Weed Control for Evergreen Trees
Weed prevention is important to the success of your tree planting project. Weeds block the sunlight that your trees use to make food, and they consume water and nutrients from the soil. But there are reasons to leave the weeds alone...best to read this page from top to bottom!
Weed control mats and landscape fabric are a very effective method for controlling weeds, and this avoids most environmental issues as described in the herbicide and thinner methods above. Weed mats are specifically engineered to allow water and nutrients to pass through the mesh, while blocking 95% of weeds. There are many online stores which offer weed mats, and some are eco-friendly.
Lawn Mowers and Weed Wappers
Obviously very fast and effective, but a noisy two stroke lawnmower puts out as much pollution as ELEVEN late model cars!
Roundup is a popular, effective and widely available herbicide, but it kills EVERYTHING, so use with caution. To make an "after planting" spray barrier, you can cut the bottom off a 2 liter [or cut a length of PVC pipe or carpet tube], slide that over each tree until it is flush with the ground, and then carefully spray around the 2 liter...but be careful that the Roundup doesn't drip off the 2 liter and onto the tree as you remove it. It is also important to avoid creating a fine mist which can blow onto the trees by the wind. Set the sprayer to produce droplets which fall to the ground.
Preen is a pre-emergent weedkiller, meaning it does not kill germinated/growing plants. Apply Preen to bare soil around each tree and then cover the Preen with mulch or sawdust to create an effective barrier against weeds and a good insulator against moisture loss. Preen is widely available at garden centers and big box stores.
A product new to us is Hi-Yield Grass Killer, which can be sprayed over most evergreen species without harm, while killing most grassy weeds. We have no experience with this product, but it sure seems to be a promising way to keep your young trees from getting overcrowded by tall grasses [download Hi-Yield Grass Killer PDF].
There are commercial herbicides which can be directly applied to evergreens without hurting them, but you may need a license to buy them. Goal is quite popular in the wholesale industry, and in our opinion it has revolutionized the nursery industry:
Believe it or not, you can spray weeds with paint thinner in the late morning of a hot/dry day, and most weeds will be dead by evening. Paint thinner strips the protective wax off the leaves of the weeds, resulting in rapid dehydration of the weeds while having little or no effect on the trees [I don't know why]. Try spraying very closely around [but not directly on] a few trees before noon, and check the results later that same day. When I was participating in spring sports in high school and college, the weeds always got ahead of me. I sprayed paint thinner directly onto my own seedling and transplant beds, killing 90% of the weeds while losing just a small percentage of the trees [thus saving myself many, many hours of Spring weeding time which I did not have]. However, I don't recommend spraying thinner directly on all of your own trees until you do some small-scale tests. For extra safety, you can use the 2 liter trick [as described above] with paint thinner too.
Please keep in mind that spraying herbicides or paint thinner is not environmentally friendly, but it can get you through a tough spot. Take a look at the weed prevention methods below.
The Safe, Environmentally Friendly, Time Tested Method
We don't endorse chemical applications for various reasons. Nothing is safer or more environmentally friendly than a sharp hoe. It's actually quite easy and effective once you  get the hang of it and  get ahead of the weeds. My grandfather walked casually through his many beds of trees uprooting weeds, and he never bent over or strained his back. He weeded regularly to keep the weeds from getting too big to quickly uproot with a quick flick of the hoe. Funny thing is, he always worked right-handed, and wore down his favorite hoe far more on one side than on the other. He also lived to be 85, and perhaps one of his secrets to longevity was working smarter [not harder] and avoiding chemicals whenever possible.
The Flipside: Weeds Can Protect
Mother Nature rarely plants evergreen seedlings in open areas the way we humans do, but rather she plants them very near and somewhat protected by the parent tree, or in an area where competing plants are already established. Many seedlings slowly flourish in the protection and partial shade of other plants, then become stronger/taller trees which tower over everything around them and soak up the full sun.
One suggestion is to NOT kill all of the weeds in your planting area if the weeds are only one to two feet tall, especially in the fall...they will help provide protection for your evergreen seedling from winterburn.
If the weeds are more than 3 ft tall, then consider spraying round patches perhaps a couple of feet across, with the idea of planting the seedling in the center of the dead plant material later on after the herbicide is no longer active. The dead/surrounding weed husks will provide some protection from the effects of winterburn if the snowcover is inadequate.