What Does NPK Mean? The Best Fertiliser for Flowers Explained

Confused by NPK? Wondering whether to use Tomorite, chicken manure pellets, Fish, Blood & Bone or liquid seaweed? This guide explains what the NPK numbers really mean, when your flowers need each nutrient and which natural fertilisers I recommend to keep your garden blooming all season.

Growing beautiful flowers isn’t just about choosing the right varieties. It’s about replacing what your plants use.

That’s where so many gardeners get confused. Walk into any garden centre and you’ll find shelves full of products claiming to grow bigger flowers, stronger roots or healthier plants.

They all promise wonderful results, but which one should you actually use?

The answer starts with understanding NPK, and don’t worry—it isn’t complicated.

In fact, once you understand these three little letters, choosing the right fertiliser becomes much easier.

As a flower farmer, I don’t think about feeding my flowers because the calendar tells me to. I feed them because I know what they’ve used.

Every stem I cut removes nutrients from the soil. Every bunch I harvest takes away tiny amounts of nitrogen, phosphorus and potassium. If I don’t replace them, the next flush of flowers will always be weaker.

Think of your soil like your kitchen pantry. Every time you cook a meal, you use some of the ingredients. Eventually, the shelves become empty, but you don’t throw random ingredients back into the cupboard. You replace what you’ve used. Plant nutrition works exactly the same way.

NPK simply tells you which nutrients you’re replacing.

What does NPK actually mean?

NPK stands for the three nutrients plants need in the largest quantities.

N = Nitrogen

Responsible for leafy growth, healthy stems and vigorous new shoots.

P = Phosphorus

Supports strong roots, healthy establishment and energy transfer throughout the plant.

K = Potassium

Encourages flower production, stem quality and resilient, healthy plants.

The numbers on a fertiliser packet tell you the ratio of each nutrient.

For example:

10-10-10 is a balanced fertiliser - all of the numbers are equal

4-3-8 (NPK) contains much more potassium than nitrogen, making it better suited to flowering and fruiting plants.

The biggest misconception?

Higher numbers don’t automatically mean a better fertiliser. The best fertiliser is simply the one that replaces the nutrients your flowers have used.

Nitrogen (N): The nutrient that builds your plant

If you grow flowers for cutting, nitrogen is one of the hardest-working nutrients in your garden.

Nitrogen is responsible for producing:

• Healthy green leaves

• Strong stems

• Vigorous new shoots

• Fast regrowth after cutting

Every time you harvest a bunch of flowers, you’re removing far more than just the blooms. You’re taking away stems, leaves and plant tissue.

All of those contain nitrogen. That’s why cut-and-come-again flowers often slow down later in the season if they aren’t fed. They simply run out of the nutrients needed to rebuild themselves.

Signs your plants may be short of nitrogen include:

• Pale green or yellowing older leaves

• Smaller leaves than usual

• Thin stems

• Slower regrowth after cutting

• Reduced vigour

Too much nitrogen can also cause problems. Plants produce lots of lush green growth but fewer flowers. Like everything in gardening, it’s about balance.

When do flowers need more nitrogen?

Most flowering plants need their biggest nitrogen boost:

• During spring as they burst into growth.

• After pinching, when they’re producing lots of new side shoots.

• During periods of repeated harvesting when they’re constantly replacing stems and leaves.

Once plants are flowering heavily, nitrogen is still important, but it shouldn’t be the only nutrient you’re replacing.

What I recommend

If you’re trying to grow healthy, productive flowers naturally, these are the products I’d reach for first.

Chicken manure pellets

My go-to organic fertiliser for most flower gardens. They provide a balanced supply of nutrients, release them gradually and are ideal for improving fertility throughout the growing season. Apply around actively growing plants and follow the application rate on the packet.

Fish, Blood & Bone

An excellent all-purpose organic fertiliser that supplies nitrogen, phosphorus and potassium.

It’s particularly useful before planting or early in the growing season while plants are establishing. Because it contains all three nutrients, think of it as a complete feed rather than simply a nitrogen fertiliser.

Nettle tea

If you enjoy making your own plant feeds, nettle tea is a fantastic homemade option that’s generally associated with boosting leafy growth. It’s a great way to recycle garden waste while feeding hungry plants. Read my full guide to making NETTLE TEA .

Grass clippings

Fresh grass clippings make an excellent mulch around ornamental beds. As they break down, they slowly return nitrogen to the soil while helping conserve moisture during dry weather. Just apply them in thin layers to avoid creating a slimy mat.

