Fertilizer and Butterfly Host Plants

Whether using organic or non-organic fertilizer, it is important to look at the ingredient list when we purchase fertilizer. Some fertilizers incorporate pesticides in the fertilizer.

Pesticides are used more often in lawn fertilizers than garden fertilizers. There are many pests that kill lawns, from beetle grubs to chinch bugs to mole crickets. By incorporating pesticides in the fertilizer, it saves the homeowner time and money to apply fertilizer and pesticide at the same time.

Pesticides move through the soil downward, of course. What many of us don’t realize is that it also moves horizontally. As irrigation and rain wets the soil, it moves underneath the soil, like water soaks through a sponge when placed under a dripping faucet. It doesn’t only go downward, it spreads sideways as well. A heavy rain will wash fertilizers and pesticides as far as the rain washes across the top of the soil, spreading it further.

As a plant’s roots draw up water from the soil, it draws up pesticides into itself. Some pesticides will stay in the plant for months. Others for a few days.

Even if we fertilize our lawn and are careful not to spread it within several feet of our butterfly host plants, it can seriously affect the health and lives of butterfly and moth caterpillars that feed on these plants.

Moving away from pesticides, commercial fertilizers in general are often mixed to benefit specific plants. High acid fertilizers are best for blueberry plants. Basic fertilizers are best for peanut plants. High nitrogen is best for non-blooming plants, like mustard greens. High phosphorus is best for flower and/or fruit producing plants, like roses and tomatoes.

Fertilizers normally carry three numbers, usually prominently displayed on the front of the container or bag. These numbers stand for, in this order, Nitrogen-Phosphorus-Potassium. Nitrogen feeds the leaves. Phosphorus promotes root growth and blooms (which become fruit on fruit-producing plants). Potassium also promotes root growth and strengthens disease and drought resistance.

These two water hyssop plants are the same age, planted at the same time. One was fertilized and the other was not.

Why fertilize plants?
Left: fertilized Right: not fertilized Water hyssop

Nectar plants should be fertilized with a higher middle and last number – phosphorus and potassium. Host plants (even if they bloom) should be fertilized with a higher first number mix (nitrogen). The primary exception is when the blooms themselves are the part of a plant the caterpillar eats. Caterpillars of Gray Hairstreaks, Cassius Blues, Ceraunus Blues, and other species tend to feed on flower buds. A soil test is advisable before adding fertilizer. Some soils are naturally high in one or the other and do not need more added.

Sleepy Orange butterflies drinking from Black Kow composted manure
Sleepy Orange butterflies drinking from Black Kow composted manure

Don’t be surprised to see butterflies visiting fertilizer that you have spread on the soil or even to land on a fertilizer bag to drink. Male butterflies drink salts and minerals, found in fertilizer. We recommend covering fertilizer with mulch.

Red-spotted Purple male drinking from the outside of a fertilizer bag
Red-spotted Purple male drinking from the outside of a fertilizer bag

Time release (controlled release) fertilizer was created to save people time. When spread around plants, it slowly releases fertilizer. Some formulations will last for a month and others for several months. Hot weather causes the fertilizer to be released more quickly. Special time-release fertilizers are made for states like Florida.

Because silt and clay bond with fertilizer as it washes into the soil, plants growing in silt/clay can draw from fertilizer for a longer period of time than plants growing in sand. Sandy soil will need fertilizer more often because it doesn’t bind fertilizer/water well. It somewhat compares to pouring water over a sieve full of pebbles as opposed to a sieve full of tiny bits of sponges. Our soil is so sandy that I can lightly fertilize every two weeks without harming plants.

Too much fertilizer will burn and/or kill plants. Too much can refer to how often fertilizer is applied as well as how much fertilizer is applied. If too much fertilizer has been applied, one can scrape any visible fertilizer off the top of the soil. Next, a water sprinkler can be placed to wash over the remaining fertilizer (assuming the soil isn’t too wet or water restrictions are present in the area). The more water washes over the remaining fertilizer, the more it is washed away from the roots of the plant.

Over-fertilization is serious. Most of the time, when plants are burned by over-fertilization, they cannot be saved.

Remember, fertilizer washes into bodies of water, promoting algae growth. Fertilize only as needed.