Geranium fertiliser for pots, window boxes and baskets

Updated: October 2026

The basket looked fine in June and tired by August, and the compost, not the variety, is usually the reason.

The basket looked fine in June and tired by August, and the compost, not the variety, is usually the reason. Geranium fertiliser for pots is a container problem: pelargoniums in patio pots, window boxes, and hanging baskets use up a small volume of compost, and they also rot if that compost stays wet. Feed on the pelargonium calendar, at the label rate, and do not buy a 20 kg sack for a balcony. Border cranesbills are a different plant, covered on pelargonium or hardy geranium. The table is the geranium fertiliser comparison. A qualifying purchase can earn a commission at no extra cost to you.

Our top picks

Badge	Product	Form	Stated NPK	Dose on the listing	Note	Price	Offer
Best overall	Cultivers Cultivers organic geranium and flowering plant fertiliser 250 g	Granules	8-1-5	Not stated	10.0	about £11.62	View offer
Alternative	Fertiligène Fertiligène Osmocote Max geranium fertiliser 700 g	Granules	Not stated	3 to 6 g per pot	9.4	about £23.00	View offer
Alternative	Cultivers Cultivers organic liquid geraniums fertiliser 5 litres	Liquid	4-1-7	Not stated	8.8	about £35.29	View offer

Best overall

Cultivers Cultivers organic geranium and flowering plant fertiliser 250 g

10.0/10about £11.62

View offer

Alternative

Fertiligène Fertiligène Osmocote Max geranium fertiliser 700 g

9.4/10about £23.00

View offer

Alternative

Cultivers Cultivers organic liquid geraniums fertiliser 5 litres

8.8/10about £35.29

View offer

On this page

1.Why pots run out
2.Baskets and window boxes
3.Grams in the pot or a cap in the can
4.Packs that do not belong on a balcony

Why pots run out

Roots in a pot cannot forage, so compost runs out. John Innes No. 2 or fresh multipurpose compost carries a short reserve. Many containers can wait about six weeks after planting. A bedding note of three or four feeds all summer is a looser average than the pelargonium guide: balanced liquid every 10 to 14 days in spring, then high-potassium feed every fortnight once flowers form. Pelargoniums must not sit in a saucer. Yellow leaves after a wet week mean check the roots. Plants go out in late May or early June in much of the UK, later in Scotland.

Baskets and window boxes

Ivy-leaved pelargoniums, sold as ivy geraniums, stay pelargoniums. A basket of Balcon Mixed wants a high-potassium feed once flowers start, not a cranesbill regime. Window boxes are the same plant in less compost. Fortnightly high-potassium liquid is the summer habit, unless the label says less often. A cap at every watering is another brand's cadence. Do not copy it onto a Cultivers bottle with no dilution in the product data. Four baskets fed every fortnight will empty a 250 ml bottle faster than two patio pots. The 4-1-7 liquids lean toward potassium. The 8-1-5 packs print more nitrogen.

Testsieger

Cultivers Cultivers organic geranium and flowering plant fertiliser 250 g

10.0/10about £11.62

View offer

Grams in the pot or a cap in the can

The liquid dose is the carton. Cultivers liquids state NPK 4-1-7 and no dilution in the product data. Do not borrow a millilitre figure from another brand. Trace elements are named on the 1, 5 and 10 litre listings, not on the 250 ml listing. Fertiligène Osmocote Max prints 3 to 6 g per plant in a pot. The 3.5 handfuls per square metre line is for the ground. The one-year and February-to-June claims are the listing's, and the product data has no NPK. The 250 g granules do not print 3 to 6 g. Which pack matches the job is on best geranium fertiliser.

Packs that do not belong on a balcony

Ten litres is about £5.46 per litre at about £54.59, and the can is listed at 12.5 kg. Twenty kilograms at about £60.42 is still the wrong object next to a window box. The 5 litre can at about £35.29 suits many baskets, not three 20 cm pots. The 250 ml, 1 litre and 250 g packs are the balcony sizes. Keep liquid on the compost and granules off the leaves. Winter storage belongs on how to feed geraniums.

Common questions

When do I start feeding a new basket?

About six weeks after planting into fresh compost, if the plants are growing and the compost is moist rather than wet. That pause is the RHS container guidance and the Gardeners' World pot note. After that, the pelargonium guide is fortnightly high-potassium feed once flowers form, not a wait until August. Do not start the day you pot up a sodden basket in a cold May.

How often through summer?

Every fortnight with a high-potassium liquid once flowers are out, on the pelargonium guide. A broader bedding note of three or four feeds all summer is a looser average. Follow the pelargonium interval for zonal and ivy-leaved plants, and follow the label if it says a weaker mix or a longer gap. Every watering is someone else's product habit.

Liquid or a few grams of granules in a pot?

Use a liquid if you will repeat a feed and you want the stated 4-1-7 bottles. Use 3 to 6 g of the Osmocote tub per plant if you want the printed granule dose and a once-in-the-window claim. Do not use both at full strength. The 250 g Cultivers granules do not print that 3 to 6 g figure.

How much Osmocote per plant?

The listing says 3 to 6 g per plant in a pot. A handful is about 20 g, which is several plants, not one basket. The 3.5 handfuls per square metre line is for the ground. Number of items 12 on the same listing is unexplained, so do not treat the order as twelve tubs.

Is a 20 kg bag useful for window boxes?

No. It is about £60.42, the form is not stated, and the listing NPK of 8-1-5 is challenged by one review that says 8-7-5. A window box wants a small liquid or the 250 g pack. The unit price of a sack is not a reason to store 20 kg on a balcony.

What if leaves go yellow after rain?

Check the drainage and the saucer before you add fertiliser. Pelargoniums hate sitting wet, even though baskets dry out fast in sun. Yellow, limp growth in a heavy compost is often roots, not hunger. Feed when the compost is moist again and the plant is actually growing.

Can I splash feed on the leaves?

Put liquid on the compost, and keep granules off the foliage. A wet leaf with concentrate on it marks easily, and the RHS links over-strong feed to scorch. Water the compost first if it is dusty and dry, then apply the labelled rate. Do not foliar-feed these bottles because a basket is awkward to water at the crown.

Geranium fertiliser buying guide