Experiments in Rock Gardening: Raised gravel planters
My 10' wide side yard is too narrow to accommodate the planters I envisioned if I used the standard 8"x8"x16" cement blocks. Instead, I chose slimmer 4"x8"x16" blocks. I used construction adhesive to bind two rows of blocks together, filled the voids with gravel, and topped the planter with red brick.
My planters were built over a gravel base topped with a double layer of weed cloth meant to thwart my neighbor's 20' evergreen hedge that abuts the property line. I set the north planters a foot away from the fence and the south planters the same distance away from the house, leaving me a four-foot path in-between. The planters are not anchored into the ground, so in a powerful earthquake (aka "The Big One") they'll probably dance around a bit.
Planter before filling
For several years, shallow sand beds framed by 2"x8" lumber and sited on the north side of my house, proved to be good platforms for staging pots of rare plants, seedlings, and temperamental primulas and alpines. The plants liked the low beds, but my back and knees told me something had to change. Inspired by fellow club members' sand beds and raised planters, I decided to build planters that would allow me to view and tend pots more comfortably.
Filling the planters started with several inches of sand recycled from my old sand bed frames to act as a reservoir of moisture. After tamping down the sand, I filled the planters the rest of the way with "sand screw sand" from a local supplier of aggregates and minerals. This is essentially pea gravel with coarser bits and finer bits. It worked well for staging pots where the roots can grow into the gravel. Three planters are filled with pots.
One planter was reserved for putting plants directly into the gravel. In this summer's heat, gravel alone proved too porous for the planted bed. Hindsight tells me I should have used more sand at the bottom for a deeper reservoir. So I top-dressed the planted bed with sand, watered it in well, then used larger gravel as a mulch. It was a great improvement.
When the blocks were delivered, I discovered that a forklift had gone through the bottom layer on the palette. The sales rep brought replacement blocks but declined to take back the broken ones. The broken blocks were put to use to add height and interest to the planted bed and gave me slightly different planting habitats. The first plants -- celmisias, iris, penstemons, lewisias, campanulas, even paeonias -- went in during May of 2014. Since then, I've added eriogonum, leontopodium, erigeron, carlinum, globularia, fritillaria, edrianthus, collimia, phlox, petrorhagia and onosma.
Trillium rivale roots
The remaining three planters are currently being used for staging pots, but I plan to turn one on the shady side of the walk into a planted bed filled with a loamy mix over a bottom layer of gravel. I want to get some of my shade-loving plants, that would be lost in the garden, out of pots. They might thrive with more root run and better drainage, and be less likely to rot in plastic pots that can trap too much moisture. This is a big problem with high alpine Asian primulas in the summer. In the winter, all of the beds are covered with polycarbonate sheets, more for protection from rain than from cold.
These planters were easy to build and definitely meet the requirement of being easy on the back and knees. As a bonus, they've provided me with new ways of planting, allowing me to experiment with alpines that I could not grow under previous conditions.
Iris ruthenica nana and Celmisia sp.
Lewisia cotyledon hybrid