Flower Farmer tip

Don’t feed because the packet says you should. Feed because your flowers have used something. Every bucket of stems you harvest takes nutrients away from your garden.

Healthy, productive flower gardens aren’t about pouring on more fertiliser. They’re about replacing what you’ve removed.

Phosphorus (P): Building strong roots

If nitrogen builds the plant above the soil, phosphorus builds what’s happening underneath.

Phosphorus is essential for:

• Strong root development

• Healthy seedlings

• Establishing young plants

• Energy transfer throughout the plant

Think of it as laying the foundations of a house.

The stronger the roots, the better your flowers can absorb water and nutrients throughout the growing season.

Plants that are short of phosphorus may:

• Establish slowly

• Stay smaller than expected

• Develop weaker root systems

• Produce less vigorous growth

  
  

Fortunately, true phosphorus deficiencies are relatively uncommon in most UK garden soils.

That means most gardeners don’t need to rush out and buy a phosphorus-only fertiliser.

Instead, focus on building healthy soil and using a good all-purpose organic fertiliser.

When do flowers need more phosphorus?

Flowers benefit from phosphorus most:

• When sowing seeds.

• When planting young seedlings.

• After planting out into the garden.

• When establishing new perennial borders.

Once plants have developed a healthy root system, phosphorus becomes less of a focus than nitrogen and potassium.

What I recommend

Bone meal

Bone meal is one of the best-known organic sources of phosphorus.

I like using it when planting bulbs, bare-root plants and long-lived perennials because it supports healthy root development.

It isn’t something I’d apply every few weeks. One application at planting is usually all that’s needed.

Fish, Blood & Bone

Fish, Blood & Bone also supplies phosphorus alongside nitrogen and potassium, making it an excellent all-round fertiliser for establishing young plants.

Chicken manure pellets

Chicken manure pellets also contain phosphorus as part of their balanced nutrient profile, making them another excellent choice for general feeding without needing separate products.

Potassium (K): The nutrient that keeps the flowers coming

If I had to choose one nutrient that flower growers often underestimate, it would be potassium.

Once your plants stop growing leaves and start producing flowers, their priorities change.

Instead of building stems and foliage, they’re putting huge amounts of energy into producing buds, blooms and seed.

That’s where potassium comes in.

Potassium helps support:

• More flowers

• Better flower quality

• Stronger stems

• Richer colour

• Better drought tolerance

• Greater resilience to stress
  

As a flower farmer, this is the nutrient I pay the closest attention to once the cutting season begins. Every stem you cut is another flower the plant has to replace.

That takes energy. And potassium plays a major role in keeping that cycle going.

Plants that are short of potassium often produce:

• Fewer flowers

• Smaller blooms

• Weaker stems

• Plants that seem to lose momentum halfway through summer

Sound familiar?

It’s often not because the plant is “finished.”

It’s because it’s used up the nutrients needed to keep producing.

When do flowers need more potassium?

Flowering plants benefit from extra potassium:

• As flower buds begin to form.

• Throughout the flowering season.

• During periods of heavy cutting.

• While producing repeat flushes of flowers.

For many cut flowers, this becomes the most important nutrient once the season is in full swing.

What I recommend:

Tomorite (and other tomato fertilisers)

Despite the name, Tomorite isn’t just for tomatoes. Tomatoes and flowering plants have something important in common—they both need plenty of potassium once they begin producing flowers or fruit.

That’s why many flower growers use tomato fertilisers on flowering annuals and cut flowers during the blooming season.

Always follow the manufacturer’s instructions for dilution and application.

Comfrey tea

If you prefer homemade plant feeds, comfrey tea is a fantastic natural alternative that’s generally associated with supplying potassium. It’s one of my favourite homemade feeds once flowering gets underway.

Read my complete guide to making comfrey tea.

Sulphate of potash

If you specifically need to increase just the potassium levels in your soil , sulphate of potash is another excellent option. It’s particularly useful for hungry flowering plants and fruiting crops, but it should always be used according to the instructions on the packet.

Flower Farmer tip

Once your flowers are blooming, stop thinking about growing bigger plants.

Start thinking about supporting better flowers. That’s when potassium really comes into its own.

The aim isn’t simply to keep plants alive. It’s to keep them flowering beautifully for as long as possible. Once your flowers are blooming, stop thinking about growing bigger plants.

Start thinking about supporting better flowers. That’s when potassium really comes into its own. The aim isn’t simply to keep plants alive. It’s to keep them flowering beautifully for as long as possible.

