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Fertilizing Flower Gardens and Avoid Too Much Phosphorus

Most home garden fertilizers are complete fertilizers, which contain the macronutrients required by plants in the largest amounts.  The numbers on a fertilizer bag refer to the percentage of nitrogen (N), phosphorus (P2O5) and potassium (K2O) (in this order).

Complete fertilizers sold as “all-purpose” fertilizers for gardens, such as 24-8-16 or 12-4-8 often contain higher amounts of nitrogen (the first number) than phosphorus or potassium. However, complete fertilizers sold for flowering plants (including roses and bulbs) such as 15-30-50 or 10-30-20 contain higher amounts of phosphorus (the second number) than nitrogen or potassium and are often labeled as “blossom or bloom booster”. The use of high phosphorus fertilizers originates from the need for phosphorus on agricultural fields heavily used for crop production which were deficient in phosphorus.  However, most non-agricultural soils contain adequate amounts of phosphorus.

All three of the major nutrients are necessary for plant health. In general, Nitrogen is often required in the largest amount. Nitrogen is an integral part of chlorophyll manufacture through photosynthesis, stimulates green leafy growth and promotes fruit and seed development; Phosphorus supports the transfer of energy throughout the plant for root development and flowering; Potassium is essential for photosynthesis and regulates many metabolic processes required for growth, fruit and seed development.

Avoid Over-Applications of Phosphorus

Do home gardeners and landscapers really need to apply high phosphorus fertilizers to get gorgeous blooms?

Answer – it depends.  Most non-agricultural soils (unless acid sandy) contain adequate amounts of phosphorus.  A soil test is the only way to know for sure if a flower garden needs phosphorus.

What harm could it do to apply extra phosphorus?

Answer - Excess phosphorus (and potassium) can be detrimental to the environment by moving in runoff water and posing a threat to water quality. Aquatic plants are limited by phosphate and the addition of phosphate will induce algal blooms (eutrophication).  Algal blooms are followed by increased bacterial activity, resulting in lowered oxygen levels and the eventual death of fish and other animals.

Also, high levels of phosphorus, either from chemical fertilizers or natural sources such as bone meal or rock phosphate, can inhibit growth of beneficial soil organisms called mycorrhizal fungi. Without beneficial organisms, plants must put additional resources into root growth at the expense of other tissues and functions.

Nitrogen is much more likely to be limiting in gardens.  Nitrogen deficiency is characterized by overall leaf yellowing (chlorosis).  Among other things, the lack of nitrogen reduces the plant's ability to take up phosphorus.  When nitrogen is restored to optimal levels, the plant's ability to use phosphorus from the soil is markedly improved.  It's important to realize that when nitrogen is deficient it does not necessarily follow that other nutrients must be deficient as well.

So, don’t guess, soil test. The UMass Soil and Plant Nutrient Testing Laboratory offers soil testing for landscapes and gardens.

Once you receive your soil test report and recommendations from the UMass Extension Soil Test Laboratory:

  • Calculate the square foot area of the flower garden.
  • Calculate how much fertilizer to use.
  • Apply limestone, if needed according to the soil test recommendation.
  • Apply fertilizer based on the recommendation and calculation described below.

How to Calculate the Square Foot Area of the Flower Garden

For flower beds, lime and fertilizer recommendations by the UMass Extension Soil Test Laboratory are provided for a 100 square foot area. Therefore, you need to determine the size of your garden before spreading lime or fertilizer. The length multiplied by the width of the garden will give you the total area. For example, a garden 5 feet wide and 10 feet across would be an area of 50 square feet. Often flower gardens are irregularly shaped and the square footage would need to be estimated.

How to Determine How Much Fertilizer to Use Based on Soil Test Recommendations

The numbers on a fertilizer bag refer to the percentage of nitrogen (N), phosphorus (P2O5) and potassium (K2O) (in this order).

To determine how much fertilizer to use, divide the lbs. of the specific fertilizer material recommended in the soil test result by the percent of that same material in the fertilizer being used. First, convert the number to its decimal form when using percentages in calculations.

For example

The soil test recommends 0.25 lb. of nitrogen per 100 sq. ft. and you plan to use a 10-5-10 fertilizer, then this is how to figure how much fertilizer to use to supply 0.25 lb. of nitrogen.

0.25 (lbs. of nitrogen based on recommendations) divided by 0.10 (10% nitrogen in fertilizer) = 2.5 lbs. of 10-5-10 fertilizer to supply 0.25 lb. nitrogen per 100 sq. ft.

Use of Organic Matter

The addition of organic matter to very sandy soils or those low in organic matter can be very beneficial. Organic matter will increase both the water and nutrient holding capacity of the soil, moderate soil temperatures, encourage earthworm and other soil organism activity, increase soil nutrient levels, enhance the soil’s structure and make it easier for plant roots to penetrate the soil. Sources of organic matter include peat moss, leaf mold, rotted manure, bagged humus or compost. The use of organic mulch such as straw or bark mulch will also add soil organic matter as it decomposes.

