In part 2 of our maintenance article, we are going to focus on the best practices for keeping green roof vegetation healthy. As we mentioned in part 1 of this article, having a regular maintenance schedule and a to-do list will give you a great head start. Many green roofs fail during the establishment period due to lack of surveillance and upkeep. But what exactly do you need to do to make sure your green roof thrives? Weed control, fertilization (if needed), and quickly identifying pest problems or disease.
Weed seeds can be transferred to your green roof via many methods: birds, wind, growing medium, tracked in by human feet, and more. Even a with a well-covered green roof, weeds can still germinate and overgrow the beneficial plants that are trying to establish themselves. Luckily there are a few ways to keep them under control. The first, most effective method is manual weed control. This is especially true for small roofs or roofs with a lot of ornamentation. Just make sure the maintenance team can identify weeds from budding plants.
The second method is mulching. As long as the mulching material is weed free and it is applied with sufficient depth, mulch can prevent weed germination as well as smoother existing weeds. The last and least popular method is chemical control or using an herbicide. They tend to be species-specific, and many chemicals in these products have been known to degrade waterproofing membranes or drainage systems. It is also possible that the chemicals can leach out and pollute local waterways. Because of these factors, this method is controversial and seen as a last-ditch effort to remove seriously overgrown weed problems.
Over the course of the establishment period, the nutrient-rich part of your growing medium will start to biodegrade. The rate of decomposition and nutrient loss varies by climate and the species of vegetation. Typically the decision to use fertilizer is made during the design phase in order to provide specific nutrients to the plants that will be used. Many plants, such as sedums, are not as sensitive to nutrient availability, and may not need any fertilization. If fertilizers are not being used, continually check for signs of nutrient stress in the plants, especially in the first 2-3 years after the installation of the roof.
Identifying Pest and Disease
Weeds and lack of nutrients are relatively easy problems to identify and fix, and once resolved many plants will regain their strength quickly. But pest problems and disease can spread very quickly if not identified, and if the issues are not resolved soon after the entire roof system can be compromised. Many green roofs are less prone to these problems since they are a much more controlled environment, but they are not completely immune to them. Although we can't go over every possible scenario, we can help you look for tell-tale signs of a problem. Evidence of pests on your green roof can include: the pest itself, shed skins, droppings or frass (insect debris), webbing, honeydew, sooty mold, pitch, gummosis, galls (abnormal outgrowths of plant tissues), or slime trails. Evidence of disease can include mushrooms or fungal growths, galls, powdery substances (white, orange or black), leaf spots, water-soaked areas, cankers, and discolored stem or root tissue. If you see any of these on or around plants in your green roof, document it and have a professional take a look.
After the establishment period of 2-3 years, the amount of maintenance and observation will be reduced considerably. Once plants have been kept healthy they are much less likely to be affected. Keep these tips in mind and you are sure to have a lush and beautiful green roof!