I’ve been thinking about this a lot lately as I work on the sidewalk boxes. When I first took them on, they mostly had weeds, or even no plants at all. A couple at the east end had a smattering of myrtle and other tough non-native ground covers; two at the west end had irises, day lilies and hosta. The soil conditions for all of them were awful: dry, compacted, filled with rocks and bits of trash and, for most of them, tree roots.
For the two that had been without trees, I was able to dig up the soil down to 8 – 10 inches below the sidewalk, remove lots of big rocks, and add in lots of good compost, but then there was the street traffic and intensely sunny and dry conditions to contend with. For the rest, I had to start with the soil conditions as is. So my usual approach of first, preparing the beds, adding in lots of good soil amendments, then planting small plants and nurturing them along, simply wasn’t feasible.
Instead, I’ve begun to think about succession planning/planting, like this:
First, plant vigorous non-native plants suitable to the light/shade conditions. This doesn’t mean any of the invasive thugs! Use plants that are really tough, that survive just about anywhere, regardless of soil condition, and surround them with spreading plants that will rapidly cover the ground, but will be fairly easy to remove later on. Irises, day lilies, geranium, and thyme have all thrived in the sunny boxes, and Ajuga has filled in some shady boxes that were earlier declared un-plantable. Because these were really inhospitable and public spaces, I started with plants that didn’t cost me any money, that I could get from transplanting or dividing other plants I already have. Add compost and water diligently. Try to have a little bit of something flowering all the time, and keep tending the boxes, so even if there isn’t much there, passersby are less inclined to walk all over them or leave behind dog poop or trash.
Second, once you see what’s surviving or not, supplement poorly faring plants with new additions, and start to do some re-arranging for aesthetics. Add in successful taller or shorter plants to build shapes. Take the most successful plants and create some repeating notes across all of the spaces. Start to add in hardy natives suitable for the space. I’ve now added some of the Stella d’Oro lilies to every box on the street side, and the magenta geranium on the sidewalk side. These are unexciting, quotidian plants, but they are making it in every sidewalk box, shady or sunny, to one degree or another, and their blooms tie together the whole stretch of sidewalk along our property. My first bits of European Wild Ginger barely survived, but after the Ajuga got going, I added some new clumps that have stayed good-looking. Boltonia and Liatris are made it in my first two sunny boxes, so I’ve added them into the rest. They really struggle when there is nothing around them to protect them and hold in some moisture, but when they’ve been surrounded by other plants, they’re beginning to get established.
Third, start to transition away from the non-natives to native plants, continuing with low cost/low risk plantings. I started with some natives in the first two boxes I worked on, but those turned out to be the boxes that were later dug up when they got new trees added by the City of Cambridge. I was able to pull out and re-use most of the plants I actually paid good money for, but it was a reminder that these are ultimately public spaces and I may be pouring money into the dirt with no return. So for now, I’m continuing with plants that reproduce freely, such as the Boltonia and Liatris, and plan to start adding in asters, Coreopsis, Heuchera and barren strawberry later this season.
I hope that in another year or two, these plantings will be well enough established that passersby will recognize and respect them (for the most part), and the soil conditions will be improved enough to support a broader range of plants. Ultimately, I envision central plantings of taller, native plants suited to the dry sunny or shady conditions in each box. I think I will continue with the edge plantings of lilies, irises, geranium, thyme and the like – cheap, plentiful, tough flowering plants that can tolerate getting stepped on, and offer some protection to the inside plants.