Best controlled-release fertilisers

A decent controlled-release fertiliser will keep your plants fed for months on end, but a bad one will leave them performing poorly. Our expert tests have uncovered the best options for you.

Controlled-release fertilisers are the ultimate low-maintenance option for feeding plants in patio pots and baskets, because you add the granules to the compost when you plant up. The containers only need liquid feeding at the end of the summer when the original feed runs out.

Below, we reveal which controlled-release fertilisers have topped our tests. Plus, we explain the differences between controlled-release and liquid feeds.

We recommend mixing this feed into a Best Buy compost for containers

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What is controlled-release fertiliser?

Usually arriving in the form of little ball-bearing-like granules, controlled-release fertiliser dissolves when the compost in your pots is moist and warm enough for plants to grow. These fertilisers are just as effective as liquid feeds at keeping your plants healthy, but are far easier to use, as they only need to be applied once per season.

Mix them into compost when you pot up young plants in the spring. Read the instructions carefully, as you risk over- or under-feeding your plants if you guess how much to add.

Some products are also referred to as slow-release fertilisers. Technically speaking, there is a difference between slow-release and controlled-release fertilisers. Slow-release fertilisers will gradually release their nutrients over time, regardless of conditions.

Controlled-release fertilisers work when the soil temperature and moisture are high enough, feeding plants when they need it through the warmer months and giving out less food in the colder months when growth slows. However, many products are still called slow-release, as this is the more commonly understood term.

Fancy making your own compost? See our guide to the best and worst compost bins.

Why is controlled-release feed useful? 

Plants need regular feeding. In the ground, nutrients are often readily available, but in a pot plants are reliant on the compost. Most composts have a base fertiliser, but this only lasts three to four weeks. Liquid feeding is very effective, but you need to feed every week or two throughout the spring, summer and autumn. One application of CRF at planting time provides nutrients over a long period, typically around four months. Our Best Buys lasted longer than this. 

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Controlled-release versus liquid feed

We compared our highest-scoring Best Buy controlled-release feed with our top rated Best Buy liquid feed. We found that the liquid feed grew larger plants that flowered more. However, it was more time-consuming as we had to mix the feed into a watering can, and remember to do this every week. 

The plants grown with controlled-release were nevertheless excellent plants that would grace any patio for most of the summer and the feeding was taken care of at the start of the season. For the very best plants that will continue to flower well from late summer into autumn, you could top up with a liquid feed every two weeks, starting in mid-to late-August. 

Using a controlled-release was by far the cheaper option, costing just 42p per pot over the course of the summer, compared to £3.20 for each pot fed with the highest-scoring liquid feed. 

How we test controlled-release fertilisers

For our most recent round of testing, we chose 11 controlled-release fertilisers that claimed to feed plants all summer long. Our shortlist was made up of products from popular brands including Miracle-Gro, Westland, Wilko and Thompson & Morgan.

Ten of these controlled-release fertilisers were loose granules and one was tablets made from the granules. In order to see the difference between a poor fertiliser and not adding one at all, we grew one set of plants with no fertiliser as a control. We mixed each feed into a peat-free Best Buy compost for patio pots, following the instructions on the packaging.

Each fertiliser was used to feed four 10-litre pots of bedding geraniums (pelargoniums) and five pots of potatoes. We also planted up four pots of pelargoniums and five of potatoes that we didn’t feed at all. We assessed our pelargoniums every two weeks for leaf colour, the number and size of the flowers, and the size of the plants. We counted how many flower heads we removed from each pot when we deadheaded the plants. When we harvested the potatoes, we looked for plenty of good-sized tubers with smooth skins and no sign of scab.

As our compost tests continue to prove, the most expensive options aren't always the most effective.

To see which compost we recommend, see our table of the best composts.