Landscaping to eliminate mold spores

Landscaping to eliminate mold spores

Landscaping to Eliminate Allergenic Mold Spores

There are many things we can do in our gardens and landscapes to eliminate allergy-causing mold spores. All molds produce tiny reproductive spores and the trick is to find ways to get rid of the molds themselves.

What we plant, and where, has a large influence. I continually see the flat out dumb practice of planting tall evergreen trees and shrubs on the South sides of house. In the winter the sun is low on the horizon and we get most of our light, and warmth, from the sunlight that shines from the South. Our warm morning light comes from the East and it is never a good idea to block that with tall evergreens either.

The best place for tall evergreens is on the North side of our houses. There they can act as a windbreak and not rob us of any needed winter sunlight.

A house with tall evergreen trees on the Southeast side, is one that will always be cold, and damp, in the winter months. And cold and damp is exactly what mold thrives on.

Recently I was at a store, standing outside waiting for a friend of mine to finish up inside. It was a cool wintery day and I was in the full, deep cold shade of a very large Canary Island Pine tree. I walked over about thirty feet and stood in a spot, in between the trees where the sun was shining through. There it was nice and warm. To my left was the big pine shading that store, and just to my right was another huge evergreen tree, a Ficus retusa, the Indian Laurel Fig.

The big fig cast a shade even deeper, and colder, than did the pine. I looked down at the sidewalk to my left and right, and sure enough, you could see mold growing in the cracks and along the edges. The north side of the trees, where I was, also had a good deal of mold growing on the tree leaves themselves.

Deciduous trees are perfect for these locations. In the hot summer they will be all leafed out and will cool down the buildings behind them. In the cold winter months they will be bare of leaves, and the low sunlight will come through and warm things up. In this day and age of exploding energy costs, it is just plain ignorant to plant evergreens where they don't belong. For stopping mold spores, deciduous trees on the South-Eastern exposures is the only way to go.


Many people seem unclear on just exactly what is a mulch. Very simply, a mulch is anything that covers the soil. They can be made of old leaves, straw, rocks, bark, gravel, boards, bricks, even plastic.

Mulches are almost always a very good idea but when it comes to mulches and molds, they aren't all created equally. Bark is a very good material on which to grow mold. Gravel mulches are good because they don't encourage mold growth. I like smooth gravels, river gravel, and please! No white gravel.

Flat stones and pavers work well for this too and in the right spot, they look good as well. Mulch holds down weeds and cuts down on summer water loss. Earthworms often thrive under mulch and in general mulches usually help plants grow better. The one spot where mulches are less effective is in those cold, always shaded areas. Here mulch will keep the soil from ever warming up. Every where else though, mulch is useful.

Newspaper mulches by the way, not only look trashy, they also grow lots of mold.

Buggy Plants and Mold

Plants that are not being grown right will usually get infested with insects. The insects secrete "honeydew" and on this very nutrient rich gooey substance molds grow quickly. The molds then start producing spores and pretty soon there is a serious allergy situation in the landscape. The insect dander itself is highly allergenic and just adds to the problem.

Buggy plants often look dirty and this is because they are covered with honeydew, mold, and yuck! They are dirty. Clean, healthy plants are what we want in our yards.

Why Are the Plants Covered With Insects?

If a tree is native to the cold, damp forests of Japan or Minnesota, it just won't thrive in a place like Los Angeles. It certainly might grow in Los Angles though, and that's the problem. It will grow there but it won't thrive. Because it doesn't have the conditions it needs it will always be somewhat weakened, and pests always prey on the weak. Remember, insect pests equal mold spores.

If an area is very deficient in fertilizer the plants there won't thrive. As they grow weaker, the insects start to prey on them.

If plants are getting far too much fertilizer they will also become weak.

If a tree is a type that needs regular water in the summer but never gets it, again it will become weak and soon be a target for the white flies, aphids, scale, spider mites, and mealybugs.

If shrubs or trees are native to an area with acid soil and you're growing them in alkaline dirt, sure enough they'll probably become bug infested.

If a tree is simply not tolerant of urban smog and it is planted right smack in the middle of a great metropolis, it will draw the pests.

If a row of shrubs are all the kind that loves bright sunshine, but someone has planted a fast-growing tree over them, perhaps a pine, when the whole row of shrubs is now growing in deep shade, if they live, they will certainly become an insect magnet. I know of a hedge just like this near where I live. A large old hedge of lantana, now shaded by a big pine, it is literally covered top to bottom in white flies and mold. It is growing right outside the back entrance to a health clinic!

There are many other cultural reasons for plants not to thrive and any one of them can result in weak plants and mold.

A Word to the Wise on Natives

Judicious use of natives is often one of the very best ways to avoid many of these weak plants-mold problems. However, make sure the "natives" you buy are endemic to your own particular area. Also, make sure you're not getting a bunch of male ( pollen- producing ) clones. Many of the native trees, shrubs, and ground covers sold now are male clones.

Air Flow

In every place there are prevailing winds. The breeze generally blows mostly from one direction. Many landscapes are so plugged up, so crowded, that the breeze simply can't penetrate the mess. A landscape with no air flow is one where molds will thrive. Molds grow best in conditions with poor air circulation.

If your own yards are over-grown and choked for lack of fresh air, then get out the pruning saw and start thinning them out. Clean, fresh air, free to move about, equals less mold and fewer mold spores.

Sunlight and Molds

Bright light and fresh air are the enemies of mold. Many landscapes have huge trees overhead that let in little light. Consider hiring a tree trimmer to thin out some of the branches overhead. Open the trees up so that the sunlight can come through. Perhaps it would be a good idea to actually remove a tree or two if they're growing too close. Let the light shine!

