Do bottles sealed with plastic corks have to be stored horizontally in my cellar?
No. But you might have a hard time discerning which are sealed with synthetic versus natural cork.
Unlike screw caps, synthetic corks fit into the bottle neck and look like traditional cylindrical corks. (Some are coloured neon purple or green, but increasingly they're disguised in the beige hue of traditional cork.)
In contrast to the natural stuff, they need not be kept moist to stay resilient and maintain their seal. At least that's the conclusion of Nomacorc, the world's largest producer, which turns out 2.4 billion closures a year.
According to Jeff Slater, Nomacorc's director of global marketing, the company has measured the so-called oxygen-transmission rate, or OTR, of sample bottles over 10 years using a Mocon permeation-testing device. He says there is no difference between bottles stored horizontally or vertically.
Nomacorcs simply do not dry out. And I suspect that's the case for all other synthetic closures.