All plants need sustenance to keep them healthy and herbs are no different. Soil and potting compost tire quickly and regularly need replenishing or feeding.
From: DK Books - Herbs
The signs of nutrient deficiency are typically a reduction in general vitality sometimes combined with a yellowing or reddish-brown mottling of the leaves. These symptoms can easily be confused with wind scorch, drought, or water logging and it would be a mistake to reach for the fertilizer or mineral supplements without first checking for these disorders. Dig a small hole under the canopy of larger herbs or next to the rootball, if newly planted, and feel how wet the soil is.
Types of Fertilizer
Finding the right type of fertilizer can be bewildering when faced with the range of ready-mixed liquid, granular, or slow or rapid release fertilizers, none of which are really a substitute for digging in compost or farmyard manure. Ask for advice and remember that different plants tend to need different formulations. Regular transplanting will minimize the amount of extra feeding needed and this can be supplemented by using foliar applications. A scattering of general-purpose, balanced fertilizer will keep most of your plants quite content.
Add granular formulations to the soil surface or mix with compost when transplanting—follow the manufacturer’s recommendations. (image 1)
Liquid fertilizers come in powder or concentrated liquid form; use a dedicated watering can to avoid mix-ups with weedkillers. (image 2)
The amount and frequency of watering that your herbs require will vary depending on climate and season, but all are likely to need extra moisture when recently planted and when in active growth. Try to apply the water directly but gently to the soil or mulch rather than soaking the leafy canopy as this can cause damage and promote fungal diseases.
Containerized herbs are likely to need daily watering, especially in hot weather, but some, such as basil, will resent going to bed at night with wet leaves or waterlogged compost, so water in the morning before they are in direct sunlight.
In winter, watering probably won’t be needed, but continue to check your herbs for signs of waterlogging and even drought as plants can rapidly dry out in cold windy weather. Don’t forget your over-wintering, tender herbs and spring seedlings in the conservatory or greenhouse, but avoid using water straight from the water butt or tap on these as the sudden cold can shock them to a standstill.
The surface layer of a pot often looks dry or is covered with a gravel mulch. Pushing a finger through into the compost gives a good indication of how wet or dry the compost really is. Lifting the pot and gauging its weight also gives an accurate indication. (image 2)
Mulches are available in a wide range of natural and recycled materials and help reduce evaporation from the soil. This can mean less watering, but also makes watering harder to gauge. Take extra care in winter and wet weather to keep the compost just moist. (image 3)