When planting any new tree, consider the shade that it will cast when it is full-grown. Certain trees always develop very thick canopies while others will be light and airy.

Watering and Irrigation

Perhaps as important as any other single mold factor is the watering. Too little water makes for weak plants that attract insects. Too much water will also always produce weak plants.

Automatic irrigation systems, on clocks, are responsible for a great deal of mold growth. Allergists in desert areas often find very high mold spore counts, in the middle of the summer! Much of this is being directly caused by irrigation systems that are not being monitored closely enough. Often they are set to irrigate lawns that are already still soggy from the last watering. Over-watered lawns will quickly become mold factories and will shower everyone near them with an abundance of mold spores.

Plant Diseases and Spores

Many pests of our plants are not insects but are fungal type diseases such as mildew, rust, black spot, scab, and leaf blight. These organisms also produce allergenic airborne spores. The very best way to avoid these diseases and their spores is by planting disease resistant plants. The second most valuable approach is to keep plants growing cleanly and strongly.

Insect-attacked plants will often later be attacked by fungus diseases, and visa-versa. Healthy plants go a long way to keeping our air clean.

Certain plants if grown in the wrong area can almost be counted on to harbor disease. Evergreen viburnum growing in the shade will certainly get moldy and full of mildew.

Crape Myrtle trees grown in an area that doesn't have hot summers will always have mildew.

A cold, wet spring frequently brings out a huge flush of both mildew and anthracnose on the leaves of California Sycamore trees.

In areas with cool, foggy nights and warm days, rust will surely grow on any roses, hollyhocks, or snapdragons that are not rust-resistant.

Most roses grown in too much shade will quickly mildew. Actually almost any plant that thrives in full sun will run into problems in too much shade.

Insecticides and Fungicides

When you see a plant covered with insects or fungus, fight the urge to go get out the chemical sprays. Many chemical sprays will themselves trigger allergies. They may also weaken your immune system.

A shrub full of insects can often be helped immensely by just blasting off the bugs with a strong jet of water from the garden hose. Spider mites on plants can also often be brought under control with this same stiff spray of water.

Many insect pests can be killed with a simple, non-toxic homemade spray of vegetable oil, water, and liquid dish soap. For a gallon of water add two tablespoons of vegetable oil and two to four tablespoons of soap. I like Ivory Liquid.

For fungus diseases spay them with a mix of baking soda and water. I use from two to six tablespoons of baking soda per gallon of water, depending on how bad the infestation of disease is. This often needs to be repeated all summer long. The baking soda will also kill some aphids. If you like you can just add some baking soda to the insecticide mix of soap and oil and have an all-around insecticide-fungicide spray mix.

Do not expect these homemade sprays to be just as effective as the most powerful chemical killers. Often they're not. But they do work and they are much safer and a whole lot less likely to cause allergies.


This stands for Integrated Pest Management and one of the basic themes of IPM is that we are not looking to eliminate insect pests, just to control them. Using beneficial insects such as ladybugs, mealy bug destroyers, tiny parasitic wasps, and green lacewings is always worth a try. It would be worthwhile for any gardener interested in allergy control to read a book or two on organic pest control.

Ants, Aphids and Scale

Ants will farm out aphids and scale and will protect them from their natural predators. When the aphids and scale have ruined one part of a plant, the ants will move them to another fresh spot.

Frequently we can't seem to get rid of the insects because there are so many ants on the trees and to kill the ants I use a slow-acting but effective mix of powdered sugar and borax. Look for the borax in a box in the grocery store where they sell laundry products. Mix the sugar and borax fifty-fifty. Sometimes I like to flood the area under where the ants are thick with a hose and then when they're all over the place, I sprinkle the sugar and borax mix.

A few types of ants don't much care for sugar and for these try mixing corn meal and borax. This bait mix will also kill some other garden pests such as slugs, earwigs and roaches. I have also had good luck killing ants with a mix of non-dairy creamer and borax. Cockroaches by the way, inside the house cause plenty of allergies and the best way to kill them is with a mix of boric acid and powdered sugar as a bait. Sprinkle this powder down where the roaches will walk through it. You can buy boric acid in almost any drug store. These baits are cheap, safer than other poisons, and they work.

Out in the yard don't put these baits where the dog will eat them. Sometimes it works well to hide them under old boards or flat rocks.

A Note about Ferns

Ferns don't produce mold spores but they sure can produce fern spores. Often these spores from the ferns can be just as allergenic as the mold spores. Fern spores usually shoot out and land fairly close to the fern. Small ferns growing in a shady part of the garden rarely trigger much allergy. But people love to grow ferns in hanging baskets and then they often hang these over patio chairs, tables, right where someone will be sitting.

When these overhead ferns cast off their miniscule spores, they will land directly on the unsuspecting victim underneath. Hanging basket ferns are fine, but watch where you hang them!

Tree ferns are handsome creatures but again we need to watch where we plant them. All too often they are planted right next to front doors where with their added height, they can shower spores on the people coming and going. Another consideration with tree ferns is that they have millions of tiny reddish-brown colored, needle-sharp hairs on their trunks. These little fern hairs can make you itch and they can also cause plenty of irritation of the throat and nose when they're inhaled.

Plant tree ferns back away from most human traffic.

About the Author

Thomas Leo Ogren, is the author of Allergy-Free Gardening, July 2000, Ten Speed Press. He has an MS in Agriculture/Horticulture, taught Landscape-Gardening for 20 years, and hosted the popular "Tom Ogren's Wild World of Plants" on Minnesota Public Radio. Allergy-Free Gardening is his third published book.

About this Author