Look for a 1-800 Customer Service phone number on your container of Weed-B-Gone, and see if they would agree that the best thing you can do is water the ground around the trees to wash the herbicide down below that 3 feet level or so that the roots extend down.
Also, you want to ensure that you don't walk on that really wet ground much as your compressing the soil with your weight can push the oxygen out of it so that tree roots can drown.
You don't need to know the rest.
Most people aren't aware that a tree's (and most plant's) root system are almost entirely within the top 3 feet of soil.
People see the roots of tiny plants, like grass, and presume that the root system of a large tree is similar, only correspondingly larger, and that's simply not true. When a tree starts growing, it starts with a tap root that penetrates deep into the ground (given the size of the sapling). But that tap root soon develops into a system of LATERAL roots that extend mostly horizontally within the top 3 feet of soil. The root system of a tree extends far beyond it's branches, but the roots seldom extend more than 3feet down.
The reason why is that the nutrients in the soil the roots are intended to absorb require the aerobic decay of organic matter (compost, if you will), and you only have enough air in the soil for aerobic decay to occur within a few feet of the surface. Deeper than that, you don't get enough air in the soil for aerobic decay, and things don't rot. It's the rotting of organic matter that produces the nutrients that plants thrive, and that makes the soil fertile. Below that top thin layer of fertile soil, there is no aerobic decay and roots that grow into that deeper soil stop growing because they're not finding the kind of soil the tree likes.
That's the same reason why they can harvest 200 year old logs from trees that were cut down in the 1800's and have been sitting on the bottoms of lakes for literally two centuries. The water deep down doesn't have enough oxygen in it for that decay to occur, so even after 200 years soaking wet, the wood won't have rotted.
This is also the reason why it's comparatively rare to have tree roots growing into a sewer pipe given the number of trees in our cities and their proximity to sewer pipes. The tree roots are almost entirely within the top 3 feet of soil where aerobic decay occurs, but the sewer pipes are much deeper where there is no aerobic decay and there are far fewer nutrients in the soil.
Trees can also grow anchor roots that do extend deeper into the ground than the roots that nourish the tree, but these anchor roots don't grow very deep as the tree grows taller. It's primarily the "feeder" roots that grow horizontally away from the tree that anchor the tree so that it can resist wind.
This web page (from the City of Winnipeg's web site) contains the following:
About trees, tree roots and sewer pipes:
Roots from trees growing near sewer lines do not actively penetrate sewer pipes and cause blockages. Roots gain entry through previously cracked portions of sewer pipes. Sewer pipes inevitably deteriorate through old age or separate and crack due to ground shifting and heaving.
Sewer pipe is laid approximately 2 metres, or more, deep.
The only tree roots at that depth are anchor roots, as the finer and fibrous feeder roots are located within the first metre of soil.
Anchor roots can co-exist with intact sewer pipes indefinitely without causing blockages.
A sewer line leak allows sewage and air to escape into the soil, creating a ratio of air, water and nutrients at that depth that becomes similar to those found near the surface. Anchor roots at the site of the leak produce very fine, opportunistic feeder roots that can enter the sewer pipe.
And, Winnipeg ain't no dry desert like Las Vegas or Phoenix. I got trees growing on or near my property that have trunks 3 feet in diameter and are better than 40 feet tall. It's hard to imagine that the root system of such trees only extends down a few feet, but what we expect and what we discover has always been a source of amazement and opportunity to us.
It's entirely the fact that despite their large size, a tree's roots are almost entirely in the same top 18 inches of soil that you "hit hard" with weed killer that's affecting the health of your trees.