Green Roof Weed Pressure
Weed pressure is a measure of the relative abundance of weeds, weed seed, and vigor with which weeds are spreading. Our stewardship team scores each living roof’s weed pressure on each visit. This page illustrates and describes levels of weed pressure to clarify terms and set expectations.
Green Roof Weed Pressure: Very Low to None
No weed pressure is a coveted goal for living roofs! We strive for very low weed pressure, and our EcoCline green roof system incorporates a weed suppression layer, which is effective at lowering germination rates for most common weed seeds. Very low weed pressure is characterized by the presence of a few weeds “here and there” on the green roof. Our crews and our clients love roofs in this category!
Very Low to No Weed Pressure: We count 4 dandelions (that have not bloomed or seeded yet) and 1 taller weed in the back. This is “very low” easily controlled weed pressure.
Very Low to No Weed Pressure: There might be a few small weeds in this green roof if we look hard enough. This may be “no” weed pressure.
Very Low to No Weed Pressure: One lone invader in a sea of healthy Sedum.
Very Low to No Weed Pressure: We see 3 mature horseweeds in this large green roof, a very low and manageable weed pressure.
Green Roof Weed Pressure: Low
Low weed pressure is characterized by the presence of limited quantities of weeds, or maybe easily removed areas of weeds that have not yet seeded.
Low Weed Pressure: We count 4 or 5 young weeds in this photo, which is fairly low weed pressure.
Low Weed Pressure: Weeds are distributed throughout the green roof, but at a low concentration that is easily controlled.
Low Weed Pressure: Here is an example of how weed pressure is averaged throughout the green roof. A clump of horseweed in the background is an area of moderate or high weed pressure. But the forground has very low weed pressure. We average this to low, but as you can see the horseweed is going to seed, so we need to prevent the spread of horseweed.
Low Weed Pressure: There are just enough weeds in this photo to constitute “low” versus “very low”. These are easily controlled.
Green Roof Weed Pressure: Moderate
Moderate weed pressure often represents a turning point: either the weeds are controlled and we reduce pressure to “low” or the weeds spread and pressure moves to “high”.
Moderate Weed Pressure: Horseweed and prickly lettuce quantities in this photo can be controlled to prevent weed pressure escalation.
Moderate Weed Pressure: Crabgrass is in patches, but is starting to spread more broadly, so it needs to be removed now.
Moderate Weed Pressure: Horseweed and weed grasses are still at relatively low densities that can be easily controlled.
Moderate Weed Pressure: Horseweed density is still relatively low, but it is going to seed, so action is required quickly.
Green Roof Weed Pressure: High
Living roofs with high weed pressure typically require multiple years in order to exhaust the accumulated weed seed bank. Some living roofs are exposed to continual deposits of weed seed from nearby areas, which may make weed pressure difficult to reduce. If extensive green roof weed pressure persists at high levels, this often indicates that the green roof does not incorporate some of the “stress” that naturally minimizes weed (e.g. thin-profile extensive green roofs may become hot enough and dry enough to “burn” weeds, but leave Sedums undamaged). Sometimes it is necessary to change the plant palette to something better suited to compete with aggressive weeds.
High Weed Pressure: Several colonies of weed grasses are present. This requires corrective action to prevent an infestation. At this point grasses have not crowded out desirable plants, so if treated early, this roof should only require minimal plant additions.
High Weed Pressure: This green roof is experiencing late-summer drough-induced stress, and the horseweed is taking full advantage of the situation. Fortunately most of these horseweeds have not seeded yet, so if removed now, the condition can be easily corrected.
High Weed Pressure: Here is another green roof experiencing late-summer drought-induced stress, and it is populated by a wide variety of grasses, prickly lettuce, and other weeds.
High Weed Pressure: This green roof is populated by horseweed that is starting to go to seed. Action is required now to prevent decline.
Green Roof Weed Pressure: Total Infestation
We reserve this weed pressure category for green roofs that are at the point of no return, and simply must be replanted. We don’t often encounter total weed infestations, but when we have, the reasons have usually been prolonged lack of maintenance, and on occasion the reasons have been improper plant selection.
Total Weed Infestation: Weed grasses overtook this green roof after lack of maintenance for several years.
Total Weed Infestation: (Same rooftop as above.) Weeds are simply “mown” as a stopgap measure until the green roof can be replanted.
Total Weed Infestation: This green roof was designed to use 8 inches of media, planted with Sedum. Sedums prefer thinner profiles, as they are “stress tolerant” plants versus “competitors”. This green roof is one story above grade, adjacent to mown grass areas, and has become completely overtaken by competitor grasses. This green roof requires restoration, but probably not using the original design.
Total Weed Infestation: In this case, our stewardship team inherited this project with an army of horseweed present, but with diligence we were able to reduce weed pressure to moderate without a full blown restoration.