CITRUS HARDINESS — Chuck Ingels, UC Farm Advisor

December’s cold snap reminded us how sensitive citrus fruits are to the cold. This was more than just a frost problem that a simple cover could solve. Most locations had temperatures down to the low 20s for several hours, several days in a row, with occasional 19º readings. Most of the fruit was destroyed, and damage to many trees occurred, but most trees will re-grow this spring.

Below is a compilation of the cold hardiness of trees, fruit, and rootstocks. These are not hard and fast temperatures, since damage is dependent on the duration of exposure to cold; also, sources disagree on the exact temperatures and rankings.  Meyer lemon (Citrus meyeri) is not a true lemon (Citrus limon); it was discovered in China in 1908 by Frank Meyer.

Cold Hardiness of Citrus Varieties (Approx. temperature below which tree damage occurs) Mexican Lime 29° Meyer Lemon 22° Bearss Lime 28° Sweet Orange 21° Regular Lemon 26° Mandarin/Tangerine 20° Grapefruit 25° Kumquat 19° Cold Hardiness of Citrus Fruit Orange Lemon Green 29° Green 28-29° Ripe 27-28° Ripe 29-30° Cold Hardiness of Citrus Rootstocks Standard Semi-Dwarf Dwarf Rough Lemon -low Sour Orange -high Flying Dragon -high Sweet Orange -mod. Trifoliate Orange-highest