Plants & Growing

The UK’s Best Slow-Release Fertiliser For Continuous Plant Nutrient Provision

fertliser granules being dropped by hand into soil

What Slow-Release Fertiliser does is precisely described by its name. Its benefits, however, are not. It allows you to fertilise once and forget, ‘automates’ consistent long-term feeding, eliminates the guesswork, and does away with fertiliser burn. As a gardener, shouldn’t you be looking into Slow-Release Fertiliser?

Why apply a teeny-weeny bit of fertiliser to your plants every few days, even assuming you could do so, when technologically-advanced fertiliser can do it for you by itself? This is exactly what Slow-Release Fertiliser does.

Feeding plants is best achieved in small, controlled ‘titbits’ administered over time. Oh— that word, ‘controlled.’ Come to think of it, strictly speaking, Slow-Release Fertiliser and Controlled-Release Fertiliser are two different chemical technologies, both being a type of Enhanced-Efficiency Fertiliser. (These are Fertiliser Industry distinctions and terminologies, but it is important at least to be aware of them.)

Slow-Release Fertilisers are uncoated products in which nutrient release is uncontrolled but gradual by way of slow solubility using ongoing urea-aldehyde reaction or some other slow chemical reaction. In contrast, Controlled-Release Fertiliser is coated; it comprises of granules that contain nutrients at their core, coated by layers of polymer, sulphur, and/or wax that effect a controlled, steady supply of small amounts of nutrients over many months. In common usage, though it is erroneous, Slow-Release Fertiliser has come to mean both Slow-Release and Controlled-Release Fertiliser. 

Granted, Slow-Release Fertilisers are rather more expensive than your day-to-day ‘garden variety’ fertilisers but they bring a set of major benefits as a result of which they are rising in popularity. These benefits are obvious and apparent as well as less well-known and technical.

First, Slow-Release Fertiliser saves labour and also compensates for human forgetfulness and neglect. But it also guards against human error: an over-application of regular fertiliser often causes plants’ roots, and consequently the plants themselves, to get burnt (‘fertiliser burn’). Slow-Release Fertiliser by dint of its very purpose does away with any such possibility. An invisible advantage is that these fertilisers are environmentally-friendly. Whereas any over-application of regular fertilisers can – and often does – result in fertiliser runoff in waterways, Slow-Release Fertiliser eliminates or at least reduces fertiliser runoff.

The leading market for horticultural Slow-Release Fertilisers is the United States while for agricultural applications China is the primary market. As of Autumn 2019, Slow-Release Fertilisers are gaining traction in both the horticultural and agricultural sectors in the United Kingdom.

Our #1 Top Pick
Gro-Sure 6 Month Slow Release Plant Food, 2 kg
Value Pick
Miracle-Gro Continuous Release Plant Food 2 kg Tub
Also Consider...
Gro-Sure Ericaceous Slow Release Plant Food, 900 grams
Suitable Plant Types
All Non-Ericaceous Plants
All Plants
All Ericaceous Plants
Feeding Length
6 Months
6 Months
3 Months
Container Size
1.1 - 2kg
2kg
900g
Price
Price not available
£10.29
£4.99
Our #1 Top Pick
Gro-Sure 6 Month Slow Release Plant Food, 2 kg
Suitable Plant Types
All Non-Ericaceous Plants
Feeding Length
6 Months
Container Size
1.1 - 2kg
Price
Price not available
Value Pick
Miracle-Gro Continuous Release Plant Food 2 kg Tub
Suitable Plant Types
All Plants
Feeding Length
6 Months
Container Size
2kg
Price
£10.29
Also Consider...
Gro-Sure Ericaceous Slow Release Plant Food, 900 grams
Suitable Plant Types
All Ericaceous Plants
Feeding Length
3 Months
Container Size
900g
Price
£4.99

Last update on 2020-08-14 / Affiliate links / Images / Pricing from Amazon Product Advertising API

Let’s start with our top pick slow-release fertiliser –

Best Pick: Gro-Sure 6 Month Slow Release Plant Food

A top-of-the-line product, Gro-Sure’s slow-release fertiliser does double duty – injecting an initial burst of energy and then slowly releasing nutrients over 6 months.

Westland’s Gro-Sure 6-month plant food has N-P-K proportions of 12-7-19 with micronutrients like iron, zinc and manganese. It comes in a 1.1-kilogramme canister. The same product is also available in a 1.33 size and 2-kilogramme tub.

This Gro-Sure product is designed to release an ‘initial burst of energy’ and then slowly release nutrients over six months. Billed as a fertiliser for all non-ericaceous plants ‘in pots, containers and hanging baskets,’ in a pinch this fertiliser can even be used for certain ericaceous plants like strawberry plants (forbs) and works wonders on them too.

