Green Roof Maintenance - Weed control

Using this forum for the first time - trying to better understand the maintenance requirements for green roof systems - specifically weed control. We have several roof systems on campus however a few are failing due to weeds. Thoughts are welcome on practical maintenance practices for weed control beyond labor intense hand weeding.

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We have a few residential green roof installations that have gone through some highs and lows over the last five years.  I don’t have any magic bullet answers but, do have a few observations:

  • I don’t think that there is any way to avoid hand weeding entirely.  Like any garden, green roofs will always require some level of weeding.  The plant mix and environmental conditions may influence just how much weeding is necessary.
  • Green roofs benefit from regular attention.  While many of the succulents and seedums that we place on green roofs are hearty and don’t require the attention of other plants, this doesn’t translate into longer periods between maintenance visits — an aggressive weed only needs a a week to wreak havoc on any garden or green roof.
  • While I haven’t tried this yet, it’s been shared that the application of a weed pre-emergent, such as Preen, in the Spring can dramatically reduce the amount of hand weeding required later in the growing season.
The extreme weather that we have been experiencing — periods of extreme rainfall followed by periods of prolonged drought and, increasingly milder winters — seem to be creating more opportunities for undesirable weeds, plants and trees, making our jobs more difficult.

There is also a related discussion on this topic in the archives, see:

http://capitolgreenroofs.groupsite.com/beta/discussion/topics/575241/messages

Hi Mark,

In my experience, the best looking green roofs are the product of a simple preventative maintenance regimen performed routinely as follows:

1.  Frequency -  twice a month, walk the roof and pull weeds while they're young, before they've flowered and set seed.  (may need shorter intervals until the weeds are under control)

2. Fertility Management - by applying a controlled release fertilizer to the green roof in the spring (*typically at no more than 3lbs Nitrogen per 1000 square feet), you provide long term nutrition over the growing season.  This helps ensure denser coverage of plants that act as "living mulch."

3. Water Management - overwatering or underwatering can cause plants to thin and expose the soil to weed seeds.  Most succulent green roof plantings grow best with irrigation run once weekly during the summer months.  Check with your local supplier to confirm recommended watering routine.

Because it sounds like you may already be dealing with a substantial seed bank built up in the soil, quite a bit of labor may be needed to get the roofs back into manageable condition.  However, with a regular routine, the overall time and cost needed to manage weeds should be greatly minimized.

Best of luck in addressing your weed concerns!  Please keep the forum in the loop as to what you implemented and how it worked for you.

Amber

Around 6 years ago I wrote: "From an economic perspective applying nursery maintenance (and experience) on extensive green roofs will sustainable damage the growth of the extensive green roof industry and can have multiple environmental issues."

I guess my English wasn´t that advanced back then to express more clearly what I want to say.
 

Today I try to rephrase:

Growing only Sedums is considered a mono culture and as all mono cultures you will have increased maintenance with an aging green roof. If you decide to go with any pre-vegetated solution you have to continue the same very high maintenance as the plants experienced in the nursery. This might be necessary over a few years until the plants transitioned to the location. Especially succulents might take more time because their "metabolism" is slower than for example of grasses.

Applying artificial water to an extensive green roof will washout nutrients faster and reduce the water holding capacity of the system. Basically it is counter productive to the intention of extensive green roofs (reduce runoff and nutrient load). If you will I can talk about fertilizers also....

For any climate location on Earth a diverse extensive green roof can be designed that does not require supplemental irrigation - this assumes that you allow a controlled succession, plant diversity, and maintenance adjusted to existing conditions. There is no one-size-fits-all maintenance.

With a plant diversity (including unwanted plants - these are not weeds) you have the best indicator of what is going on right in front of your eyes. These and other indicators tell you what to do and when.

Listening to maintenance manuals might only work as a general guideline. Working with living elements requires three things: 1. experience 2. experience  3. experience

If a manual would help I would buy one about how to maintain people (and be politically correct here).

With experience comes the knowledge that a nursery sells plants for the point of sale (they don´t want to keep them, thus they need to grow fast) and the green roof owner wants to keep the plants as long as the building lasts - for me this is a tremendous conflict of interest. Potentially a good advise but certainly not shared experiences.

Jorg
 

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