Fertiliser buying guide

Give your plants a long and healthy life with garden fertilisers

All flowers, plants and trees get most of the nutrients that keep them alive and in bloom from the soil you plant them in, but there are nutrients your plant won’t be able to get enough of to really flourish. That’s where garden fertilisers for plants can help! In this guide we’ll take you through choosing the best product for your plants, including tips about what kind to choose, and special advice about lawns and houseplants.

What is a fertiliser and why should I use it?

Plants need many nutrients to grow strong and healthy. These might not be available in the soil they’re planted in. Adding fertiliser, also known as plant food, gives nature a helping hand, and often encourages stronger growth.

There are six primary food groups that plants need to thrive. The first three; Carbon, hydrogen and oxygen, are easy for a plant to receive from the atmosphere.

The other three; nitrogen, phosphorous and potassium, can be more difficult for plants to find naturally, and are important for different reasons:

  • Nitrogen (N): good for plant growth
  • Phosphorous (P): strengthens roots
  • Potassium (K): increases flower and fruit production

This is important to know as plant foods are labelled with an NPK number, and this helps you understand what kind of support you’ll be giving your plants.

What is a fertiliser and why should I use it?What is a fertiliser and why should I use it?What is a fertiliser and why should I use it?What is a fertiliser and why should I use it?

NPK Numbers

The NPK number is found on the label of most fertilisers. It shows the percentages of Nitrogen (N), Phosphorous (P) and Potassium (K) that the fertiliser contains and the order of the nutrient content is always the same.

For example, if the number reads 4-5-8 that fertiliser contains 4% nitrogen, 5% phosphorous and 8% potassium. The rest is usually clay, limestone or another inert material designed to ensure even distribution and prevent chemical burn.

If the ratios are about the same, it is a general-purpose fertiliser. A fertiliser designed for tomatoes and plump fruit would be higher in potassium (K). For green leafy growth, a plant would need more nitrogen in the fertiliser (N).

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Slow release vs fast acting

You can choose to fertilise your plants with slow release or fast acting fertiliser, which will act in different ways to deliver the nutrients to your plants.

Slow release food can feed your plants for months from one application. The fertiliser is usually sold as granules which are forked into the top soil or mixed into the compost when potting up. The granules slowly break down and the feed is absorbed by the plant. Nutrients from slow release garden fertilisers won’t be immediately available to the plant and can take some time to take effect, but it will provide long-lasting and even distribution of nutrients.

Fast acting kinds are often sold as a liquid fertiliser (either as a concentrate and mixed with water or as a ready-mixed solution) and are poured on to the soil at the base of the plant. This makes feeding as easy as watering! This is a good way to deliver an instant boost of nutrients to plants, especially those that are showing signs of disease or deficiency. One application will last a short amount of time so regular feeding is required. Over use can sometimes cause damage to the roots of the plant, so always read the product instructions for the correct application.

Choosing the right type or fertiliser

When buying plant food, you have different choices depending on what you’re growing, what your preference is, and how you want to use it. The table below lays out the kinds you’ll find at B&Q.

Product What it is
Bone meal & roasted horn Derived from animal bones. Used to increase phosphorus in the soil, to help root growth and help flowering plants to grow more plentiful flowers.
Fish, blood & bone Balanced ingredients - encourages strong, healthy rooting, helps build soil fertility and increase crop yield.
Chicken Manure Pellets Ideal for improving soil, as well as planting and the feeding of established plants.
Mycorrihizal A fungus that works with the plant's roots to increase water and nutrient absorption.
Seaweed Non animal based natural soil improver and food high in nitrogen.
Edibles Designed specifically for growing healthy fruit and vegetables, with increased yield and taste.
Universal Ideal for use with all plant varieties, will help to ensure your plants stay healthy and strong.
Specialist Specifically formulated to meet the needs of the particular plant, available for a range of flowers, fruit and vegetables, such as tomatoes, rhododendrons, cacti, orchids etc.

Fertiliser comes in different formats, the two most common types being liquid and granular. Both have advantages and disadvantages, so pick the one that best suits your needs.

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Liquid or Granular

Liquid feed usually comes in the form of a concentrate that can be added to water, either in a watering can or with a hosepipe attachment. This type is fast acting and will need to be reapplied every few weeks.

