Grass Overtaking my Rooftop Garden Plot

(Question)

I live in Vancouver and have access to a plot on our rooftop garden in my building. Last year was my first year (and I’m a very beginning gardener). I turned the existing soil over, added a bit more and then planted a variety of different things – both food and flower. Everything was great for a month or so and then the grass started. It overtook the garden very quickly and I spent most of last summer trying to methodically pull it up without ripping out everything I planted but to no avail – it was always ahead of me. It wasn’t a little bit of grass – it looked like a lawn with some plant and flowers poking up. Fast forward to now and I’m wondering what I have to do to avoid this this year. I should note that it gets a fair bit of sun although I’m the only plot with any shade because of the vine that is growing up the pergola.

I’ve started to dig down about six inches but that is going to pose some soil disposal challenges for this apartment dweller. Will it be enough to just till it over and over, take out the big clumps and hope for the best?? I should note that there are 12 plots on the roof and mine was the only one with a grass problem.

I’ve attached a picture from last summer to show you the extent of the problem. We had a lot of snow (for us) this winter so everything now has a post winter look you’ll be quite familiar with in Ontario. It’s actually quite a big plot so what is dug up so far this year (not the attached picture – that’s from last years clean up attempt) is just 4 heavy buckets of dirt and only about 1/10th to 1/12th of the whole garden. If you tell me I should dig the whole thing down 6 inches or whatever the right depth is then that’s fine but I’ll have to find a disposal strategy as were not allowed to fill the communal compost bins with dirt.  Any suggestions you have would be great.

(Answer)

I’m not sure where the grass came from — it would be interesting to find out!  As grass is usually quite dense and competitive for space, I don’t see any way for it to “get along” with other plants in the space. Your best bet would be to get rid of the grass, by “lasagna” gardening (also called sheet mulching, which sounds like much less fun!). This avoids lugging soil clumps downstairs and you will create great compost on-site.  The best time to do this is in early fall, to prepare the area for gardening the following spring — but you can go ahead now (it’s early spring).

Use a thick layer of newspapers to cover the area of grass you want to kill – mow or clip the area well so that the newspaper can lie flat/close to the nasty grass.  Water the area well and put 10 layers or so of newspaper sheeting over the entire spot. Keep the whole mass wet so nothing blows off your rooftop. Once the area is covered, continue with a layer of composted organic material. The grass will eventually die as no light will be able to reach it.

As it’s now spring and you want to take advantage of the upcoming growing season, after you complete the layering, add a thick layer of your preferred soil (e.g., triple mix, or topsoil)– and go ahead and plant your garden. (See also Gardening in raised beds, which discusses best soils to use).  You might want to talk to your neighbours who have the other rooftop plots — about the best soils to use, how to compost together to enrich everyone’s garden, etc.

There is lots of good information on lasagna gardening on-line (including via YouTube), but here are some helpful resources:

Finally, this season, remember not to “turn over” the existing garden soil, which may contain grass lying-in-wait to appear — and to spread throughout your plot.  However, once the grass has died, it will decompose and nourish the soil, and you can turn over the soil to your heart’s content.

In the future, you may want to consult the Master Gardeners Association of BC – they have an on-line site for gardening questions, Ask us here.