Weed Barriers: Things You Want to Know
The first question is whether you're trying to prevent weeds from moving upwards in your garden plantings or invading from the sides (as in grass into flower beds)
Lateral Movement of Weeds
If you want to stop weeds from moving sideways into your garden beds, there are two basic options you can use. The first (and most common) is a a barrier that goes down into the ground at least six-inches as a way to stop weeds. These are referred to as edging materials
The second is less well known but is a weed mat or material (I'm currently using old shingles) that goes from the retaining wall or garden edge outwards onto the lawn for a few inches and is often covered with a mulch (if it's ugly or left bare if it's brick or expensive) :-) The majority of weeds are stopped before they get to the garden bed.
Invasion of weeds from below or air
There are two basic weed barrier methods used in this case. The first is a weed mat or fabric made of a porous plastic material to allow water to move downwards but prevent weeds and roots from moving up into the garden area to throw leaves.
The second is a heavy layer of mulch. This does an admirable job of preventing air-born seeds from becoming established but doesn't work on invading roots.
Landscaping edging materials come in all manner of shapes, sizes and materials. The most commonly seen are the black plastic edging hose-shaped top style. The trench is dug, the edging material inserted and spiked into place and only the hose-shaped round top is left above ground. This is a good style for quick and easy installation but do understand that even though this material is UV-treated, it will break down in sunlight. The heavier grades will last for a significant number of years but the lighter (cheaper) grades will deteriorate much faster.
Note the cheaper grades of plastic weed barriers will crimp or twist on installation and it's a really good idea to lay them out in the hot sun for a few hours to soften them up before you try to bury them. Also use a lot of stakes with them to hold them in place - picking up some extra is a cheap investment compared to having to redo the job when frost heaves them out of the ground.
The only problem with the plastic edging is that it is easily damaged by power equipment. If you drop a mower wheel into a hole and the blades and edging have a contest to see who's tougher - the mower will win every time.
Edging also comes in aluminum as well as brick and stone. The more permanent a system you install, the more expensive it becomes but ... it's permanent.
Weed Mats or Landscape Fabric
This comes in different "weights" with the very heavy-duty used by landscape nurseries for weed prevention. This heavy-duty material is strong enough for foot and machinery traffic and will last for years even if left uncovered.
Note: All landscape fabrics will deteriorate in the sunlight if left uncovered. This is why they are always mulched in the home garden.
Professional grade landscape fabric will prevent most roots and grasses from penetrating. It is indeed a weed block.
Lighter grade home fabrics sold in many box stores (even that labelled "professional" grade) is far lighter but less expensive. It will not stop tree roots but it will prevent most grasses from penetrating. Aggressive grasses (bamboo) will indeed send roots and shoots through the lighter grades of landscape weed barrier.
Caution: Do not use weed mats or landscape fabric in perennial or annual flower beds where you want to dig and move plants around. It just doesn't work from a practical point of view. Use weed barriers of this kind in permanent shrub beds or under walkways to stop invading weeds.
No edging is going to stop weed seeds from flying in or being dropped by birds so it will not eliminate all weeds. Good weed barriers do however stop invading grasses from moving laterally into the garden.
Sources Resource for This Article
Lawn Edging options are here - a wide variety of styles, prices and suppliers.
You can use Landscape Fabric as weed barriers - multiple sources and prices here
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