Yellow Leaves on Alliums

Normal Yellowing

• While many allium varieties grow as perennials, common harvesting practices generally restrict regrowth. Near the end of the growing season, usually around September and October, the plants begin to enter their dormant stage. Like other bulb plants, the leaves stop growing and begin turning yellow. This signals the time to prepare for the allium harvest by cutting back the leaves. Wait two to three weeks before digging up the edible onions or garlic bulbs. This type of yellowing is normal and not a sign of illness.

Soil Nutrients

• Inadequate levels of soil nutrients are a common cause of leaf yellowing in alliums. These plants have a high requirement for nitrogen, and large plants can deplete the existing amount. The main symptom of low nitrogen levels involves leaves that begin yellowing near the tips. Without treatment, the yellowing can continue along the entire lengths of the leaves, withering entire plants as it progresses. Adding a nitrogen fertilizer to the soil prior to planting, as well as every two to three weeks during the growing season, helps ensure adequate levels of this nutrient.

Moisture

• Yellow leaves that look and feel dry can indicate a lack of moisture in the soil, especially during the critical stage of development that usually falls between mid-May and early July. Alliums require even soil moisture. These plants have very shallow root systems that are sensitive to dry conditions. Keeping the soil slightly damp near the developing bulbs reduces the risk of dehydration. A layer of mulch helps hold in moisture and minimize leaf yellowing.

Pests

• Onion maggot is a type of larvae that bores into the underground stems of allium plants, causing the leaves to yellow and wilt. The small maggots feed on the bulbs and destroy the plants. Remove affected plants as soon as they appear to reduce the chance of widespread infestations. Alternating the crops helps to avoid planting onions and garlic plants in the same soil every year. Planting new allium in a site that last contained onions or garlic exposes the new plant to nutrient-deficient soil and possible colonies of onion maggots.