The results of our test of controlled-release fertilisers will show you which will help your plants maintain their healthy appearance throughout the summer.

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Your plants will continue to thrive for months on end if you use a controlled-release fertiliser of sufficient quality, but if you use one of inferior quality, their performance will suffer. Our in-depth research and analysis have revealed the options that are ideal for you. Because the granules of controlled-release fertilisers are added to the compost at the time of planting, this type of fertiliser offers the lowest-maintenance option for feeding plants that are grown in containers. Continue reading to find out which controlled-release fertilisers performed the best in the most recent round of tests that we ran. In addition to this, we delve deeper into the distinctions that exist between controlled-release and liquid feeds.

The very best fertilisers with controlled release

Fertilizers were used to provide nutrients to potted pelargoniums and potatoes as part of our effort to determine which controlled-release fertiliser would be most beneficial for your garden. Before we started planting our pots, we gave the compost and the granules of fertiliser a good mix.


best compost

Miracle-All-Purpose Gro’s Plant Food with Continuous Release

Price: £5.99

Overall score: 91%

The pelargoniums performed exceptionally well in comparison to the other plants. They developed into large plants very quickly and bloomed nonstop throughout the entire summer; even at the end of September, their leaves were still vibrantly dark green and healthy looking. We averaged about two large potatoes in each pot, which resulted in a fantastic overall yield of almost three kilogrammes. Scab appeared on a few of the potatoes, but not on very many of them.



Chempak Yearlong Fertiliser

Price: £9.99

Overall score: 90%

The pelargonium eventually developed into large plants that were, up until the end of September, completely covered in flowers. The colour of the leaves on the plants was great up until the final few weeks of the test, when the nitrogen began to run low, turning the leaves a pale green colour instead. We had a wonderful harvest of about 3 kilogrammes of potatoes, all of which were of an exceptional quality. The majority of these were of a medium size, but each pot contained one or two potatoes that were suitable for baking.


Osmocote is a plant food that has a controlled release.

Price: £5.49

Overall score: 89%

These pelargoniums, which were grown in this fertiliser, turned out to be wonderful plants. They were quite big, maintained their dark green leaves until the test was over in late September, and produced large flowers for an extended period of time. This feed outperformed the other Best Buys in the potato test, producing three or more potatoes of baking size and coming within a few grammes of producing 3 kilogrammes of tubers in each container.

What exactly is meant by the term “controlled-release fertiliser”?

The controlled-release fertiliser, which typically takes the form of tiny granules resembling ball bearings and dissolves when the compost in your pots is wet and the temperature is high enough for plants to grow, is typically delivered. These fertilisers are just as effective as liquid feeds in maintaining the health of your plants; however, they are much simpler to use because you only need to apply them once during each growing season. When you are repotting young plants in the spring, incorporate them into the compost you are using. If you try to guess how much to add, you run the risk of either over- or under-feeding your plants. The instructions should be read carefully.

Controlled-release fertiliser

Some of these products are also known as fertilisers that have a slow release rate. There is a distinction that can be made between fertilisers that have a controlled release and those that have a slow release.

In spite of the environmental conditions, the nutrients contained in slow-release fertilisers will be gradually released over time. When the soil’s temperature and moisture levels are just right, controlled-release fertilisers become effective. This type of fertiliser provides plants with the nutrients they require during the warmer months, but provides less nourishment during the winter months when growth is slower.

Despite this, many products continue to be referred to as slow-release because this is the term that is most widely understood.

Comparison of controlled-release and liquid feeding systems

In previous years, we put both a Best Buy liquid feed and a Best Buy controlled-release fertiliser through their paces in a series of experiments. We discovered that the plants that were given liquid nutrition grew larger, but they produced fewer flowers; the majority of the growth occurred in the leaves. However, the containers that were initially fertilised with a controlled-release fertiliser and then supplemented with a liquid feed whenever they stopped producing growth that appeared to be healthy produced the most impressive displays throughout the entire summer. Utilize a nutrient delivery system that has a controlled rate of release, but supplement it with a liquid feed if your plants show any signs of becoming stunted.

Methods that we use to evaluate slow-release fertilisers

Our most recent round of testing consisted of selecting 10 controlled-release fertilisers that boasted the ability to provide nourishment to plants throughout the entire summer. The products that made it onto our shortlist were manufactured by well-known companies such as Chempak, Garden Direct, Osmocote, and Thompson & Morgan.

These controlled-release fertilisers came in a total of ten different forms, eight of which were loose granules and two of which were tablets made from the granules. As a control, we grew one set of plants with no fertiliser at all so that we could compare the results of using a poor fertiliser to the results of not adding any at all. Following the directions on the product packaging, we combined all of the feeds into a peat-free compost that was available at Best Buy for use in patio pots.

Which? controlled-release fertilisers tests

Each fertiliser was utilised to provide nourishment to five 10-liter pots of bedding geraniums (also known as pelargoniums) and five pots of potatoes. In addition, we planted five pots of pelargoniums and five pots of potatoes, but we did not provide any kind of nutrition for them.

Three separate times, we examined our pelargoniums to determine their leaf colour, the number of flowers, their size, and the overall size of the plants. When we were harvesting the potatoes, we looked for plenty of tubers that were a good size and had smooth skins. There were no signs of scab on the potatoes.

Near the end of our study, we gathered compost samples from both one of the pots that had performed exceptionally well and from one of the pots that had performed less well.

These were sent off to have their nutrient content analysed in order to determine why the plants that were growing in the pots that were not performing well appeared to be in such poor health. Our ongoing tests with compost continue to demonstrate that the most pricey options are not necessarily the ones that are the most effective.