June 19th, 2011
In a perfect world...the best, most effective time/way to control weeds in a tree planting is to apply herbicides in late March to early April while trees are dormant and weeds have not emerged. Unfortunately that doesn't always work out when time and weather prevent the application of herbicides before bud break.
I finally had time to some spraying caught up in a hardwood/conifer/shrub planting this past week...a far more difficult job when many trees are hidden below a mass of weeds but also still possible. Glyphosate is out this time of year because unless trees are covered, even a slight unexpected breeze can cause wind drift and a fatal mist of herbicide to a few leaves on a seedling tree.
Instead I use heavier rates of crop oil which can indeed burn tree leaves but unless given a full load over the top, unlikely to cause severe injury. I always use a 3 gallon Solo backpack sprayer and taking my time, I direct spray around and not over young trees this time of year. I cleaned house, so to speak and emptied nearly every herbicide jug getting the job done but that will allow me to compare and then share results of different combinations and rates.
The following are the herbicides and surfactants I used in different combinations and amounts on emerged weeds, primarily broadleaves. I have used Oust on these trees for nearly three years so grasses are a non-factor at this point.
Princep 4l (Simazine) mixed at 1 and 2 quarts in my 3 gallon BP sprayer
Atrazine 4l mixed at same rates as Princep
Surflan (oryzalin) at 1-2 quarts per sprayer load (sprays orange)
Stealth (Pendimethalin, Pendulum, Prowl) at 1-2 quarts per sprayer load (sprays yellow)
Oust XP...used ONLY on cedars and oaks and at very light rates of 1/2 ounce per sprayer mix. Oust can cause severe injury to any trees when combined with crop oil and sprayed over the top so I used extreme caution to NOT allow it to get on tree leaves.
Clethodim at 8 ounces per sprayer mix only where grasses were present
I used a concentrated crop oil in ALL herbicide mixes to act as a burn down for emerged weeds. It's not as effective of course as using gly but then next best option without danger of killing trees. 8 ounces per mix would be normal but I used double that to burn down weeds.
In the past Oust XP has controlled 100% of all grasses and weeds except...for pigweed. Pendulum (prowl) is better suited for pigweed control so I'll compare the different treatments and rates and report back the results this summer....