excess water to move through the soil. This will also assist the soil in retaining enough moisture for the plant to thrive. Water fennel when the soil is dry to at least two inches deep. To check the soil moisture stick your index finger in the ground. If you feel moist soil before the soil line reaches your knuckle, it is not yet time to water.