I confess that I have never mastered the art of keeping a year-round supply of basil going in my garden. I have, however, been successful with a number of strategies that, if implemented together, might just work. So give this a try.
First, think of basil plants as a kind of long-lasting grocery store item rather than something you buy from the garden center once a year. In other words, plan on picking up basil plants whenever you see them throughout the year to replace the one that might be on its way out.
Second, choose the right variety. The giant, fragrant ‘Genovese’ is wonderful, but it won’t last as long. ‘Finissimo Verde’ produces smaller leaves, but behaves more like a perennial and might even make it through the winter indoors. ‘Pesto Perpetuo’ is also long-lasting. Garden centers will probably start selling the new ‘Bonsai’ grafted basil this year, which, as the name suggests, is grafted onto sturdier rootstock and trained into a bonsai shape. And there’s always the awesome, and surprisingly cold-tolerant, purple Thai basil.
Third, try to find a place to grow them indoors. A sunny, south-facing window is ideal. If you’re really committed to this, you’ll get a little heated seed mat (available at garden centers and hydro shops for about twenty bucks) and put that under your plant. You can also pull a potted basil from its pot, rinse the roots off, and stick it in a glass of water. It won’t live forever, but keep a few going and you’ll be surprised at how well they do.
And finally, if you’re dying to plant some basil outside but it’s not quite warm enough, try this trick, which I learned from Niki Jabbour’s new book The Year-Round Vegetable Gardener: put an upturned wine glass or punchbowl over the plant, cloche-style. It’s a cocktail-ish way to grow cocktail herbs.