The Grower’s Corner is intended to provide and share timely information on relevant poplar and willow crop management and related topics.
Projects supported by IPS VI
(Article posted May 2015)
by Michelle Sulz (Alberta-Pacific Forest Industries Inc.) and Cees van Oosten (SilviConsult Woody Crops Technology Inc.)
We start with the general topic of why we do weed control and the type of weeds we encounter. It is important to understand that the best time to begin weed control is during the site preparation phase well before the crop is ever established. Growers have many more options for weed control at a lower cost during site preparation than at any point after establishment. So why not control weeds right from the start before the crop is planted? Site preparation will be the topic for our next article.
Why Control Weeds?
There are several reasons we carry out weed control; most importantly because it benefits the crop, while other reasons include meeting legal or even aesthetic requirements.
Poplars and willows are extremely shade intolerant and require full exposure to sunlight to grow well. Any weeds threatening to overtop and shade out the young crop must be removed. Poplar and willow crops that experience weed competition at time of establishment will grow poorly and suffer mortality. Severe weed problems may even result in complete crop failure shortly after planting (see photo).
Even if complete crop failure does not occur, poor weed management during the early stages of establishment (especially the first two years) will have a very negative long-term impact on crop productivity. In areas of low survival the crop will fail to close canopy and weeds will remain a problem for years to come.
Many growers make the assumption that once the crop canopy is above the weeds there is no more need for additional weed control. From a survival point of view that may be correct, but from a growth perspective yield potential depends on the crop having access to sufficient water and nutrients (see figure).
This requires ongoing weed control until the crop canopy completely dominates the growing site. The extent and duration to which we want to continue weed control depends on our objectives and budget
Another important reason we should be thorough in our weed control is crop protection. Control of weeds removes habitat for damaging rodents, such as voles (several Microtus species) and pocket gophers (various species). These rodents can cause serious damage in a tree crop by girdling stems just above ground level and by eating tree roots. Voles prefer habitat with perennial grasses and lots of litter for cover. Their main food sources are grasses, sedges and forbs. If weeds can be controlled from day one, we have removed the potential for good habitat and can largely prevent rodent problems.
An R-4 (rising 4) hybrid poplar crop near Birch Hills (SK) in early July 2005 (a Forest 2020 project). This site was site prepared and planted in the spring of 2002 (the same year). It was replanted spring 2003 and again in the spring 2004 with little or no effort to control the weeds. Inadequate site preparation in 2002, just before planting, resulted in an uncontrollable weed problem and a doomed crop right from the start. Persons in the photo point to some of the remaining poplars. Photo: Cees van Oosten
The results of a small trial after three years of growth, comparing ‘no weed control’ with ‘one year complete weed control’ and ‘two years complete weed control’. Weed control was achieved using glyphosate applied with a backpack sprayer while shielding the trees. Trial results: Cees van Oosten
Weed competition also leads to stress, which in turn can make the crop more susceptible to insect damage.
In areas where ungulates such as moose and deer are a problem, reduced growth due to weed competition extends the opportunity and time for browse and damage to the crop, before crop height and canopy closure reduce susceptibility.
In areas with a drier climate the need for extended weed control in already established crops can also be driven by the need to reduce the fire hazard. In the Prairie Region for instance, dense areas of dried weeds, such as perennial grasses, can represent a significant fire hazard in the spring and fall. This is especially true along the headlands surrounding the crop. Under some circumstances weed control to establish a fire break may be warranted.
As farmers we are obligated to control noxious weeds such as Canada thistle (Cirsium arvense) and scentless chamomile or mayweed (Tripleurospermum perforatum), just to name a few on Provincial noxious weeds lists. Local, Regional and Provincial regulations may require us to control noxious weeds, or at least control their spread, and more often than not we do not have any labelled herbicides we can use without damaging the crop. Having to comply with noxious weed regulations can be a daunting and expensive proposition; so, as with all weeds, the most inexpensive and effective time to begin controlling noxious weeds is at the site preparation phase.
Types of Weeds & Weed Identification
Weeds are classified using definitions from ‘Weeds of the Prairies’:
- Annual: A plant that germinates in the spring, sets seed in the same year and then dies.
- Winter annual: A plant that germinates in the fall and survives the winter as a dormant rosette. It resumes growth in the spring, sets seed in early summer and then dies.
- Biennial: A plant that germinates in the spring of the first year, producing a rosette that survives the winter in a dormant state. It resumes growth in the second year, flowers, sets seed and then dies.
- Simple perennial: A plant that survives for three or more seasons. Each spring the plant re-grows from stored root and crown reserves. Seed production may occur in the first season and in each subsequent year. Spread of a simple perennial weed species is primarily by seed.
- Creeping perennial: A plant that survives for three or more seasons and, in that way, is similar to a simple perennial. However, a creeping perennial has a specialized method of vegetative propagation through rhizomes, stolons (runners) and budding rootstocks in addition to seed production.
It is very important to know what class of weeds we are trying to control. The best timing and method of weed control depend quite a bit on the classes of weeds we are dealing with. For example, application of an herbicide to annual weeds in the early fall after they have gone to seed is a waste of both time and money, while spraying perennial weeds in the early fall can be very effective. This concept will be covered in more detail in future articles.
In general, creeping perennials are the most difficult weeds to control. These include such species as quackgrass (Agropyron repens), field bindweed or morningglory (Convolvulus arvensis), Canada thistle (Cirsium arvense), etc. Any plant that is not intended to be part of the crop can be considered a weed. For example, volunteer barley (Hordeum vulgare), wheat (Triticum aestivum), oats (Avena sativa) and canola (Brassica spp.) in a poplar or willow crop are weeds and should be controlled.
Knowing the classification of weeds we are dealing with is a great first step, but for many weeds it is even better if we can identify the species of some of the common problem weeds found in our crop. For example, knowing the difference between canola and wild mustard may make a huge difference to what method of weed control works best as the canola could easily be “Roundup Ready” (resistant to the herbicide glyphosate) while wild mustard will easily be killed by glyphosate. Books and the internet are both great resources for weed identification and it is a good idea for us to learn how to identify common weeds found in our area.
Scientific Names vs. Common Names
We will mention both the ‘most common’ of common names and scientific names of the weeds we discuss. Common names vary widely from region to region, whereas scientific names are much more standardized. To illustrate, stinkweed, penny cress, Frenchweed, field pennycress, fanweed (and probably a few other regional names) are all common names for the weed with the scientific name Thlaspi arvense. We will be using the USDA Natural Resources Conservation Service ‘Plants Database’ as the reference.
We hope the first article in the Grower’s Corner series has provided a good starting point for further weed control discussions and articles. There are a variety of reasons that we may wish to control weeds and it is important that growers are well informed about the reasons for weed control and the types and species of weeds common to our area so that the right decisions for the situation can be made. If you have comments or questions please contact Cees (“Case”) van Oosten at firstname.lastname@example.org under the subject 'Grower’s Corner'; we want to hear from you.
 USDA Natural Resources Conservation Service ‘Plants Database’: http://plants.usda.gov/java/nameSearch