She had her epiphany early - not long after graduating from the University of Victoria and going to work in her chosen avocation, anthropology.
"It was two to three months out in the field, and the rest of the time in the office analyzing data. I'm good at the in-the-field stuff. Being in an office, not as much."
She did a residential course of study in small-scale organic gardening and farming at Linnaea Farm on Cortes Island, and there she found her passion.
"For a lot of people, the environmental movement is full of scary, negative news. It's a lot of what we're doing wrong, and people feel inconsequential and powerless to do anything.
"Food is an easy way for people to become engaged in a meaningful way. It's full of positives: it's better for their health, the environment, and for their children."
As the project coordinator for the Edible Garden Project, and as chair of the FarmFolk CityFolk Society board, Johnstone gets to spread the message to urbanites all over the Lower Mainland.
The EGP oversees sharing gardens - in backyards, on boulevards, and in community gardens - which produce food to donate to people in need. It also works to support community gardens and hosts a series of workshops on all things related to growing food.
Perhaps its best-known venture is Loutet Farm, the Grand Boulevardarea field that lets residents get in on the farming, and in on the fruits and veggies that grow there. It's the first step in building a plant-to-plate local food system on the North Shore.
"As a society, we have no idea what it takes to sustain how we live. It's so easy to go into a grocery store and buy an apple that was flown here from New Zealand, or garlic from China. We don't realize the energy required," says Johnstone.
"It takes skill to produce good food, and not many people have those skills. It's been wonderful to see the community-garden community come into its own."
It's wonderful too, for her to be out in the field, getting her hands dirty.
For more information about the Edible Garden Project, visit ediblegardenproject.com.