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To Fertilize. Or Not.

by Lawrence Rozas, President

and Phyllis Griffard, Education and Outreach

Native plants, which are well adapted to the local environment, do not require fertilizer to thrive in most soils. When used at all, fertilizer is best used sparingly and only at the right time. Over-fertilizing is probably second only to over-watering as the leading cause of plant death by the home gardener. Furthermore, your fertilizer does not stop at your desirable plants or your property line. The great majority of what you apply does not get into your plants, even when applied correctly. It ends up in weeds and waterways. Nutrient load in waterways impairs them and are the cause of dead zones. That’s why heavy mulching with natural mulch is such a good alternative to fertilizer. 

To decide what fertilization regime is appropriate for your garden, think about what fertilizer is and what plants use it for. Fertilizer is a mixture of inorganic nutrients. The N:P:K numbers you see on the label refer to the nitrogen, phosphorus and potassium needed in high amounts by plant cells. They need N and P to make every DNA and protein molecule from sugar, and K as their main electrolyte. They need much smaller amounts of micronutrients like the metals copper and selenium, needed as catalysts for enzymes. Some species need more or less acid (specific soil pH) to take up or use the nutrients efficiently. Most soils and mulches provide enough of these nutrients, but they are so inexpensive that fertilizer producers add them “just in case” one of them is limiting in your soil.

So with that reminder, recommendations from gardening experts make sense. Fertilizer may be beneficial for leaf growth, but it is not good for root growth. Remember that roots grow in response to chemical cues. If roots have access to ample nutrients without having to reach, they get “spoiled” and don’t invest in growing roots. That’s why propagators, including ANPP, don’t fertilize when planting seeds. It’s not until seedlings have true leaves that we bump them into bigger pots with some slow-release fertilizer on the pot’s periphery (we use osmocote). Plants that receive fertilizer too early have stunted root development. When is too early? Avoid fertilizing woody plants (trees and shrubs) until one year after they are planted. Then only fertilize young woodies for only a few years. Continuing to fertilize older woodies will result in weak stems and branches prone to breakage during high winds. Fertilize herbaceous plants (perennials) and young woody plants only at the beginning of their growing season (late winter to early spring for most species). Avoid applying fertilizer to plants under stressful conditions. For example, fertilizing plants during a drought will do more harm than good and may lead to plant mortality. Avoid fertilizing plants late in the growing season as well, as they are preparing for rough times ahead.

If and when you do fertilize, Bill Fontenot recommends using fish emulsion or a manure tea for herbaceous plants and small woodies. This tea is made by mixing manure in water at a 1:3 ratio (1/3 manure to 2/3 water). Alternatively, a water-soluble fertilizer (5 tablespoons in 5 gallons of water) can be applied directly to the root zone of plants. Louisiana irises respond well to water-soluble fertilizer recommended for acid-loving plants (e.g., azaleas). As with other plants, fertilize irises only during their growing season. Begin fertilizing irises early in the year (January - February) and cease once the bloom stalks emerge in Spring (usually March). Fertilize every week or two during this period. Fertilizer can be applied to Louisiana irises using several methods. A hose-end sprayer is a convenient, quick method for applying fertilizer to irises in a raised bed. With this method, the fertilizer is absorbed by both the leaves and roots of the plants. Isolated patches of irises can be fertilized using a watering can by applying fertilizer directly to each plant. In water gardens, especially those containing fish, irises can be safely fertilized using organic and spike-type fertilizers inserted directly into the soil around the plants rather than applied to the water, which will lead to an algal bloom and die-offs of animals.

Thinking about natural fertilizer can also help you make wise decisions about mulching. Leaves such as oak and pine needles contain more diverse molecules than wood and bark, which are just the thick cell walls of dead cells. Cell walls are almost all sugar, with little DNA or protein, therefore very few of those other good minerals like N, P, K and metals. So when decomposers break down wood, their own little cell bodies pull N, P and K from the soil, leaving little for your plants. That's the difference between compost and mulch and we don't use mulch to plant into. Leaves from your yard are also better than purchased mulch because they are not transported far from their cheap source. More importantly, especially here in Louisiana, we should never purchase cypress mulch. The sources are not documented, and some are harvested from our few bald cypress forests left standing, which property owners have every right to sell to producers for pennies. Use your own leaves and shred or sift them if you are going for a particular neat look. Or get free mulch from your parish yard waste facility (Dugas Road in Lafayette Parish, in LeBeau in St. Landry Parish). Amazing how good your own leaves or nice black mulch looks when piled up neatly around your plants, leaching decent free fertilizer all the while (unlike bagged mulch).

Native plants are adapted to take up nutrients efficiently from soil, so fertilizing your entire yard “because it’s February” is not smart for your plants nor for weeds and watersheds. When possible, let the nitrogen, phosphorus, potassium and metals come from the dead leaves in your natural mulch.


Caillet, M., J. F. Campbell, K. C. Vaughn, and D. Vercher. 2000. The Louisiana Iris. The Society for Louisiana Irises, Timber Press, Portland, OR. 211 p.

Fontenot, W. R. 1992. Native Gardening in the South. A Prairie Basse Publication, Carencro, LA. 176 p.

From the ANPP January 2021 Newsletter.

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