University of Maryland Extension

Herb Problems

Abiotic Problems

1. Bolting

Cilantro bolting
Photo: Howard F. Schwartz, Colorado State University, Bugwood.org
Flowering cilantro 

Bolting is defined as flowering and going to seed prematurely, usually due to unsuitable climatic conditions at certain stages of growth. Cilantro/Coriander is very prone to bolting in response to high temperatures.

Basil also begins to flower soon after planting. Pinch off the flowers as soon as they appear to prolong production and keep harvesting the leaves. 

2. Overwatering 

Poorly drained soil, water that pools due to the slope or plants being overwatered can cause irreversible damage to herbs. Excess water reduces oxygen in the soil, which damages fine roots and renders the plant unable to take up water. Overwatered plants show the same symptoms as plants with root rots, crown rots (crown rot from overwatering) or drought stress. The primary symptom of excess moisture is wilting or yellowing of lower and inner leaves. If excess water continues, plants may experience symptoms such as scorch, leaf drop, and/ or plant death. 

Plants should be watered when needed. Factors influencing plant watering include stage of growth, time of year, humidity, and temperature.

3. Winter Injury

See Overwintering

Excessively wet, cold soil can cause Mediterranean herbs such as rosemary, thymes, and lavenders to die over the winter. 

Insects

1. Aphids

Aphids on the undersides of leaves lady beetles controlling aphids on a plant 

Aphids on the undersides of leaves

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Lady bird beetles controlling aphids

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2. Caterpillars (Black swallowtail or parsley worm caterpillar)

black swallowtail caterpillar on eaten parsley
Photo: David Cappaert, Bugwood.org
Black swallowtail caterpillar 

Parsley, dill, and fennel are some of the host plants of the black swallowtail caterpillar. They can have voracious appetites. But because these caterpillars turn into black swallowtail butterflies, a native butterfly to most of the eastern United States and beyond, the damage is often tolerated. One solution is to plant extra so there will be enough herbs for you and for the caterpillars. 

3. Four-lined plant bug

four lined plant bug adult
damage caused by four lined plant bug

Four-lined plant bug

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Damage caused by four-lined plant bug

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The four-lined plant bug is a pest in the garden in May and June (one generation per year). The adults are 3/16 of an inch long and yellowish green with 4 black stripes down the back. The nymphs (immature bugs) are red with the yellow and black wing pads partially covering the body. This bug feeds on over 250 herbaceous plants. Damage from this plant bug begins as yellow stipples or spots, eventually becoming brown to black necrotic spots. In large infestations, these spots coalesce and cause leaves to turn brown. Four-lined plant bugs are quick and difficult to detect, and only a few can cause noticeable damage.  Spraying an insecticide is not recommended. Lightly prune damaged plants to encourage new growth. 

4. Japanese beetles

 Japanese beetle adult

Can be a problem on basil. Handpick adults or tap infested leaves over a container of soapy water – the beetles will fall in and drown. Avoid baited traps: they attract extra beetles that will increase damage. Do not spray insecticides on herbs. 

5. Spider mites

Spider mites on a leaf
close up of adult spider mites 

Spider mite eggs

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Close-up of adult spider mites on leaf

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Diseases

1. Downy mildew of basil

downy mildew spores on underside of basil leaf
Basil plant with downy mildew

Downy mildew on underside of leaf

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Downy mildew infected basil

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2.  Southern blight

southern blight on thymesouthern blight lesion on stem

Southern blight on thyme

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Southern blight lesion on stem

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