Stinknet is a descriptive name for an invasive plant from Africa that has quickly made a home for itself in our low deserts. Between Phoenix and Tucson, it can be found just about anywhere you want to look, including right here in Pinal County.

The round bright yellow flowers can be judged attractive by those who like color in the garden but it carries juices that can cause allergic reactions to those with sensitive skin and asthma in others. Because it burns hot once it is dried out, and because it tends to grow in dense clumps that crowd out desirable plants in the garden, it really is a plant out of place in our yards. Health and fire hazards have recently let the Arizona Department of Agriculture to declare it a noxious weed under Arizona laws.

The preferred name for stinknet is globe chamomile but many people find it easier to use the shorter and more descriptive name. If you are doing a web search for more information, which I wholeheartedly recommend, you can use any of its names, including its scientific name, Oncosiphon piluliferum. It should pop right up.

No, it is not the same plant that is used to make chamomile tea. I just knew that you were going to ask that question! While some have listed stinknet as a source of some medicinal value, I personally have not seen any literature that convinces me that it can be trusted for such purposes. Moreover, I am quick to rely on the good instincts of cattle and goats. They don’t’ like to eat the stuff. Yes, the plants used for chamomile tea are distantly related to stinknet; they are in the same plant family of course; but the differences are great and globe chamomile, or stinknet, simply should not be considered, or used, as a source for chamomile tea.

“How would I recognize it if I saw it?” you ask. Like we said earlier, it is everywhere so you should have no problem finding it. The easiest characteristic used to identify the plant is the brilliant yellow-colored flower. These round, globe-shaped blossoms are quick to catch the eye, once you know what to look for. Pea-sized or smaller, the flower heads are actually composed of somewhere around four hundred individual flowers, each capable of producing seed. If we let the plants flower and distribute their seeds, we help them get established. The end result of that choice is that we will be fighting them for many years to come. The plant also can be recognized by its fern or “carrot-like” leaves.

Stinknet is a winter annual that germinates, grows, flowers, and produces seed all in only one growing season. The first new seedlings begin to appear in November and the season continues through mid-March. Flowering usually begins in February, but can start a little earlier. The seeds are quite small, less than two small marks on your metric ruler, and extremely light. Their size and weight mean that they blow easily with the wind. They can also stick to feet, tires, clothing, and animal fur. For these reasons, they are quite mobile and can quickly colonize new areas.

How prevalent are they in Pinal County? If you step outdoors and walk around, there is a good chance that you will find it lurking out in a lawn or among other garden plants. If the seeds fall in an irrigated area, or if we have a moist winter season, a huge population of plants can emerge in just a short period of time. I have seen heavy populations of plants in the northern areas of the county and it is well distributed in our low desert communities.

Be careful in your search for these undesirable neighbors. They can be sneaky. Look for them for sure in lawn and garden areas, but I am finding them also growing in cracks in concrete driveways, asphalt parking lots, under landscape shrubs, and many other hidden or unexpected places. Keep a watchful eye out to find them. It only takes one to cause a problem.

You will want to know how to control this invasive weed, of course. That can be difficult, but there are some effective techniques. Let me share the good news first. Because it is an annual plant, it only grows from seed. It is not like some of our difficult perennial weeds that redevelop from year to year from underground stems. Stinknet does not have any of those. What this means is that once it is removed, roots and all, it will not grow back.

The second piece of good news is that it only grows during the cooler part of the year. Once it begins to warm up, seeds stop sprouting and the plants themselves die. The summer months fortunately are a time to rest and recuperate from this weedy pest but for sure it will be back when the temperatures begin to cool off.

Our first line of defense is to pull them out roots and all. There are several points that should be remembered when this technique is used. First, the fluids in the plant can cause people with sensitive skin to react with an itchy rash. I wouldn’t accept the risk so use gloves whenever you work with the plant.

Second, when you remove the plant, be sure to put the plant immediately into a garbage bag, especially if it has flowers. Tossing the plant onto the ground and then raking up could very well spread more seeds for future problems down the road. Use good garden sanitation practices for best results.

Third, do not just pull off the tops of the weeds and leave the roots. If we leave the spot on the plant where the leaves meet the roots, the weed will probably grow back. I carry a linoleum knife with me in the yard and use it to cut off these undesirable plants down in the roots. You can also dig out the entire plant, including the roots, if you are careful not to disturb the plants that you want to keep. If there are no flowers present; now be sure for they can be small; I will use my string trimmer to whack them down to ground level. They might grow back, but it is faster and easier than digging. In this case, I am just trying to prevent flowers and seeds from forming.

You may not be physically able to perform that kind of labor in the yard and would prefer to use some kind of chemical control. That can be a challenge also. There are not a lot of chemicals that will kill the plant. Glyphosate, the active ingredient of many trade names, does have the ability to kill stinknet, but the spray droplets tend to run off the waxy and sticky leaves unless there is some kind of agent in the spray that will smooth out the droplets and cause them to stick to the plant. If you choose this route, be sure that the spray has some kind of surfactant.

What about pre-emergent herbicides? They will work fine as far as they go. You will remember that pre-emergent herbicides are those that are applied to the soil before the seeds begin to germinate and kill the seeds as they take on water during the germination stage. Oryzalin, trifluralin, and pendimethalin are each common names of pre-emergent herbicides that regularly can be found on nursery shelves. Look for them on the label’s active ingredient list. Be wary now. Each have active lives of periods less than the weed’s five to six-month germination period so multiple applications may be necessary depending upon the conditions in your yard and how often you irrigate. Some licensed applicators might be able to use some products that are longer lasting, but make sure you know what they are using and that the materials will be safe for other plants and pets.

While the invasive weed has become firmly naturalized from Phoenix to Tucson, it is not too late to slow down the spread of this gardening challenge. It will take a concerted effort on the part of everyone in Arizona, neighbors and strangers alike, to minimize its takeover. By learning to recognize the weed and by quickly removing it before it can produce seeds, it is possible to launch a successful fight against this invader.

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If you have questions, you can reach one of our Master Gardener volunteers every week day except Saturday and Sunday between 9 am and 12 pm by calling the Master Gardener hotline at (520) 374-6263. You can also call the Cooperative Extension office, 820 E. Cottonwood Lane, Building C, in Casa Grande at (520) 836-5221, extension 204 and leave a message.

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