The free pattern will be ready soon on the yarn company’s Web site (www.lionbrand.com).But for those who need a quicker fix, a knockoff poncho on eBay is going for $152.50.

You might call it the Martha Effect. In the 127 years since Ruben Blumenthal founded Lion Brand Yarn, the New York-based company has never quite experienced a frenzy like the one set off by Martha ...