Which? uses cookies to improve our sites and by continuing you agree to our cookies policy

Controlled-release fertilisers

How we test controlled-release fertilisers

Article 3 of 3

Put us to the test

Our Test Labs compare features and prices on a range of products. Try Which? to unlock our reviews. You'll instantly be able to compare our test scores, so you can make sure you don't get stuck with a Don't Buy.

How we test controlled-release fertilisers

Find out about how we test controlled-release fertilisers to find the best ones to keep your plants fed for months on end.


Controlled-release fertilisers are the ultimate low-maintenance option for feeding plants in containers, because you add the granules to the compost when you plant up. Then, as the weather warms up and the containers get watered, the granules gradually release their nutrients for the plant roots to seek out.

One application of controlled-release fertiliser should keep your plants in pots looking lush for most of the growing season.

A decent controlled-release fertiliser will keep your plants fed for months on end, but a bad one will leave them performing poorly. Keep reading to find out what criteria the Which? experts used when testing them.

If you already know what you're looking for, go straight to our Best Buy controlled-release fertilisers.

What we tested

We put five brand-leading controlled-release granular fertilisers to the test using a Which? Best Buy compost. We used each one to feed four 9-litre pots of bedding geraniums (pelargoniums).

Our pots were planted up in mid-May 2012. We mixed the feeds into the compost before planting up. We also planted up four pots of pelargoniums that we didn’t feed at all.

Our pots were put in a sunny spot and watered as necessary using an automatic-irrigation system. Our pelargoniums were assessed for vigour and flowering impact in late June, late July and late August.