How to Fertilize Your Perennials

Fertilizing perennials is a different challenge from dealing with annuals. Here’s what you need to know to get the timing and ratios right for the healthiest plants possible.

You want your garden to look its best, which often means applying fertilizer. The rules are different for annual plants and perennials, which makes it challenging to know what to fertilize, and when.

While annuals must be replanted yearly, perennials come back every spring. This means their planting bed must contain all the nutrients they need for long-term growth without overwhelming the plants. It can be a tricky balance that stimies many gardeners.

Here’s a guide to understanding the fertilizer needs of perennials to give them the best shot at success.

Understanding Fertilizer Basics

First, a refresher on fertilizer. This plant amendment provides necessary nutrients to encourage growth. Mixed fertilizers contain many micro and macronutrients but always include a blend of the following three:

Nitrogen: This plant nutrient promotes leaf growth by stimulating chlorophyll production — the primary chemical involved in photosynthesis.
Phosphorus: Ample amounts of phosphorus support the development of roots, stems, flowers, and fruit.
Potassium: Not found only in bananas, potassium helps plants process and manufacture their own food sources for fuel production.

Each type of fertilizer will have different ratios of these three macronutrients, making it possible to choose the style that best addresses your plant concerns.

Why Fertilize Perennials?

Like all plants, perennials need nutrients to thrive. Perennial beds tend to be crowded and competitive, meaning the plants are fighting over limited soil nutrients and can benefit from an infusion of fertilizer.

However, since they live for multiple years, you need to take caution not to overfeed them or offer the wrong ratios of nutrients.

Many perennials, including ground covers and grasses, need minimal amounts of fertilizer and will react badly if given excessive amounts. Excessive fertilizer can cause perennials to grow too quickly, resulting in leggy growth that flops over before the flowers bloom.

Not only will this look unsightly in the moment, it can affect the plant’s health for the following years. In extreme cases, too much fertilizer makes perennials more susceptible to insect and disease problems.

But, as long as you’re careful, fertilizing perennials will lead to healthy plants and encourage them to produce more blooms for longer over the growing season.

What Perennials Need Fertilizer?

It can be challenging to determine which perennials most need fertilizer. For example, some varieties prefer acidic soil, while others thrive in alkaline. Overall though, these plants tend to be forgiving and can thrive in various environments.

Soil tests usually aren’t necessary, although it’s a good idea to research your choice of perennials before planting to learn about any specific nutrient needs. If you’ve planted them in a well-prepared planting bed, they may only need a dose of compost in the spring.

Perennials that require minimal fertilizer include ornamental grasses, false indigo, asters, sea holly, bee balm, and butterfly weed.

On the flip side, some do better with extra nutrients. Heavy feeders include mums, lilies, lupines, tall phlox, peonies, delphiniums, and astilbes.

How to Fertilize Perennials

Fertilizing perennials doesn’t need to be complicated. Most are happy with all-purpose flower food. Organic blends are best because they run the lowest risk of burning the plants.

Overfertilizing perennials is as damaging as forgoing fertilizer altogether. The general rule is to apply twice per year — first in the early spring when new growth emerges and again in the fall before plants go dormant.

While application requirements will vary, plan to apply around one pound of nitrogen or less per 1,000 square feet. For context, that’s about one-quarter of what’s needed for the same amount of lawn space.

Fertilizers vary based on delivery and application method.

Slow Release

Also known as timed release, these encapsulated fertilizers break down slowly for added nutrients for up to six months after application. They work well in containers or established gardens where the risk of soil erosion is low.

Sidedress

This method of fertilizer application requires you to apply several tablespoons of fertilizer (refer to the manufacturer’s instructions) around the root zone of each plant. Ensure the fertilizer isn’t close to the stem or crown, as it could burn the plant. If the plant’s leaves stay yellow throughout the growing season, consider supplementing with liquid fertilizer around the plant.

Topdress

If you want to add fertilizer to an already established perennial bed, topdressing is an easy, effective option. Just add one to two inches of compost to the bed in the spring for a boost of nutrients all growing season. The heaviest feeders (mums, peonies, and tall phlox) can benefit from a secondary summer application.

6 Fertilizer Application Tips

Here’s some final advice for correctly applying fertilizer to perennials.

Never apply fertilizer too heavily to established beds, as perennials are susceptible to tissue burn that will damage or even kill the plants. Read all labels carefully and start with the lowest recommended concentration.
Generally, the slower the perennial grows, the less fertilizer it needs. In contrast, plants that produce lots of flowers or fruit tend to be the heaviest feeders.
Stick to natural and organic fertilizers when possible to reduce the risk of burning plants. Composted manures are ideal.
Avoid fertilizing in the late summer or fall, as it triggers new growth that will die off in the winter.
Plan to provide more fertilizer in more frequent doses for perennials in sandy soil, as it tends to wash away quickly.
Generally, the less you mess with fertilizer in nutrient-rich, established perennial beds, the better.
When planting perennial bulbs, incorporate some phosphorus into the hole to encourage blooming in the spring.
Most healthy shrubs and trees don’t need fertilizer, but consider adding some when you see signs of dead branches, disease, or insect damage to give the plant more fuel for fighting back.