Watering

When should I fertilise my houseplants?

March 04, 2026 1 min read

Fertilise houseplants from March to September. This is the active growing season in the UK, when day length and light levels are sufficient to support active growth and the plant can actually use the nutrients you provide. A general rule is to fertilise every two to four weeks through the growing season with a balanced liquid fertiliser, and not at all through winter.

Plants only absorb nutrients when they are actively growing and photosynthesising. In winter, with reduced light and day length, most houseplants' metabolic processes slow significantly. 

What Type of Fertiliser to Use

A balanced liquid fertiliser with roughly equal parts nitrogen, phosphorus, and potassium (NPK) is suitable for most tropical houseplants through the growing season. Our BioBizz Bio-Grow is an organic liquid feed that provides the main macronutrients alongside micronutrients, at a concentration that makes over-application unlikely. Apply at the rate on the product label, there is no benefit to exceeding the recommended dose, and it increases the risk of salt accumulation.

High-phosphorus fertilisers are sometimes recommended to promote flowering in orchids and flowering houseplants. Organic worm casting teas and compost teas are increasingly popular for regular use as a gentle, microbe-rich supplement. Our Worm Castings can be used as a top-dressing or brewed into a liquid feed that is gentle enough to use at every watering without risk of over-fertilisation.

Signs of Under-Fertilising and Over-Fertilising

Under-fertilised plants show progressive yellowing and fading, particularly of older leaves first (nitrogen deficiency), or interveinal chlorosis where leaf areas between veins fade while veins stay green (magnesium deficiency). Growth slows and new leaves are smaller and paler than expected. Addressing this with a balanced liquid fertiliser during the growing season typically shows improvement within a few weeks.

Over-fertilised plants show brown leaf tip burn (salt damage from excess fertiliser in the root zone), crusty white deposits on the compost surface (salt crystallisation), and paradoxically may show yellow leaves as root damage prevents nutrient uptake despite high soil fertility. If over-fertilisation is suspected, flush the compost with plain water several times to remove excess salts, and reduce fertiliser frequency.

Frequently Asked Questions

Fresh compost provides nutrients for approximately three to six months before they are depleted by root uptake and watering. After that point, even a recently repotted plant benefits from supplemental feeding. If you repot annually in spring and fertilise through the growing season, the plant receives a complete nutrient supply through both channels.
Yes, but less frequently and at lower concentrations. Succulents and cacti have evolved in nutrient-poor soils and do not require the same feeding intensity as tropical foliage plants. A low-nitrogen, higher-phosphorus fertiliser at half the normal rate, applied once a month in summer only, is appropriate. See our guide to the best soil for houseplants which covers substrate and fertility needs for different plant types.
Slow-release fertilisers are granules or pellets that release nutrients gradually over weeks or months as they break down. They are convenient for outdoor containers and large plant collections where liquid feeding is time-consuming. For houseplants, liquid fertilisers are generally preferable because they allow more precise control over timing and quantity, and they do not accumulate in the way that multiple cycles of granule application can. Mixed into compost at potting time, slow-release granules can supplement liquid feeding through the early part of the growing season.
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