Stevia

Stevia is a tender perennial that most North American gardeners grow as an annual, and the reason to grow it at all is the sweetness in its leaves — thirty times sweeter than sugar when dried, two hundred times sweeter when extracted. But that sweetness accumulates only in warm weather and long days. A stevia plant grown in a cool, overcast summer produces leaves that taste mildly herbal and faintly sweet, like someone waved a sugar packet near the pot. The plant needs real heat — consistent temperatures in the seventies and eighties — to build the steviol glycosides that make the leaves worth harvesting.

Start seeds indoors eight to ten weeks before your last frost date. Stevia seeds are tiny and germinate slowly, often taking two weeks or more. They need light to germinate, so press them onto the surface of damp soil but don't cover them. Once the seedlings have two sets of true leaves, transplant them into individual pots and grow them on under lights until the soil outdoors is reliably warm. Two weeks after your last frost is usually the earliest safe date; the plants can handle a light frost once established, but they sulk badly in cold soil and may not recover.

The critical thing most gardeners miss is timing the harvest. Stevia is photoperiod-sensitive — it starts to flower when the days begin to shorten in late summer — and once flowering starts, the leaves lose much of their sweetness. The plant redirects its energy to reproduction, and the steviol glycosides that made the leaves valuable drop sharply. Harvest all the leaves you intend to use for the season just before the flower buds open. If you're growing stevia primarily for leaf production, pinch out flower buds as soon as they appear to keep the plant in vegetative growth a few weeks longer.

Fresh stevia leaves taste different from dried ones. Fresh, they have a licorice-like aftertaste and a complex herbal note that many people find off-putting in tea or baked goods. Drying concentrates the sweetness and mellows the flavor. Hang the cut stems in a warm, dry place out of direct sun until the leaves are brittle, then strip them and store them in a jar. A single dried leaf is enough to sweeten a cup of tea.

Stevia tends to grow into a small, somewhat leggy shrub if left unpinched. Pinching the growing tips every few weeks encourages branching and produces a bushier plant with more harvestable leaves. The plant is a light feeder — too much nitrogen produces lush foliage that is less sweet — so avoid heavy fertilization. A moderate dressing of compost at planting is usually sufficient for the season.