The canister has a helpful chart on the back label specifying usage for different types of plants and containers. Application is quick and easy. The difference Gro-Sure 6 Month makes is noticeable, especially in growth and robustness. It also helps in producing stunning blooms.

The 1.1-kilogramme canister’s lid has a built-in sprinkler.

For what the product does, it is not at all costly and can even be considered good value.

The 2-kilogramme tub’s packaging is insecure and simply unacceptable. The lid pops open in transit or even during transportation, and the granules escape the tub, falling into the external packaging. If you do not unpack very carefully you may get fertiliser granules all over your floor! In any event, there is some degree of wastage. If ordering this product for delivery, opt for the 1.1 or 1.33-kilo size.

This is a top-of-the-line product that is all-purpose.

Pros

  • Brings a very quick and noticeable improvement to both foliage and flowers.
  • Can be used outdoors as well as indoors for containerised plants.
  • Works very well even for some ericaceous plants.

Cons

  • The 2-kilo tub’s packaging is defective and results in waste and causes inconvenience; do not buy this size.
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Value Pick: Miracle-Gro All Purpose Continuous Release Plant Food

High-Tech, Low-Price, is an apt descriptor for Miracle-Gro’s product that is truly ‘all-purpose;’ just throw it around in your garden – hey, it’s cheap enough!

This Miracle-Gro product releases food for 6 months. The 2-kilo tub contains 4 500g packets. The same fertiliser is also available in a 1-kilo shaker jar.

The bio-degradable resin-coated granules contain fertiliser with an N-P-K balance of 17-9-11 plus various other nutrients. The high-tech coating’s pores are triggered by soil temperature changes to release or cut off nutrient supply; nutrient release is proportional to rising soil warmth.

The makers position this fertiliser as being ‘ideal for flowers, fruit and vegetables in fact, for everything in the garden’ – and it really is. It produces bigger and healthier plants, and more luxuriant blooms and longer blooming on annuals. It perks up perennials very well too.

It can be mixed with compost as prescribed or used as an alternative to compost. Different dosages are prescribed depending on the maturity of the plant, its size, and whether it is in open ground or a container. Smaller amounts are to be used for young plants.

The lid is easy enough to open if you know the trick, otherwise you may fiddle around with it for an hour or two!

Includes a measuring scoop.

This excellent all-purpose slow-release fertiliser can’t be beaten on quality and it pips the competition on price per kilo. Even the packaging is very sensible.

Pros

  • Genuinely all-purpose formulation eliminates nit-picky decision-making.
  • High-tech chemistry and resultant temperature-triggered food supply.
  • Smart packaging: product is supplied in 4 packets in one box so that what is not required stays sealed and fresh.

Cons

  • The lid can be a bother to open but if you learn the trick, it’s a ‘snap.’
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Gro-Sure Ericaceous Slow-Release Plant Food

Formulated for acid-loving plants, Gro-Sure does the job for 3 months and produces distinct improvement, with blueberry bushes in particular just loving it.

Gro-Sure Ericaceous food is supplied in a 900g shaker jar. It is for Rhododendrons, Azaleas, Camellias, and other acidophilic plants.

As this fertiliser is expressly designed for acid-loving or lime-hating plants, it should not be used for plants which prefer a pH towards neutral, let alone for lime-loving plants.

Its N-P-K proportion is 12-11-18 and it includes micronutrients like Magnesium and Sulphur in trioxide form.

Plants get a distinct lift soon after an application. Any traces of yellow or limpness become history as plants clearly become healthier and foliage becomes glossier.

A single application feeds plants for 3 months.

Mix it in with soil when preparing beds or containers as specified on the back label, or simply sprinkle it on the surface.

Although this fertiliser is meant for ericaceous plants, try it on Hydrangeas too if you want them to turn a pleasing blue. And talking of blue, blueberry bushes just love this product; apply it and they will express their gratitude with an abundance of berries.

Pros

  • A quick and convenient plant food especially for acid-loving plants.
  • Yellowing or limp plants are actually turned around, becoming healthier and brighter.
  • An unexpected bonus is that Hydrangea lovers can use it to blue their blooms.

Cons

  • None, but don’t be careless and apply this on lilac, forsythia, legumes, and other lime-loving plants.
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Miracle-Gro Azalea, Camellia & Rhododendron Continuous Release Food

Especially for acid-loving plants, Miracle-Gro’s technologically advanced ericaceous fertiliser contains additional nutrients, and it is a boon specially for Azaleas.

Miracle-Gro’s ericaceous fertiliser comes in a 1-kilogramme shaker jar and in a 1-kilo carton. It is also available as a liquid in a 1-litre bottle. Note that the jar, carton, and bottle’s contain fertiliser in slightly different formulations from one another.

This product is meant for acid-loving or lime-hating plants and should not be used for plants which prefer a pH towards neutral, let alone alkaline-oriented plants.

