Weeds compete with plants for water, nutrients, sun, and space—the key to defeating them is to remove them as soon as you see them. One of the virtues of many herbs is that the fragrant volatile oils they exude are primarily produced to keep insects away; so many of the plants in your herb garden will naturally protect themselves against pests.
Once weeds begin to appear on the surface of the soil in spring, you should remove them by hand or using a hand-fork for those that have long roots. To help prevent the weeds from returning, you have to remove them before they set seed and make sure you get as much of the root out of the soil as possible, especially with perennial weeds. Check the soil regularly for any new shoots that may be appearing and remove them immediately.
If you want to clear a large area of weeds from the soil before planting in it, try solarizing: clear a patch of soil, water it thoroughly, then cover the area with a piece of clear plastic sheet and bury it at the edges to prevent any air from getting out and in. In the next six to eight weeks, the sun will create killing heat and steam under the plastic sheet. After that time, remove the plastic and plant your herb plants immediately.
Caterpillars: Pick off by hand, wearing a glove, and eliminate them. Encourage parasitic wasps to eat them by planting flowering herbs, or spray infested plants with two cups of water mixed with a peeled clove of garlic (strain the liquid before use).
Slugs and snails: Set traps of shallow bowls of beer to drown them, or pick off by hand at night when they are most active. You can also use non-toxic pellets or apply copper tape to containers to give them a mild electric shock as they pass over it.
Aphids: Along with scale, mealy bugs, whitefly, spittle bugs, thrips, and red spider mites, aphids weaken a plant’s growth. Spritz them off plants with water from a hose or use an organic, insecticidal soap.
Vine weevils: A major pest, these grubs appear at night and chew notches out of leaf margins and devour roots. They are tricky to control biologically, but you can add nematodes to the soil to kill them.
Avoiding diseases and bacteria
Most herbs are remarkably free of plant diseases and lots of them can be prevented by good care and maintenance, but sometimes diseases do develop. Deal with them rapidly to avoid lasting damage.
Diseases: Grow resistant varieties and apply good cultural practices and hygiene in the garden. Try not to work in the garden when it’s wet because you may inadvertently spread diseases. Examine plants frequently, removing infected leaves as they fall. Dispose of any infected plant material carefully—it’s best to burn it.
Bacteria: If bacteria enters a plant through a wound, spray the affected area with a simple homemade organic solution. To prepare the solution, purée a dozen peeled garlic cloves and then mix the garlic purée with 11⁄4 quarts (1.2 liters) of water. Strain the liquid, pour it into a bottle with a spray mist nozzle attachment, and apply as necessary. If the plant doesn’t respond within a couple of days, clip off the infected parts and dispose of them. Make sure that you sterilize your clippers or pruning shears before you use them again on other plants.
Although fungi is fairly rare among herbs, mint is prone to rust and downy and powdery mildews can develop in warm, humid, and wet weather conditions. Spray with an alkaline solution to prevent fungi from taking hold, and use organic controls for fungal diseases.
Mosaic viruses cause yellow, white, or light green dots on leaves. Other viruses cause curled leaves, and ring spot viruses cause pale, yellowed ringed spots on leaves. They don’t cause serious damage, but you can remove affected plants and destroy them.