Care should be taken when using composts, manures or other materials that are potentially high in nutrients as a source of organic matter. Heavy continuous use of compost can lead to imbalances or excess levels of some nutrients after a number of years. As with any soil amendment, it is advisable to periodically test your soil for nutrient levels, pH and organic matter and adjust your fertilizer and organic matter applications accordingly.

Guidelines for How and When to Fertilize Flowering Plants

Use the amount of fertilizer recommended on the soil test report at the times of year listed below for the type of flowering plants being grown.  Note that these are guidelines. Plant health, types of fertilizers, weather conditions, soil types and other factors influence plant nutrient needs and timing of application.

For new flower beds, work the fertilizer into the top 4 to 6 inches of soil before planting.  For established plantings, spread the fertilizer evenly around the plants and lightly rake it into the soil, then water thoroughly.  If possible, pull back the mulch around plants so the fertilizer is applied to the soil and not on top of the mulch.

Annuals - Apply fertilizer during flower bed preparation. Make a second application at the same rate 6 to 8 weeks later. Annual selections that will continue blooming into fall may benefit from a third application at the same rate made in late August.

Perennials and Ornamental Grasses (new plantings) - Apply fertilizer during flower bed preparation. Make a second application at the same rate 6 to 8 weeks later.

Perennials and Ornamental Grasses (established plantings) - Apply fertilizer when growth resumes in the spring. Perennials with long lasting foliage or extended bloom periods may benefit from a second application at the same rate 6 to 8 weeks later.

Spring Flowering Bulbs – Do not apply bone meal or other source of phosphorus unless a soil test  indicates it is needed.  Apply fertilizer as soon as new growth emerges in the spring. Also apply fertilizer at the same rate when preparing beds in late August or early September. 

Summer Flowering Bulbs - Apply fertilizer at planting time or, in the case of hardy summer flowering bulbs, when growth resumes in the spring. Make a second application at the same rate after flowering for plants with short flowering periods. For plants with long flowering periods such as cannas and dahlias, make a second fertilizer application at the same rate in mid-July.

Roses - Make separate applications of fertilizer in May, June and July. Do not fertilize after mid-July as new growth may be encouraged. It most likely will not have time to harden off properly in the fall and will be very susceptible to winter kill.

Wildflowers – Wildflowers that are native to New England's woodlands or meadows generally have low nutrient requirements. Apply fertilizer once in the spring as new growth begins, or during bed preparation.

Types of Fertilizers for Flowers

There are several ways to supply nutrients to flowering plants. These include granular chemical fertilizers, which may or may not be controlled-release, water soluble fertilizer and organic fertilizers. Controlled-release fertilizers are also called continuous feed, slow-release or timed-release.

Granular fertilizer formulations that are not controlled - release will generally supply nutrients to the plants for about 6 to 8 weeks. During periods of excessive rainfall or frequent irrigation, the nutrients may be leached out of the soil and fertilizer may need to be reapplied.

Controlled (Continuous) - release granular fertilizers consist of water soluble fertilizer that is encased in a semi-permeable resin coating. When they come in contact with water, small amounts of nutrients are released to the soil for use by the plant. The rate of nutrient release for most of these fertilizers is regulated by temperature. The warmer the temperature the faster nutrients are released. When the initial fertilizer has been depleted, fertilizer will need to be re-applied. Many of the products for use with flowers will supply nutrients for 3-4 months depending on the temperature and amount of moisture.

Some gardeners may also prefer to use water-soluble fertilizer formulations. Water-soluble fertilizers are purchased ready to use or as a concentrated powder or liquid fertilizer that is mixed with water and applied to either the soil, or to both the soil and the plant's foliage. Since the nutrients are in a soluble form, they are subject to leaching (movement through the soil). Because these nutrients are available for only a short period of time, the label of a water-soluble fertilizer will direct you to apply it at more frequent intervals than when using a granular or controlled- release formulation. In the flower garden liquid fertilizer is useful for a quick boost or to supplement granular or controlled-release fertilizers when then they have been depleted. However, most gardeners prefer products that do not have to be constantly reapplied.

Organic fertilizers can also be used to supply nutrients to flowering plants.  They can be purchased as complete fertilizers or for individual nutrients and as liquids or solid bulk forms. Organic fertilizers are often lower in nutrient analysis and solubility than synthetic fertilizers.  So, they may need to be applied at higher rates and greater attention should be given to soil preparation during the initial stages of bed preparation to ensure uniform distribution. Thoroughly incorporate organic fertilizers into the soil.

Tina Smith & Doug Cox
Last Updated: 
June 2015