Growing Herbs - Pests & Other Problems - By Kate Wall

Growing Herbs - Pests & Other Problems

Many of our favourite culinary herbs also happen to be amongst the easiest plants to grow at home. Most herbs are relatively hardy plants that are forgiving of a little neglect, but as with all plants, sometimes they fail to thrive and we need to intervene.

First and foremost ensure you have planted your herb in the right growing conditions. Mint and lemongrass will be happy with and thrive in damp conditions but most other herbs will not. Almost all of the other commonly used culinary herbs will require good drainage. In particular rosemary, lavender, thyme and oregano will need excellent drainage. Poor drainage will very quickly lead to fungal problems and rot in these herbs, and is the most common reason why these herbs do not thrive. If you have heavy soils or poor drainage, create raised beds or grow your herbs in large pots. You can find out more about growing herbs in pots here.

In the wrong growing conditions, all plants will fail to thrive and will be more susceptible to pests and other diseases. Herbs are no exception. In addition to good drainage, herbs will prefer a neutral to slightly acid soil with added organic matter, and full sun. Insufficient sunlight is another very common reason why herbs fail to thrive. Herbs grown in poorly lit situations will tend to be spindly and can often have weak stems and fewer leaves to harvest. A minimum of 6 hours of sun per day is required to grow herbs well, even more for the Mediterranean herbs which like hot dry conditions (rosemary, thyme, oregano, sage and lavender).

Herbs such as basil, parsley, mint, lemongrass, dill and others will not do well if they are kept too dry so ensuring sufficient water, together with good drainage will be essential for healthy plants. In a garden situation, a soil enriched with organic matter and mulched will see herbs thrive with minimal additional watering.

 A herb which is not growing in suitable conditions will not only fail to thrive, but will also tend to have a poorer flavour than will the same herb growing in better conditions.

The high essential oil content in the leaves of many herbs is what gives the herb the flavour we appreciate. This same essential oil content tends to make the leaves of herbs less attractive to sap sucking and leaf chewing insects. For this reason herbs often make excellent companion plants to repel insects from other vegetables growing nearby. Herbs growing in good conditions will therefore have far less trouble with insect pests than will herbs which are growing under stress.

If your herbs are being attacked by mealy bug, aphids or white fly, chances are they are growing in a location which receives too much water and / or too little sunlight. Improving the growing conditions will be the most effective way of managing these pests.

Grasshoppers and sometimes also caterpillars and leaf miners will sometimes attack basil, mint or other herbs. Herbs with a stronger flavour are less likely to be attacked. It can be beneficial to grow some mild or sweeter flavoured basil or other herbs nearby to the fuller flavoured herbs you wish to harvest. The milder flavoured herbs will be more attractive to chewing insects and distract them from the herbs you wish to harvest.

Care should always be taken when using pesticides on edible plants and their use avoided where possible. Check labels for exclusion times – the amount of time after using the product until it is safe to consume the leaves of the plant. Long lasting insecticides in particular will not wash off as they are absorbed into the plant tissues. Pyrethrum is a good option for spraying pests on edible leaves as it will wash off easily leaving leaves safe for eating. If the pest problem is not easily resolved, try improving the growing conditions, or additional companion planting with other plants which are more highly attractive to the pests rather than relying on poisons to grow edible plants. Exclusion netting can work to keep grasshoppers and caterpillars off leafy herbs.

Growing herbs in a mixed planting together with other flowers and vegetables can be very effective for deterring leaf eating insects, the majority of whom find their food by sight or smell. In a mixed planting singling out one plant is far more difficult, so it is unlikely that the pests will damage the entire crop, even if they do find one or two plants.

This mixed planting technique can also be very effective for deterring possums which are well known to like parsley.  Growing a strong smelling plant such as dogbane nearby can sometimes deter possums, as can placing camphor nearby. If using camphor which is designed for use against moths in cupboards, be aware that it is soluble and toxic, so do not place it too near the edible plants and certainly not where it can dissolve and drip onto them.

Most herbs are grown as annuals and therefore are reasonably short lived. For this reason they can succumb very quickly to pest or fungal problems, and are not always easy to save if they do struggle. In most cases the problem lies in growing the plant in situations which do not suit that herbs needs such as too much water or too little sun. Annual plants are often considered easier to replace than to transplant to a better location.

Perennial herbs tend to be hardier and less prone to pest problems. Perennial herbs which are affected by pests or fungal problems are more likely to respond well if transplanted into better situations, or given improved care.

In general most herbs need no additional care than most garden plants. We tend to grow them in pots or close to the kitchen which is not always the best location for them and can make them more difficult to grow well. By addressing the growing conditions, most herbs will be very trouble free and rewarding to grow.