When it comes to fertilizer, herbs, unlike vegetables, aren’t heavy feeders. The fertilizer needs depend on the type of herb, the growing conditions and the texture and fertility of the soil.
If you grow herbs in your garden, in order to follow the basics about fertilizing herbs you need to know what type of soil you have—sandy, loamy, or clay soil—and also its composition of nutrients (a soil test is the only way to find out what your soil might be lacking for healthy plant growth).
Herbs grown in pots and hydroponic herbs have different fertilizer needs than the herbs in your garden.
Do Herbs Need Fertilizer?
The short answer is, yes, but not all herbs have the same fertilizer needs. Herbs roughly fall into two groups:
- Slow-growing herbs with small leaves or needles and fibrous, woody stems that are native to the Mediterranean where they grow in dry, infertile soil. These herbs are usually perennials such as bay laurel, culinary lavender, mint, marjoram, oregano, rosemary, sage, savory, tarragon, and thyme.
- Fast-growing herbs with larger, thinner leaves. These can be annuals such as basil, borage, cilantro, chervil, and dill; biannual herbs such as parsley; or perennials such as chives.
Herbs in the first group generally need less fertilizer than herbs in the second group.
What Type of Nutrients Do Herbs Need?
Start out by planting herbs in healthy soil rich in organic matter. In addition, they will benefit from an organic complete, slow-release fertilizer containing equal amounts of the macronutrients nitrogen, phosphorus and potassium. A slow-release fertilizer is especially important if your garden has sandy soil because nutrients wash out quickly.
To give fast-growing herbs that you harvest often an extra boost, you can also apply fish emulsion, an organic fertilizer that is high in nitrogen, with an NPK ratio of 4-1-1 or 5-1-1.
How Often Should I Fertilize Herbs?
The frequency of fertilization follows the growth pattern of the herbs. In soil of average fertility, it is usually sufficient to apply a balanced fertilizer in the spring when they break dormancy, or when the new growing season starts.
For other herbs, a light monthly application of a slow-release complete fertilizer should be enough—unless the leaves start to look yellow, which may be a sign of nitrogen deficiency. In that case, applying fish emulsion is a quick fix but before you reach for the fertilizer bottle, rule out that the yellowing of the leaves is not caused by something else.
How to Fertilize Herbs in Containers
Herbs grown in containers need fertilizer applications more often, because with the frequent watering that container plants require, the fertilizer in the potting mix washes out more quickly. Just as with sandy soil, it is important to use a slow-release fertilizer.
The roots of container plants are in a confined space, unlike herbs grown in garden soil or raised beds, which can lead to over-fertilizing if you aren't careful. Organic fertilizers are recommended over synthetic fertilizers, which contain a high level of salts that can build up in the container over time. To prevent this, it is best to use half of the strength of the fertilizer amount specified on the label for any type of fertilizer.
How to Fertilize Hydroponic Herbs
The fertilizer requirements for hydroponically grown herbs are quite different from herbs grown in garden soil or potting mix. They need regular and frequent fertilization, every two weeks or as specified in the directions of your hydroponic system, using a special hydroponic fertilizer for vegetables and herbs.
Hydroponically grown basil is susceptible to magnesium deficiency, because the water in which the plants are grown or the added fertilizer do not supply enough magnesium. This micronutrient deficiency manifests itself as chlorotic yellowing leaves, typically between the leaf veins. To fix it, add a liquid magnesium supplement, which is usually sold as a combination of calcium and magnesium, and follow the instructions on the label.
Is It Possible to Overfertilize Herbs?
Adding too much fertilizer to herbs usually leads to an excess of nitrogen, which has undesirable results, especially for slow-growing herbs. For basil and other thin-leaved herbs, the fast leaf growth induced by nitrogen is fine because you want your plants to be lush. For rosemary and other Mediterranean herbs however, rapid growth means that there is less concentration of essential oils causing the herbs to become less aromatic, and have a weaker flavor.