11 Perennials That Benefit from Fertiliser in March

Many gardeners wait until flowers have already appeared before applying fertiliser. For perennials, that is generally too late. March is when you set the course for abundant, colourful borders: roots become properly active again and absorb nutrients like a sponge. Acting purposefully now will help you get considerably more from your garden, without much extra effort.

Why March determines your perennial flowering

After winter, it is not only the soil that thaws out; your perennials’ roots also begin growing again. They produce fresh fine roots that are especially effective at taking up nutrients. Fertiliser therefore has a far greater effect at this stage than it does in high summer, when the ground is often dry and compacted.

“Fertiliser applied in March acts like a starting signal: plants have energy before they form buds and flowers – and that makes the difference between ‘quite nice’ and ‘Wow’.”

Organic fertilisers and slow-release products provide a steady supply of nutrients for three to six months. This avoids nutrient surges that encourage foliage to grow excessively while flowering falls behind. Depending on your area, the ideal time is from late February to early April, when the soil is lightly moist but no longer frozen.

These 11 perennials particularly reward fertiliser in March

Some perennials respond much more strongly to an early fertiliser application than others. If your time or budget is limited, prioritise these plants first.

1. Roses – start the flowering engine at the right time

Roses need plenty of energy to produce numerous new shoots, buds and a lengthy flowering period. March, immediately after pruning, is the ideal time for the first application of fertiliser.

• Use a balanced rose fertiliser, such as NPK 10-10-10 or 12-12-12.
• Work it lightly into the top layer of soil, then water it in.
• Apply a slightly lower dose to roses in containers to prevent root damage.

This gives plants a vigorous start and encourages strong, well-ripened shoots – the foundation for healthy flowers.

2. Peonies – strengthen them properly once a year

A good spring feed rewards peonies with lavish, heavy blooms. Their roots reach deeply into the soil and draw a great deal from it, so nutrient reserves need replenishing.

A balanced general fertiliser, or mature compost combined with a little organic fertiliser, applied in March is often sufficient for the whole season. Crucially, do not put fertiliser directly on the emerging shoots; instead, work it into the soil around the plant.

3. Dahlias – energy for a long flowering spectacle

Anyone who loves dahlias should feed them for flowers rather than leaves. Too much nitrogen produces rampant greenery and few blooms.

• Choose a fertiliser with a higher phosphorus and potassium content, for example 5-10-10.
• Make the first application in March or April, when the tubers are being prepared or planted.
• Give strongly flowering varieties a second, lighter application in early summer.

This directs the plants’ energy into buds and abundant flowers rather than endless foliage.

4. Delphiniums – towering growth needs nutrients

Delphiniums grow into impressive flower spikes within a short time. Without adequate nutrition, their stems remain thin and their flower spikes sparse.

A balanced garden fertiliser in March, combined with some compost, promotes sturdy shoots that stand firmly. If there is a second flush of flowers in late summer, you can apply a light feed after cutting the plant back.

5. Garden phlox – fragrant clouds backed by good feeding

Phlox paniculata is a genuinely hungry plant. If you want dense, scented clouds of flowers in high summer, action is needed in spring.

As with roses, a fertiliser with an even nutrient balance is ideal. Pairing this with a compost mulch helps to keep soil life and moisture levels stable, which suits phlox particularly well.

6. Coreopsis – long-flowering sunshine

Coreopsis flowers for many weeks, and that requires energy. A moderate spring feed gives it the stamina to keep going.

Apply a thin layer of organic fertiliser or a light general fertiliser in March or April. Too much nutrition can make plants unstable and may even shorten their flowering period.

7. Bearded iris – flowers are preferable to leafy growth

Bearded iris thrives in lean, well-drained sites. Even so, its rhizomes benefit from targeted feeding – provided the nitrogen level is low.

• Use a flower-focused fertiliser, around 5-10-10.
• Never scatter it directly over the rhizomes; lightly fork it into the soil beside them.
• Do not mulch too heavily after feeding, as rhizomes need air and light.

8. Hostas – hungry stars of the shade

Hostas grow slowly but create impressive layers of foliage. Although they are not exceptionally demanding feeders, they appreciate a gentle boost at the start of the season.

A mix of compost and slow-release fertiliser will usually be entirely enough. In partial shade and shade, organic matter helps the soil retain moisture and protects the leaves from stress.

