|One of three tomatillo plants in this year's garden. PHOTO/Amy W.|
Until I spoke with her, the possibility of needing more than one plant hadn't occurred to me, since tomatillos are tomato-family plants with perfect flowers, but it turns out that tomatillo flowers are self-infertile.
They need to be cross-pollinated, and best fruit-set will occur when that pollen comes from an entirely different individual plant.
Most of the University-produced information that I found did not mention this potential problem; they all just said to grow tomatillos like tomatoes. For all the small-garden people who grow just one tomato plant, and likewise decide to plant just one tomatillo, well, there will be some disappointed gardeners. The tomatoes will set fruit just fine, but there will be no source of fruit for the best salsa verde; green tomatoes will have to suffice (they work, but tomatillas work better).
Luckily, for those who are looking for more complete information about growing tomatillos, University of California's Sonoma County Master Gardeners Tomatillo page includes the essential bit about needing more than one plant for best fruit-set.