Foliar fungal infections that crop up in hot, humid weather can turn healthy plants to black mush in a few days. Herbs with woolly or hairy leaves, such as artemisia, lamb's ears and yarrow, are most susceptible. Cut away affected stalks and wait; the plant may recover. Improve air circulation and prune herbs carefully to avoid future problems.
Powdery mildew whitens the leaves of herbs, particularly if the plants are too close together or if the weather is damp. Allow for adequate air circulation and water sparingly.
Proper watering is also critical to avoid root rot, a fungal disease that kills drought-loving plants such as lavender, sage, oregano, rosemary and tarragon. Make sure drainage is adequate, keep the soil loose and don't overwater.
Laura's herb garden
My formal-looking but informal-feeling herb garden is a 4.8-by-4.8-metre square, with four two-by-two-metre beds dissected by gravel paths. Each of the four beds is enclosed by a boxwood hedge, and there's a sundial in the middle with thyme growing at its base. Hedges of lavender and lavender cotton (Santolina chamaecyparissus, Zone 6), the house and my cold frames serve as the exterior borders for the garden. The site is right outside my back door, giving me easy access to herbs all year. It's well protected, and residual heat from the house allows me to overwinter herbs, such as lemon verbena, that are apparently too tender for my climate. I established formal theme beds (culinary, medicinal, crafting and tea herbs), but mix and match informally within the beds.
Some herbs can become invasive. Control self-seeders by cutting off fading flowers before they go to seed. Those that spread by runners, rhizomes or roots can be confined by planting in a deep pot. Or, if you have an established crop you want to eliminate, cut it down when it sprouts in spring and mulch the area heavily for an entire season. The following herbs must be controlled: bergamot, comfrey, lady's mantle, lemon balm, mint, oregano, sweet Annie (though I don't find it misbehaves in my garden), sweet woodruff, sweet violet, tansy and valerian.