Young trees and large weeds are a bad combination.
Weeds which rob nutrients and water, plus harbor pests and pathogens, can jeopardize the future productivity of newly-planted pistachio trees.
“Starting clean” with a new orchard is a critical first step in successful weed management, helping ensure production potential, says Kurt Hembree, University of California Cooperative Extension weed management farm advisor at Fresno County.
The smart use of pre- and post-emergent materials provides young trees the opportunity for good growth and better weed control options through the orchard’s life.
Orchard sites infested with highly invasive weeds, including nutsedge, field bindweed, bermudagrass, and johnsongrass, should receive the most attention before planting trees since these weeds can increase management costs during orchard establishment and beyond.
The goal with a new pistachio orchard is good weed control, says Hembree, and to reduce the seed bank, and avoid tree injury from herbicides and equipment.
This year, late spring rains provided additional moisture to spur weed growth, he explains. Fast-growing weeds including horseweed, fleabane, jungle rice, and Palmer amaranth are widespread in the southern pistachio growing area and are best controlled early.
“In years like this Palmer amaranth can grow two inches a day. If you wait longer than a couple of weeks after emergence then it’s more difficult to control,” Hembree said.
Weeds which host insect pests and harbor vertebrate pests should receive close attention.
Examples include knotweed, spurge, and London rocket which are hosts for false chinch bug. Russian thistle hosts lygus and green plant bug.
Prior to planting, growers are encouraged to assess orchards for perennial weed pressure and hard-to-kill annuals.
Strategies for a clean start include mechanical weed control, and post emergent herbicides to knock down existing weeds. After irrigation, re-growth can be assessed; repeated herbicide applications and tillage are options.
Hembree says these strategies can significantly reduce the weed seed bank and increase the amount of control over perennial weeds. Using good pre-plant materials during the non-bearing years can make post emergent sprays more practical.
Once trees are planted, Hembree says it’s essential to monitor for new weeds and re-growth. Early detection and control can prevent nutrient and water losses, plus save herbicide control costs. This should be part of a comprehensive orchard control plan which considers previous weed pressure and the potential re-infestation from outside sources.
Hembree warns that herbicides can harm young trees by burning the tree foliage or damaging the bark. Even with protective tubes on trees, applicators should focus closely on proper spraying to eliminate spray contact with trees. The product Paraquat can cause bark on young trees to split, creating an opening for infection by a canker.
Herbicide applications are best made before weeds are taller than a trunk protector, he says. Applicators should properly adjust the spray pressure and drop size, and always remain conscious of the spray direction to prevent herbicide damage.
Resistance is another consideration in the weed control program. Hembree recommends using materials with two different modes of action in a tank mix, and not repeating the exact same combination for subsequent applications.
He says it’s fairly common for growers to leave resident vegetation and weeds in the middle of every row or every other row, and keep it mowed during the growing season.
Ground cover between the tree rows can provide steadier equipment footing when moving through the orchard under wet conditions, plus reduce dust common during the summer and fall months.
On sloped soils, ground cover helps prevent erosion and conserve moisture. It is also common, Hembree says, to see a mix of filaree and other resident low-growing weeds mowed into the middles.