Controlling weeds in the pecan orchard

    Over the past week, I've been working on getting the weeds and grass controlled around my pecan trees (photo at right). It is widely accepted that total vegetative control around young trees is one of the key factors in promoting rapid tree growth and greater nut production.  There are several ways to achieve weed control around pecan trees including tillage, herbicides, and hardwood chip mulch. But in my orchard, I've opted for spraying herbicides.
      When I first started working with pecans back in the early 1980's, glyphosate (Roundup) was the easy choice for total vegetation control. All you needed to do was to make sure you didn't hit pecan leaves or green bark with the spray. However, once roundup ready crops were developed, the widespread use of glyphosate has led to the proliferation of roundup resistant weeds.
   This year, I have used a combination of herbicides around my trees. I used Gramoxone (a plant desiccant) to control existing winter annual weeds, and Simazine (a pre-emergent herbicide) to control germinating weeds and grasses. Later this summer, I'll come back with a broad spectrum contract herbicide to address any additional weed problems. I'll rotate between glufosinate ammonium  (Rely) and glyphosate (Roundup) to help avoid resistance issues.

     In my orchard, most of my trees started out as volunteer seedlings. So for these trees I drive a utility vehicle from tree to tree carrying a 25 gallon sprayer in the back (photo at left).  I try to create a 6 to 7 foot weed-free zone around each tree. I do not try to drive and spray at the same time. At each tree, I stop and get out so I can spray around each tree being very careful not to get any spray drift on the tree.

    Minimizing spray drift is very important when spraying around young trees. Last fall, I planted some one-year-old seedlings in some of the gaps in my volunteer orchard. I caged these trees to ward off the deer and carefully sprayed around the seedling (photo at right). When spraying near the small tree, I always ease up on the water pressure coming out of the nozzle. This dramatically cuts down the creation of a fine spray mist that can drift onto the green bark of the seedling.

   

Vegetation control is not so critical for the performance of mature pecan trees. The oldest trees on my farm are in the pecan breeding block which is planted in rows. Here, I spray a herbicide strip down the row using a simple one-nozzle boom sprayer mounted on my tractor. The herbicide strip has two functions. First, it serves to control climbing weeds such as poison ivy, trumpet vine, and wild grape. And second, it makes mowing the ground cover much easier. I can mow in one direction without having to try to weave in and out of the trees. In the volunteer orchard, I mow in two directions to get all the ground cover mowed. This means I'm covering the same ground twice every time I mow. 

Controlling weeds around young pecan trees

   During most years, regular spring rains and muddy soil conditions keeps us out of the field at this time of year. In fact, the vast majority of floods that occur on the Neosho River happen during the months of May and June. However, a drier than normal Spring has enabled us to get an early jump on weed control this year. During periods of calm winds, we use a 30-gallon, electric sprayer to apply roundup in a seven foot circle around each tree (photo at right).
   Roundup (gyphosate) is a systemic, contact herbicide that can damage pecan trees if spray droplets contact any green tissue. To limit possible spray drift problems, we use a hand held spray wand and adjust the nozzle to a course spray pattern (large droplet size). We use a generic form of roundup at the rate of 2 oz. per gallon. To help the roundup penetrate weed leaves, we add liquid ammonium sulfate and a non-ionic surfactant to the spray tank.

    What you don't see in the photo above is the fact that we walk through the orchard a week before spraying and trim off any basal sprouts or any new shoots sprouting from the lower portion of the trunk. One of the most common causes of roundup injury to young pecans is accidentally hitting unseen basal sprouts hidden in tall weeds.
    During this preliminary walk through the orchard, we discovered several trees that were killed back to the ground by mid-winter cold. In the photo at left, you can see we had two years of strong growth on this young graft but the entire tree died back to the ground line following last winter's intense cold.  However, all is not lost. This tree has already developed a couple of basal sprouts that will provide a new place to graft in the future.
    In the mean time, we were fortunate to discover these new shoots before applying our herbicide spray. First, we pulled the weeds away from the stump sprouts by hand. Then, we very carefully sprayed the rest of the area around the tree. Roundup can penetrate both the leaves and green bark of tender pecan shoots, so be very careful.

Young trees suffer from Bermuda grass competition

Bermuda grass competes with young tree

     Bermuda grass is probably the most competitive ground cover commonly found in young pecan orchards. Try as you may to develop a weed free zone around young pecan trees; Bermuda grass will creep back, covering every inch of available soil. In the photo above, you can almost see where this pecan grower had sprayed glyphosate herbicide in a ring around his young tree. By early October, Bermuda had recovered the ground.

Light green leaves indicates low nitrogen

    Although all species of grasses are competitive with young trees, Bermuda is one of the toughest competitors for three reasons. Bermuda is a warm season perennial grass that actively competes with the tree for valuable summer soil moisture. Bermuda is also very aggressive in extracting nitrogen from the soil. In fact, the leaves on trees in this planting were light green indicating low leaf nitrogen levels (photo above).
    The third reason Bermuda grass is so competitive with trees is that it secretes allelopathic chemicals from its roots which suppresses the growth of other plants including pecan trees.  Once Bermuda grass is established in a pecan orchard it it very difficult to control. Only orchard wide and repeated applications of gylphosate seems to do the trick.
    I a twist of nature's irony, pecan trees will eventually win the war against Bermuda grass. Given enough time, pecan trees will grow so large as to completely shade out the low-light intolerant Bermuda grass.  
        

Kill the grass

    The most important cultural practice you can use to increase pecan tree growth rate is to kill all competing vegetation in a 7 foot circle around young trees. Yes, kill the grass. You can use herbicides or a thick blanket of wood chip mulch. Either way, total vegetation control is the only way to ensure fast growth during the early years of a pecan tree's life.
   In the photo at left, a young tree has a herbicide circle around the tree. Not only does vegetation control increase tree growth it also stimulates early nut production.
    Once pecan trees grow to 8 or 9 inches in trunk diameter, they seem to start dominating the ground cover and herbicide circles become less important to tree growth and nut production.