Where do compost, manure and liquid seaweed fit?

By now, you might be wondering…

“If I’m using compost or manure, do I still need fertiliser?”

The answer is yes. That’s because these products all do slightly different jobs.

Compost

Compost is one of the best things you can add to your garden.

It improves:

• Soil structure

• Water retention

• Drainage

• Soil biology

Healthy soil grows healthier plants. But compost is not a high-strength fertiliser. In a productive cutting garden, where you’re constantly removing flowers, compost alone often can’t replace nutrients quickly enough to keep up with demand. Think of compost as feeding the soil. Healthy soil then helps feed your plants.

Well-rotted manure

Well-rotted farmyard manure is a fantastic natural soil improver. Like compost, it adds organic matter while also supplying a slow release of nutrients. It’s an excellent choice for improving flower beds over winter or before planting in spring.

For many gardens, manure forms the foundation of a healthy feeding programme.

But if you’re growing flowers intensively and harvesting armfuls of stems every week, you may still need to replace nutrients during the growing season.

Liquid seaweed

Liquid seaweed often causes the biggest confusion. Despite sitting on the fertiliser shelf in the garden centre, it isn’t really a fertiliser in the traditional sense. It contains only very small amounts of nitrogen, phosphorus and potassium.

I

nstead, think of it as a plant tonic.

It supplies trace elements and naturally occurring compounds that help plants cope with stress.

I use liquid seaweed when plants are:

• Recovering after transplanting.

• Coping with hot weather.

• Under drought stress.

• Recovering from cold snaps.

• Looking generally stressed.

IT DOESN'T REPLACE NUTRIENTS

It helps plants cope while they’re under pressure.

A simple way to remember it is:

Feed hungry plants.

Support stressed plants.

Hungry plants need fertiliser.

Stressed plants benefit from seaweed.

Sometimes they need both.

The biggest mistake gardeners make

One of the biggest myths in gardening is that more fertiliser means more flowers.

It doesn’t. Plants need the right nutrients at the right time.

Too much nitrogen can produce lush green leaves but very few flowers. Adding more potassium won’t fix weak roots. And no amount of fertiliser can compensate for soil that’s desperately dry. The best results come from understanding what your plants are trying to achieve.

Growing leaves?

They’ll use more nitrogen.

Establishing roots?

Phosphorus is important.

Flowering their hearts out?

They’ll need more potassium.

My simple feeding routine

People often ask what I use on my flowers.

The truth is, I keep it surprisingly simple.

Before planting

I improve the soil with compost or well-rotted manure. I’ll use an organic general fertiliser such as chicken manure pellets or Fish, Blood & Bone.

As plants grow

I keep the soil healthy and replace nutrients as plants become established.

Once flowering begins

This is when I switch my attention to supporting flower production with a higher-potassium feed, such as Tomorite or homemade comfrey tea.

During heatwaves or after transplanting

I reach for liquid seaweed.

Not because my plants are hungry.

Because they’re stressed.

The one rule I always remember

Every stem you cut removes nutrients from your garden.

If you harvest it…

You need to replace it.

That’s all feeding really is.

You’re not giving your plants a treat.

You’re simply putting back what they’ve already used.

Once you understand that, fertiliser labels stop being confusing. And feeding your flowers becomes one of the simplest parts of gardening.

Frequently Asked Questions

What do the NPK numbers on fertiliser mean?

They show the ratio of nitrogen (N), phosphorus (P) and potassium (K) in the fertiliser.

What’s the best fertiliser for flowering plants?

It depends on the stage of growth. Plants need more nitrogen while producing leaves and stems, and more potassium once they begin flowering.

Is Tomorite only for tomatoes?

No. Tomorite is a high-potassium fertiliser that’s also widely used on flowering plants.

Is liquid seaweed a fertiliser?

Not really. It’s best thought of as a plant tonic that supports stressed plants rather than replacing nutrients.

Is compost enough on its own?

Compost is brilliant for improving soil health, but in productive cutting gardens it often needs to be combined with fertiliser to replace nutrients removed through repeated harvesting.

Growing beautiful flowers isn’t about buying the most expensive fertiliser. It’s about understanding what your plants need at each stage of growth.

Build healthy soil.

Replace the nutrients you’ve removed.

Support plants when they’re under stress.

Do those three things consistently, and your flowers will reward you with stronger stems, healthier plants and blooms that keep coming all season long.