Garlic leaves turning yellow: maturity, hunger, rust or wet roots

Garlic leaves turning yellow from the bottom up in late spring or early summer usually means the crop is finishing, not failing. Yellowing earlier in the season, or with orange spots, a pale thin plant or a soggy pot, points to a cause you can act on. Check which leaves are yellow and when, then match the pattern below.

Start with the calendar and the pattern

Ohio State Extension says garlic is ready for harvest “when the lower leaves begin to yellow”, and explains why: the leaves are connected to the bulb wrappers below. NC State sees this in mid-June in its region, and Utah State says to harvest when the tops begin to yellow or start to fall over, but before they are completely dry. If your lower leaves are yellowing in that window while the upper leaves stay green, you are almost certainly looking at maturity.

If the yellowing is earlier, hits the whole plant, or comes with spots or wilting, work through the other causes. The sections below give the detail.

Maturity: yellow lower leaves at the end of the season

This is the cause to rule in first. Penn State Extension gives a range for the harvest signal: garlic is ready when 40 to 60 percent of the leaves have yellowed, and it notes that garlic generally has six leaves. NC State is more cautious and digs a test bulb when two or three leaves on a plant are yellow.

Two things cost you if you wait too long. Ohio State warns that waiting until leaves turn brown results in rotted or missing wrappers. Penn State says bulbs harvested too late start to split open, and the RHS adds that late-lifted bulbs have a shorter storage life.

Water matters here too. Ohio State says to reduce irrigation once leaves begin to yellow, because the plant needs to dry down, and Utah State says to stop watering when the leaves start to mature. For how to read the harvest window and cure the bulbs, see growing garlic in containers.

Nitrogen: pale plants, yellow tips, thin stems

University of Minnesota Extension lists the nitrogen-deficiency pattern as yellow leaf tips, an entire plant that looks pale green to yellow, and thin stems. Cornell Cooperative Extension calls garlic a heavy feeder that requires adequate nitrogen, and Utah State side-dresses with nitrogen in spring.

Ohio State warns that over-watering or heavy rain can remove nutrients. Its advice for plants that look yellow, small-leaved or not vigorous is an additional fertiliser application before bulbing, for example a garlic or balanced vegetable fertiliser. The timing matters: the advice is for before bulbing, not for a plant that is already yellowing at maturity. Follow the label of whatever fertiliser you use. Small bulbs are a separate problem with their own causes, covered in why garlic bulbs are small.

Rust: orange spots

University of Minnesota Extension describes garlic rust as small bright orange spots on leaves and stems, with black spots developing at their edges. With many spots, leaves wilt, turn yellow and dry out, and the bulbs are small and of poor quality. UC IPM says lesions typically form first on older leaves and then spread to younger ones, and that severely affected leaves can be covered in pustules, turn yellow and then die.

The fungus is active in cool weather, between 57 and 75 °F according to UC IPM. The RHS says infection is worse on nitrogen-rich soil with low potassium, so go easy on feeding, and says not to crowd plants because crowding raises humidity. It also says no fungicide is approved for home gardeners to control leek rust, which also infects garlic. The RHS also advises avoiding wetting the leaves when you water garlic, since that can encourage fungal problems. UC IPM recommends rotating away from Allium crops for two to three years.

Rust also hits chives; see chives turning yellow for that plant.

Wet roots and rots: yellow from the tip down, wilting

Garlic in a pot that stays wet is exposed to several root and basal problems, and their leaf symptoms overlap. According to University of Minnesota Extension:

• Fusarium basal rot turns leaves yellow to brown from the tip down and wilts them, with watery brown decay at the base of the bulb.
• Pink root turns leaves yellow with a reddish tinge from the tip down because of root damage, and commonly occurs mid to late season in areas with poor soil.
• Botrytis neck rot yellows the outer leaves first, and is common in cool wet weather or in heavy wet soils.

Ohio State puts the general rule plainly: bulb rots are a problem in wet years or in poorly drained soil. So the first move is physical. Lift the pot, check every drainage hole and let the mix dry more between waterings. If the plant keeps declining, dig one and look at the roots and the base; watery brown decay at the base is the sign of Fusarium basal rot. Overwatered vs underwatered helps if you are unsure which side of watering you are on.

Pests and virus

Two less common causes are worth a glance. University of Minnesota Extension says onion maggot makes plants yellow and wilt, sometimes rapidly, with holes in the bulbs. Onion yellow dwarf virus starts as yellow streaks at the base of young leaves, spreads to all the leaves, and is spread by the green peach aphid. Confirm the pattern before putting effort into either.

If none of these fit, the broader yellow leaves cause finder covers other plants and patterns.

Sources

• Ohio State Extension, Growing garlic in the garden
• Utah State University Extension, Garlic in the garden
• Penn State Extension, Garlic production
• NC State Extension, It’s time for garlic
• Cornell Cooperative Extension Schoharie-Otsego, Guide for growing garlic
• University of Minnesota Extension, Garlic: leaves yellow then wilt
• University of Minnesota Extension, Garlic: flecks, spots, streaks or blotches on leaves
• UC IPM, Onion and garlic rust
• RHS, Leek rust
• RHS, How to grow garlic