While insect and disease problems pop up only occasionally among our plants, and may even clear up on their own, weeds are always with us. They are a constant, not occasional, problem. And a weed problem virtually never clears up on its own — it just gets worse with time.
Most gardeners have used herbicides or weed killers at some point in their gardening efforts. With our year-round growing season and abundant rainfall, weed killers are almost indispensable in maintaining our lawns and gardens. For many, they feel fortunate that there are products that can help them in their efforts to control weeds. For many others, they feel guilty or uncomfortable about using herbicides and other pesticides. All gardeners would like to use products that are effective but safer and less likely to adversely impact the environment.
I’m pleased to see a growing availability of weed control products making use of more natural and organic active ingredients. And because they are relatively new, it’s important for gardeners to become aware of them and what they can (and cannot) do to help us control weeds.
These alternative herbicides fall into several categories, including natural oils (clove oil, eugenol, and citrus oil), herbicidal soaps (various fatty acids), natural acids (acetic acid — vinegar, citric acid) or iron compounds (iron chelates). Some products are available that contain more than one of these active ingredients and others.
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Increasing attention is being paid to these alternate herbicides that are considered by some to be less-toxic alternatives. All these active ingredients are post-emergent herbicides that are applied to actively growing weeds. Corn gluten is the only organic preemergence herbicide I can find. It is applied to a weed free area to prevent weed seeds from successfully germinating and growing. While it may have some uses in lawn weed control, in beds a 2- to 3-inch layer of mulch is just as good and has added benefits.
They are all contact or burndown herbicides — they kill only the part of the plant you spray them on. They are not systemic and, when sprayed on the plant, they are not absorbed by the leaves. They don’t move within the plant to kill all parts, including underground parts like rhizomes and bulbs. This limits their effectiveness in controlling perennial weeds and woody weeds. Persistent repeated applications will be needed when trying to control perennial weeds.
These herbicides are most effective on annual weeds when they are small, young and tender. Older weeds are more resistant and may require more than one application.
Except for the iron chelate herbicides, these herbicides are nonselective — they will damage or kill whatever you apply them to. You must be careful not to spray these onto desirable plants as they will damage the part of the plant that gets sprayed.
Take your time when applying these herbicides to ensure you have good spray coverage. Slow down and make sure you have applied the product to run off covering all the surfaces of the weed’s leaves.
Natural herbicides can be a safer and effective as part of an integrated pest management program. As with any pesticide, whether natural, organic or synthetic, use them safely. Read the label directions carefully and fully (the usage label of a commercial product is a great advantage over homemade recipes). Use the any personal protection recommended, such as gloves, protective eye wear, long sleeve shirts, long pants and shoes with socks. Strong acetic acid and some of the plant-based products can cause skin, eye or lung irritation if not handled properly.
Gripeweed or chamberbitter is a summer annual weed that over the past couple of decades has become more common in southeastern Louisiana.
Acetic acid
Acetic acid, or vinegar, is an effective contact herbicide. Herbicide grade vinegar is much stronger than the vinegar in your kitchen cabinets (5% acetic acid). So, purchase a commercial vinegar herbicide that is around 10% acetic acid or more for the best results.
Use these products carefully, particularly the stronger percentages, as they can cause severe eye irritation and burn the skin.
A common homemade herbicide you may see online includes kitchen vinegar, salt and dishwashing liquid. Kitchen vinegar does not work that well by itself but adding salt and soap increases effectiveness. If you use a formulation like this over and over to kill weeds in a bed, the salt in the soil can reach levels detrimental to the desirable plants growing in the bed (the soil becomes contaminated with salt). So be cautious where and how often you use it.
Natural essential oils
Citrus oil (d-limonene) is extracted from the rinds of citrus fruits. Other botanical oils being formulated into herbicides include clove oil, cinnamon oil and eugenol. These chemicals kill the plant by dissolving the waxy cuticle of plants, causing them to desiccate and die. They are generally more effective on broadleaf weeds than grassy weeds.
Iron-chelates
Iron based herbicides are the only selective alternative herbicide I’ve found. They can be used to kill broadleaved weeds in lawns without damaging the lawn grass. This is because broadleaf plants absorb iron faster than grasses, and they end up absorbing levels of iron that are toxic to them while the grass is unharmed. The lawn grass may turn darker after they are used, and they are more likely to adversely affect the grass if they are used when temperatures are high.
If you are looking for alternatives to the commonly available herbicides, they are out there. Take a look at what is available at local businesses that sell garden products. Do research online. While there will still be situations where we need to utilize the “big guns” for effective control, it’s well worth learning about these new herbicides and seeing how you can put them to work in your landscape.
GARDEN TIPS:
- Remove spent flowers from bedding plants such as marigolds, zinnias, pentas, cosmos, salvia and rudbeckia to encourage them to continue blooming as long as possible.
- If you planted petunias this spring, don’t be surprised if they begin to look poorly this month or in July. Petunias generally do not tolerate the intense heat of summers this far south. When they languish, pull them up and replace them with more heat tolerant bedding plants such as angelonia, blue daze, celosia, coleus, gaillardia, lantana, ornamental sweet potato, Profusion zinnia, marigold, melampodium, narrow-leaf zinnia, pentas, periwinkle, purslane, salvia, scaevola or torenia.
- Spring and early summer vegetables planted back last March, like snap beans, cucumbers, squash and tomatoes, often finish up in late June or July. When you remove them, you could plant a green manure crop in the bed to improve the soil over the summer. Peanuts and Southern peas make excellent green manure crops. Just as the young plants come into flower, chop them up and till them under. They will enrich the soil with nitrogen and organic matter. Allow the bed to sit for several weeks while the organic matter decomposes, and you will be ready to plant your fall crops.
- Remove suckers that appear at the base of crape myrtles promptly as soon as they appear by cutting them off at their point of origin at the trunk or even below ground. Do not leave a stub or they are more likely to resprout. You might try a sprout inhibitor (available at local nurseries or online) to help reduce how often you have to do this. Crape myrtles that have been topped often produce more suckers than those that have not — another reason not to top your crape myrtles.
Garden columnist Dan Gill answers readers' questions each week. To send a question, email Gill at dgill@agcenter.lsu.edu.
