Our test of controlled-release fertilisers shows you which will keep your plants looking good all summer long.

BB and DB CRF 18

 

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 containers, because you add the granules to the compost when you plant up. Keep reading to find out which controlled-release fertilisers have topped our latest round of tests. We also explain more on the differences between controlled-release and liquid feeds.

Best controlled-release fertilisers

To help us on our mission to find the best controlled-release fertiliser for your garden, we used fertilisers to feed pots of pelargoniums and potatoes. We mixed the fertiliser granules into the compost before planting up our pots.

best compost

 

Miracle-Gro All Purpose Continuous Release Plant Food

Price: £5.99

Overall score: 91%

The pelargoniums were among the best in the test. They rapidly grew into large plants that flowered continuously all summer long and by the end of September, they still had healthy dark green leaves. We grew around two large potatoes in each pot and had an excellent overall yield of nearly 3kg. A few of the potatoes developed scab, but not many.

compost

 

Chempak Yearlong Fertiliser

Price: £9.99

Overall score: 90%

The pelargonium grew into large plants, which were covered in flowers until the end of September. The colour of the plants’ leaves was great until the last few weeks of the test, when the nitrogen started to run low, turning them a pale green. We had a great crop of around 3kg of potatoes, all of an excellent quality. These were mostly medium sized, although all the pots had one or two baking-sized potatoes.

compost

 

Osmocote Controlled Release Plant Food 

Price: £5.49

Overall score: 89%

The pelargoniums grown in this fertiliser were fantastic plants. They were large, had dark green leaves until the end of the test in late September, and grew large flowers for months on end. In the potato test, this feed edged out the other Best Buys, growing three or more baking-sized potatoes, with just shy of 3kg of tubers in each pot.

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.

Controlled-release fertiliser

 

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.

Controlled-release vs liquid feeds

 

In previous years, we’ve trialled a Best Buy liquid feed against a Best Buy controlled-release fertiliser. We found that the liquid-fed plants grew bigger, but had fewer flowers – most of the growth was in the leaves. However, the best displays throughout the summer came from pots initially fed with a controlled-release fertiliser, then topped up with a liquid feed if they stopped producing healthy-looking growth. Our advice is to use a controlled-release feed, but give an additional liquid feed if your plants show signs of flagging.

How we test controlled-release fertilisers

For our most recent round of testing, we chose 10 controlled-release fertilisers that claimed to feed plants all summer long. Our shortlist was made up of products from popular brands including Chempak, Garden Direct, Osmocote and Thompson & Morgan.

Eight of these controlled-release fertilisers were loose granules and two were 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.

Which? controlled-release fertilisers tests

 

Each fertiliser was used to feed five 10-litre pots of bedding geraniums (pelargoniums) and five pots of potatoes. We also planted up five pots of pelargoniums and five of potatoes that we didn’t feed at all.

We assessed our pelargoniums three times for leaf colour, the number and size of the flowers, and the size of the plants. When we harvested the potatoes, we looked for plenty of good-sized tubers with smooth skins and no sign of scab.

Towards the end of our experiment, we collected compost samples from one of our best-performing pots and also from poorly performing pots.

These were sent for nutrient analysis to see why the plants in the poorly performing pots were so unhealthy. As our compost tests continue to prove, the most expensive options aren’t always the most effective.