The Myth:  Adding potassium or magnesium to your landscape
plants will increase their cold hardiness  The Facts  Several
articles have been written recently for gardeners who like to
"push the zone" envelope. With winter weather on the way, this
information is generally useful for protecting marginally cold-
hardy plants, The addition of wood ashes (for potassium) and
Epsom salts (for magnesium) was recommended to promote cold
hardiness of plants grown outside their normal range.  In
particular, potassium is mentioned numerous times as "promoting
cold hardiness, disease resistance, and general durability."
Some horticultural consultants state "what the potassium does is
strengthen the cell walls of the plant and will displace the
amount of water in the cells making it harder for them to burst
during freezing temperatures."  They also recommend that by
adding magnesium for increased cold hardiness is less common on
the web, and the rationale behind its use is not clear. Where
did these recommendations originate