The 9-14-19 fertiliser in the shaker jar is a mix of 6 nutrients including magnesium, iron, and other micronutrients.

By virtue of its continuous release technology, a single dose feeds plants for 6 months. At the beginning of the growing season, a single application suffices. The technology then takes over as the granules’ biodegradable resin-coating works its magic by opening and closing the pores as controlled by higher and lower soil temperature.

Different dosages are prescribed depending on the maturity of the plant, its size, and whether it is in open ground or in a container.

It brings a noticeable difference in the improvement in foliage and flowering. It also gives worn-out plants a lift, making them vigorous and putting them on the road to recovery.

Miracle-Gro truly works miracles on Azalea, verily resurrecting near-death plants. This fertiliser is also ideal for other acid-loving plants like Gardenia.

Pros

  • Results in noticeable improvements in both foliage and flowers.
  • Even restores wilting and damaged plants back to health.
  • Positively works miracles with Azaleas.

Cons

  • Not available in a size any bigger than 1 kilogramme.
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Osmocote Controlled Release Plant Food

Highest-Tech, Highest-Price – a fair description for this ultra-convenient Osmocote product that you ’sow’ into the soil; it is as excellent indoors as it is outdoors.

Osmocote’s ultra-convenient plant food is not in the form of loose granules; the granules are tightly packed into a half-conical ‘tablet.’ Each package contains 25 of these 5g tablets that are to be pushed about 5 centimetres into the soil. One tablet is good for 6 months – nearly the whole growing season.

The fertiliser formulation is 14-9-11 plus magnesium and other micronutrients.

This fertiliser’s release is controlled by the temperature; warming soil releases more food whereas cooling trends reduce food supply.

Though Osmocote bills its product as being particularly useful for indoor plants in containers, it is just as good for plants in open ground, including established plants. The package has a helpful ruler along the side with which pot diameter can be measured and the dosage looked up on the chart at the back which accounts for other variables such as plant type and container type.

This product is very effective and especially fast-acting and brings about a marked improvement in both growth and flowering of plants.

These tablets are the easiest and most convenient type of fertiliser to use.

Pros

  • The easiest and most convenient fertiliser bar none.
  • It works a treat – marked improvements are readily observable very quickly.
  • Apart from indoor plants and containerised plants as advertised, this fertiliser is very good for all plants.

Cons

  • The price – on a pounds-per-kilo basis, this product is very costly.
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Should You Fertilise Outdoor Plants?

Fertilising Outdoor Plants with Slow-Release Fertiliser is an uncomplicated business. But first, you need to determine how rich and fertile your soil is in the first place, and, next, which kinds of plants you intend to fertilise. If your soil is loamy or if it is well-composted, you should fertilise sparingly and infrequently; perhaps you may not even need to fertilise at all.

On the other hand, if your soil is light, sandy, or gravelly, then regular fertilising is called for. If the plant in question is ground cover, an evergreen, or a perennial, fertilising is not required and may not even bring any joy; it depends on the soil and the state of the plant. If the plant is deciduous or an annual, or if it is unhealthy from lack of nutrients, then fertilising is very much a good idea that should yield colourful results.

How To Fertilise Outdoor Plants

Although specifics depend upon the particular plant and particular fertiliser, in general when using a Slow-Release Fertiliser you can ‘top dress’ or ‘basal dress.’ To top dress, sprinkle one tablespoon of granules around a plant, taking care to ensure that the granules are not concentrated at the base of the main stem. Sprinkle the granules so that they are about evenly distributed above its root ball or root system and work them into the soil. Depending on the fertiliser, you may need to apply it quarterly except in winter or bi-annually. 

To bottom dress, the appropriate quantity of the fertiliser is mixed with compost which is then added to the soil when bedding or when preparing new containers.

As a general rule, when fertilising it is wiser to err on the side of caution and under-fertilise than to over-fertilise, for the latter can burn and even kill plants.

When To Fertilise Your Plants

It would be tempting to say ’spring’ or perhaps ‘summer’ but such an answer would be incorrect. The best times to fertilise a plant is just before it begins fresh seasonal growth and right when it begins to flower. As such, for most plants and shrubs, spring and summer are the correct answers. But consider, as examples, snapdragons and pansies. These are well-known as autumn- and winter-flowering plants; therefore, snapdragons are best fertilised in summer and autumn, and pansies in autumn and winter. Try using a nitrogen-rich fertiliser for the new growth phase and a phosphorous-rich fertiliser for the flowering season.

On the other hand, if you use a Slow-Release Fertiliser, you can fertilise once and not have to think about it perhaps even for the rest of the season.

About the author

Kersasp 'Kersie' Shekhdar

Kersie learnt the basics of gardening as a toddler, courtesy of his grandfather. In his youth he was an active gardener with a preference for flowering plants. He is a professional and vocational writer and his freelance projects have spanned various kinds of writing.

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