Why should weeds in an orchard be managed? What problems can result if they are not managed properly?

Answer: Weeds compete directly with the tree for needed water and nutrients during the growing season.  High weed pressure can stunt tree growth and also significantly reduce fruit size and yield per tree.  Broadleaf weeds can harbor insects (i.e., stink bugs, plant bugs, etc.) that will migrate from the weeds to fruit on the tree.  Stink bug feeding can cause “cat-facing” of fruits making them unmarketable.  Also, as they probe the fruit with their long proboscis, they puncture the skin and make wounds that provide ideal sites for fungal pathogens (i.e., brown rot) to grow.  Weeds can also serve as alternate hosts for viruses or ring nematodes. 

Prior to the advent of excellent herbicides, many growers cultivated the soil of the entire orchard to eliminate weeds.  Scientific research demonstrated that most peach tree feeder roots were in the upper soil layer (i.e., 0-12 inches) and that soil cultivation actually led to many feeder roots being cut off or damaged.  In most commercial peach orchards in the U.S., cultivation is no longer used for weed control.  Both preemergent herbicides (i.e., simazine, karmex, etc.) and postemergent herbicides (i.e., glyphosate, paraquat, etc.) are commonly used to effectively eliminate weeds in the tree row ‘herbicide strip’.  Occasionally, growers may overdose with the soil applied preemergent herbicide.  High doses of simazine or karmex, especially with young trees, can cause phytotoxicity (foliar chlorosis, shortened internodes, etc.) and even kill trees. 

With young trees, many growers use milk cartons to protect the young, green, bark from contact postemergent herbicides but when overspray occurs, bark of trees can be damaged. Off-target herbicide drift is particularly a problem with glyphosate (RoundUp and other formulations).  If the grower is not using a shielded sprayer in his own orchard, or spraying when it is windy, or a nearby landowner is spraying roundup on a windy day, drift onto peach foliage can be readily taken up by the tree and significant injury or even tree death can result.  Finally, weeds should be managed to reduce the risk of damage from fire.

The occurrence of fire damaged orchards is quite rare, but when it occurs, it can be devastating. Dead, dry weeds that are present on the orchard floor in the fall/winter/spring present excellent fuel under dry conditions that could be combustible if there was a lightning strike, a cigarette from a passing motorist, or accidental fire from a piece of farm machinery.  Dry weeds combined with windy weather and one spark of fire can lead to destruction of hundreds of trees in a short period of time.  

Weeds in a peach orchard
Weeds in a peach orchard
Young tree in foreground stunted by high weed pressure.
Young tree in foreground stunted by high weed pressure.

Weeds in a peach orchard

stink bug
Mature tree in poor health because of intense weed competition.

Stink bugs that are attracted to broadleaf weeds in the orchard can migrate up into the tree.  Feeding damage to young fruits will cause cat-facing appearance.  Damage to larger, more mature fruits, may provide wounds for colonization with brown rot fungi. (source: David R. Lance, USDA APHIS PPQ, Bugwood.org).
June Gold Stinkbug clean cultivation of peach orchard
 Fruits that were damaged by stink bug feeding when they were smaller.  As fruits grow, these damaged areas take on a “cat-facing” appearance and they are unmarketable.
Orchard in Ft. Valley, GA in July, 1895.  Note that row middles have soil ridges.  No ground vegetation is visible.  The practice at the time was to cultivate down the row middles and then cultivate perpendicular to this across the tree rows.  Most commercial growers in the U.S. no longer cultivate peach orchards for weed control.
herbicide strip in peach orchard
herbicide damage above carton
Young orchard with good weed management.  Note grass in row middle to reduce erosion and weed-free herbicide strip down the tree row.  Milk cartons appear around the trunk at the base of the tree to protect against damage from contact herbicides while the bark is still green and tender.
Although a milk carton was used to shield the trunk of this young tree, the herbicide applicator accidentally got contact herbicide (i.e., paraquat) on the tree trunk above the top of the carton.  The black/brown bark above the carton shows the contact herbicide damage.
Two year old trees showing paraquat damage Close up of trunk on 2-year-old peach showing paraquat damage
Two year old trees showing paraquat damage (trunk constriction and necrosis) that occurred during the first season in the field. Shielding by milk carton or using latex paint could have limited and potentially prevented damage. Ideally, paraquat should not come into contact with "green" bark at all. Close up of trunk on 2-year-old peach showing paraquat damage (trunk constriction and necrosis) that occurred during the first season in the field. Trees with such damage will never produce properly and need to be replaced. An expensive error that was avoidable.
veinal chlorosis and internode stunting in peach interveinal chlorosis in peach
An overdose of preemergence herbicide (i.e., solicam) that was taken up by the root system of the tree and caused veinal chlorosis and internode stunting in peach. An overdose of preemergence herbicide (i.e. simazine) that was taken up by the root system of the tree and caused interveinal chlorosis in peach.
Peach trees damaged by RoundUp Accidental, non-target, herbicide drift of Roundup onto peach orchard
Accidental spraying of young peach trees with Roundup by inexperienced pesticide applicator.  Note that damaged trees have a “scorched” appearance.  All symptomatic trees on this site died. Accidental, non-target, herbicide drift of Roundup onto peach orchard the previous fall.  Fall uptake of this systemic herbicide resulted in minimal, spindly and stressed foliation the following spring. 
Scorched orchard floor and peach trees following an accidental fire.
Scorched orchard floor and peach trees following an accidental fire.  The orchard floor was covered with dry, dead weeds.  When the grass was ignited and strong winds followed, many trees were caught in the fire. All trees with brown leaves died.

 

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