February 27, 2020

WESTMINSTER, Colorado - An article in the most recent edition of the journal Weed Science shows that cover crops can play an important role in slowing the development of herbicide resistant weeds.

Researchers conducted field experiments in Pennsylvania to explore how cover cropping tactics influenced the management of horseweed in no-till grain crops. Seven cover-cropping treatments were used over two subsequent growing seasons.

There were several significant findings. In comparison to fallow control plots, cover crop treatments reduced horseweed density at the time of a pre-plant, burndown herbicide application by 52% in the first year and 86% in the next. This reduced the herbicide "workload" and lowered the selection pressure for resistant weeds. Cereal rye alone or in combination with forage radish was found to provide the most consistent horseweed suppression.

Importantly, winter hardy cover crops also reduced horseweed size inequality - meaning fewer large horseweed plants were found at the time of herbicide application. Researchers say this reduces the chance of a size-dependent fitness advantage for horseweed biotypes that develop herbicide resistance.

"Our hope is that understanding the complementary relationship between cover crops and herbicides can lead to new weed control strategies that slow the development of herbicide resistance," says John M. Wallace, Ph.D., of Pennsylvania State University.

Full text of the article "Cover crop effects on horseweed (Erigeron canadensis) density and size inequality at the time of herbicide exposure"  is now available in Weed Science Volume 67, Issue 3.

https://www.cambridge.org/core/journals/weed-science/article/cover-crop-effects-on-horseweed-erigeron-canadensis-density-and-size-inequality-at-the-time-of-herbicide-exposure/E5D181B65270F615000F53706156B2FB

The USDA - Natural Resources Conservation Service has a wealth of information on fire recovery and erosion management.  They can also do site visits to make recommendations about fire recovery:

https://offices.sc.egov.usda.gov/locator/app?state=CA

https://www.nrcs.usda.gov/wps/portal/nrcs/detail/ca/newsroom/features/?cid=nrcseprd1289661

Their publications include directions for physical barriers, such as sand bagging and wattles, but also erosion managment practices which are critical going into the rainy season.

Soil likes to be covered at all times.  It doesn't “want” to be exposed to the elements, so you either cover it (plants, asphalt, paper etc.) or it will cover it for you with plants (weeds).  If it can't be covered fast enough, it disappears – erodes.  This can be from wind or rain or just natural movement down slope.  Plants that are managed for other than their agricultural return are called cover crops, although they can also have a crop that is saleable. Often weeds can be managed to be a cover crop, as well.

We are looking at a possibly wet winter and many tree crops grown on hillsides and sloping ground are prone to soil erosion.  Covers can be grown year round, but that usually means they require water all year round.  That means they need an irrigation system dedicated to their needs.  It also means having extra water which may be limiting.

A winter cover crop that grows out in the winter, does its thing (although that “thing” can include lots of other things, e.g. insectary, nutrients, water retention, etc.), and then dies or goes dormant, can be ideal.  It also requires less water than a permanent cover.

But there is a big problem here. Establishment of an introduced cover still requires water.  Rainfall in Southern California is erratic and there may be early rains to germinate seed, but it may not be consistent enough to get the plants established.  In fact, they may die for lack of further rain or be delayed.

Delayed germination means that soil is cooler and there is less growth. The real growth may occur after there has been sufficient rainfall by January and February when the soils are cooler and there is even less chance for growth

So when rainfall is doing its worst, there's no effective cover.  Or what cover there is, is what has germinated from “native” seed.  It may not have the characteristics you want for management: low stature, low entanglement with the trees, low water use, holds the soil without holding up harvest, etc.

So what do you do?  There are several approaches. You can move the sprinklers out into the middles and irrigate up the seed.  If you are in a limited water situation, you can do alternate middles, not cover cropping the whole area, or every third middle.  Whatever it takes to break the surface flow of water.  Or you can turn to mulching.  Put down sufficient mulch in a middle or every other middle to break overland water flow. 

Cover cropping is easier than mulching, but it takes water and timing.

Below are two websites with descriptions of cover crops and how to distinguish them from “weeds”.  Often a good cover can be the residential weeds. A low–lying cover allows pickers in to get lemons without mess and fear of snakes.  It also means that it can be more easily treated (mowed, weed whipped) at the end of the rainy season to reduce fire hazard.

1)      Characteristics of different cover crops

http://asi.ucdavis.edu/programs/sarep/research-initiatives/are/nutrient-mgmt/cover-crops-database1

2)      Weed identification from the UC IPM

http://www.ipm.ucdavis.edu/PMG/WEEDS/broad_preview.html