Charles Kerr asked me the other day if burning a field, ditch, or fence row kills weed seeds. Local farmers are telling him when they burn they kill the weed seeds and don't have to spray.
In the weed science classes I took many years ago, I learned that burning a field only killed seeds in the canopy of the weeds and since most weeds dropped their seeds and they fell to the ground after maturing, burning was not effective in killing those seeds. To learn the truth about burning and elimination of weed seeds, I sent Dr. George Beck, professor of weed science at Colorado State University, an email.
"George, I'm looking for information on the effectiveness the burning of fields has on weed control. Do you have a fact sheet on that? I remember being told fire will control weed seeds that are in the canopy of the crop being burned but has little or no effect on weed seeds that have dropped to the ground due to the speed the fire moves over the field. What can you tell me or can you direct me to more info?"
"Hi Curt - Burning usually does not kill weed seeds except, as you point out, when they still are on the plant and in the canopy. Otherwise, burning can stimulate seed germination - not just weed seeds, but many different species. It is a good way to manage the soil seed reserve of weeds because fire will break weed seed dormancy and can cause large flushes of weeds that then can be managed by other means such as tillage or herbicides. Fire typically is used in combination with other tools."
George referred me to the publication "Control of invasive weeds with prescribed burning" by DiTomaso, J. M., M. L. Brooks, E. B. Allen, R. Minnich, R. M. Rice, and G. B. Kyser. This was published in the peer-reviewed scientific journal Weed Technology, volume 20, pages 535-548. I found it on the web. It is interesting reading.
So burning stimulates weed seeds into growth. That is part of the reason why you see a flush of new growth after burning a field. The dark color resulting from the ash also stimulates growth as the darkened soil absorbs heat stimulating early growth. The major point Dr. Beck makes is that when a field or ditch is burned, additional "tools" such as tillage or chemicals are needed to manage the stimulated growth of the weeds. Fire alone cannot be used to control weeds.
So if you are going to burn your fence rows, fields or ditches, go the extra step and apply a product that will keep the weeds in check. Once you get the weeds under control and restrict their growth, you won't have to burn. You might even like being able to breathe without choking on that cloud of lung-penetrating particulates you produce every time you fire up your weed burner. I'm sure the CSU Extension office can provide you a list of products you can use to restrict weed growth.
Years ago, Tom Doherty, former agronomist for CSU Extension, and I put together a series of publications on weed control in ditches and fence rows, and with the changes that have occurred in chemicals since then, I hope the Extension office has the names of more effective and less potent products. If they don't have such a list, give me a call and I'll help you develop a weed control plan for your farming operation that doesn't require burning on an annual basis. You can reach me at 970-778-7866.
Dr. Curtis E. Swift is a retired horticulture agent with the CSU Extension. Reach him at Curtis.Swift@alumni.colostate.edu or check out his blog at http://SwiftsGardeningBlog.blogspot.com. He owns Swift Horticultural Consulting and High Altitude Lavender.