9. Daylilies – each flower lasts one day, but the display lasts all summer

Daylilies open fresh blooms every day for weeks. They particularly need phosphorus and potassium to support bud formation.

Give them a first feed in March with a slightly higher phosphorus and potassium content. A second, moderate application in summer keeps the flowering going without overloading the plants.

10. Hydrangeas – influence colour and abundance

Mophead hydrangeas in particular respond directly to their nutrient supply, while blue varieties are also affected by the soil pH.

• For blue flowers in acidic soil, use a fertiliser low in phosphorus, such as 12-4-8.
• In neutral to slightly alkaline soil, blooms will tend to remain pink; fertiliser alone will do little to change that.
• Fertilise in March, then give hydrangeas in containers a weaker second feed in early summer.

11. Astilbes – long-flowering plants with a big appetite

Astilbes prefer damp, humus-rich soils and are among the more hungry perennials. Feed them well and you will be rewarded with long, dense flower plumes.

A balanced spring feed, followed by a second application in autumn, strengthens plants for new growth and the formation of future buds. Combined with a thick layer of compost, this keeps both soil moisture and nutrient availability consistent.

How to apply fertiliser correctly

Even the best fertiliser is of little use if it is applied incorrectly. Common mistakes can quickly cause leaf scorch or weak plants with soft growth.

“Never fertilise directly at the crown, and never feed plants when the soil is frozen or bone dry – it prevents plenty of disappointment.”

For perennial borders in March, follow these practical steps:

1. Lightly rake over the surface and clear away old plant debris.
2. Scatter the recommended amount of fertiliser evenly around the plants.
3. Gently work it into the top layer of soil.
4. Water thoroughly afterwards so the nutrients reach the root zone.

This prevents localised over-concentrations and ensures the fine roots can access the nutrients.

Weeds in March: irritating competition or free liquid fertiliser?

The first wild plants appearing in borders can seem irritating, but they also offer an opportunity. In March, their roots are usually still shallow, making them easy to pull out by hand. Removing them takes away competition for water and nutrients from your perennials.

For practical gardeners, there is another benefit: freshly pulled weeds can be made into a simple plant brew that works as a liquid fertiliser.

• Loosely fill a bucket with wild plants, excluding any seed heads.
• Top it up with water and leave it to ferment for several weeks.
• Dilute the resulting liquid manure at a ratio of roughly 1:10 with water.
• Apply it around the root area of perennials every 14 days.

The nutrients stored in the weeds are returned to the border rather than ending up in the garden waste. This is an inexpensive way to give depleted borders an additional feed.

Why slow-release and organic fertilisers suit perennials so well

Perennials remain in the garden for years. A continuous, moderate flow of nutrients is better suited to this rhythm than quick, short-lived boosts of nitrogen. Organic fertilisers and slow-release products release nutrients gradually, often in close cooperation with soil organisms.

With every application, you usually improve more than the nutrient status alone: soil structure benefits too. The ground retains water more effectively, is less prone to capping and lets air reach the roots. This makes plants more resilient during dry spells and spring temperature fluctuations.

Common questions: quantities, risks and sensible combinations

Many amateur gardeners are uncertain about how much fertiliser their perennials can tolerate. The general rule is to apply small amounts regularly rather than giving occasional “fertiliser shocks”.

Perennial type	Fertiliser type in March	Note
Heavy feeders (for example roses, astilbes)	General fertiliser + compost	Apply one stronger feed, then a light additional feed in summer if needed
Moderate feeders (daylilies, phlox)	Moderate general fertiliser application	During dry spells, do not apply additional fertiliser unless you also water
Light feeders / shade-loving perennials (hostas)	Compost + a small amount of slow-release fertiliser	Use sparingly, otherwise you risk overly lush growth

The main risks arise from overdosing and feeding at the wrong time. Plants given too much encouragement in cold soil produce soft new growth and become vulnerable to late frosts. Only fertilise once the soil is genuinely frost-free and can be worked easily.

A sensible combination consists of:

• an organic base fertiliser or compost in March;
• a light supplementary feed with liquid fertiliser, if required, in early summer;
• a mulch of composted bark or leaves to protect the soil and its living organisms.

Gardeners who feed their 11 key perennials in this way will often be surprised by how much the garden “gives back” in late spring and summer: more densely planted borders, stronger stems and longer flowering periods – all from a few well-timed tasks in March.