Granular fertilisers are applied dry and mixed and watered into the soil, these can be easier to use because you can see exactly how much you are using and where it has landed. This type will not need to be reapplied as often, quick release formulas will last for around 4 weeks and specially coated slow release formula can keep your soil enriched for up to 12 weeks.

Other types

  • Soluble – Soluble fertiliser is dissolved in water and applied with a hose-end sprayer or a watering can. They’re fast acting, universal, easy to apply, and provide even feeding.

  • Drip feed – These are ideal for houseplants and small potted plants. They come ready to use in a small pouch, simply detach the cap and push into the soil at the base of the plant. It’ll gradually release a fast releasing, strong feed for a variety of plants.
  • Pelletse – Easy to use, you simply push the pellets into the soil, and the pellets will slowly release nutrients, feeding plants for months on end.

Sulphate of potash

Potassium sulphate, or sulphate of potash, is commonly called potash. It aids in disease resistance and frost protection by strengthening the plant’s cell walls. It helps in seed and root development. It encourages strong new growth and helps with the formation of flower buds and fruit.

Try high potash foods if you’re trying to improve the quality of your fruit or flowers but be careful not to use it on acid loving plants like azaleas and rhododendrons.


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Feeding lawns

Fertiliser is vital for keeping a lawn lush and green. Lawns should only be fed during the growing season and usually need only two feeds a year – in spring and autumn.

The spring feed should be high in nitrogen to kick-start the growth of lots of lush new grass, whereas the autumn feed should be high in phosphorus and potassium to toughen the grass up for the winter.

Always choose a dry day for feeding, but make sure the ground is not too dry – water first if necessary.

Many lawn feed products combine lawn fertiliser with weed killer. They are applied in exactly the same way, but you need to be much more cautious to prevent the treatment spreading onto other plants, as it may kill them. The advantage of such a combined product is that well-fed grasses will grow more vigorously to fill gaps where the weeds have died back.

If you are in doubt about how much and how far to scatter, mark out a square metre with canes and twine. Then measure out the dosage per metre, as instructed on the feed packet, and spread it inside the square – this will give you a visual guide to go by. You can do the same when applying liquid feeds with a watering can or sprayer.

On smaller lawns, you can scatter granular feed by hand. Bigger lawns are best fed using granular feed in a wheeled spreader; these cover large areas quickly and ensure you are using the correct amount of feed.

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Feeding houseplants

Garden plants can search the soil outside for their nutrients, but pot-grown houseplants are totally reliant upon you to feed and water them. This is easy to do, but the hardest part is often remembering to do it!

If you can, avoid a general-purpose fertiliser. Instead, use specially formulated houseplant food, either as a liquid or slow-release solid inserted into the potting compost.

Feeding should generally begin in the spring and continue through the summer months before cutting down in autumn to allow for the plant’s natural slowdown in growth over the winter months.

As with your outdoor plants, the type of food you give them will depend on what kind of plant you’re growing. There are some general houseplant foods, and some that are geared to a specific plant or group of plants.

For houseplants with a lot of lush green foliage such as ferns and ficus’, you want to encourage this growth by using a high nitrogen fertiliser. If you have plants that flower, such as campanula, you can use a feed that’s high in potassium in the run up to the flowering season to help their beautiful blooms along. The exception to this is orchids, which don’t cope well with strong fertilisers. It’s best to use orchid feed, specially designed to give them the balance of nutrients without being too strong. As bonsai trees are specialist plants, they require the right care and nutrition with bonsai food - which will help to provide balanced and steady growth.

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Whilst cacti and succulents will grow without fertiliser, by giving them specialist feed every 2 weeks in the spring and summer, you can encourage better growth and flowering. Avoid feeds that are high in nitrogen as this will result in weaker plants.

If you’ve got a gorgeous citrus plant, you’ll find these are especially hungry. You can use citrus feeds or general houseplant food to support their growth, every two weeks in the spring and summer. Don’t feed your citrus plant in the winter unless it’s still flowering, in which case a monthly dose will be fine.

Carnivorous plants will generally look after getting their own nutrients by catching insects, so you don’t need to worry about fertilising them. Because of this, they’ve developed the ability to live in soil with very little nutrients.

Safety first

Some fertilisers are designed to work in a particular way and may be damaging if these instructions are not followed. Always check the ingredients as well and know exactly what you are applying to your garden. Wear gloves when applying fertiliser, and goggles when applying feed and weedkiller on the lawn. As always, keep fertilisers and chemicals out of reach of children.