Winter- dormant fruits need chilly weather
With an El Niño winter approaching, it is useful to wonder how our plants will adapt. Apples, wheat and temperate plants are adapted to winter cold, so they have natural mechanisms for avoiding damage from temperatures below freezing and for flowering and fruiting during warm weather. As Texas A& M’s peach researchers David Byrne and Terry Bacon put it in a classic article ( http:// aggie- horticulture. tamu. edu/ stonefruit/ chillacc. html):
“Stone- fruit trees such as peaches develop their vegetative and fruiting buds in the summer, and as winter approaches, the already- developed buds go dormant in response to both shorter day lengths and cooler temperatures. This dormancy ( or sleeping stage) protects these buds from oncoming cold weather. Once buds have entered dormancy, they will be tolerant to temperaturesmuch below freezing and will not grow in response to mid- winter warm spells.
“These buds remain dormant until they have accumulated sufficient chilling units ( CU) of cold weather. When enough chilling accumulates, the buds are ready to grow in response to warm temperatures. As long as there have been enough CUs, the flower and leaf buds develop normally.”
If enough chilling units for the particular variety of fruit tree don’t happen, dormancy is not completely released, and the tree does poorly. Leaves will be late to come out, and may be sparse and on ends of branches; and there may be only a few small, poor- quality fruit. In extreme cases, the tree dies, but more often the gardener turns the useless tree into mulch.
There are many ways to calculate chill from winter temperatures. Generally, temperatures near 45 degrees Fahrenheit contribute, and the further away from this, the less effect the temperatures have. In cold places, orchardists use the Utah model and speak of “chill hours” ( see http:// hydrology1. nmsu. edu/ nmcrops/ Trees/ apples/ chilldly. htm), but this far south, winter temperatures often are too warm for this calculation to work and temperatures above 66 degrees F tend to reverse chill ( See http:// aggie- horticulture. tamu. edu/ stonefruit/ chillacc. html).
Southeast Texas is near the extreme southern edge for growing temperate climate fruit trees such as apples, pears, peaches, cherries, plums, chestnuts and kiwis. In very warm winters like the last one, such fruit trees did poorly all over the area unless they were “very low- chill varieties.”
But three years ago, we had one of the coldest winters in decades, so high- chill varieties did well, and very low- chill varieties were in danger of getting their new leaves and buds killed by late freezes.
So what should the fruit grower do when choosing fruit varieties to plant?
I have done a reviewof winter temperatures all over southeast Texas since 1991, and have been able to compile a table of winter- chill numbers for each National Weather Service site. See http:// yearroundgardening. me/ category/ fruit- trees- winterweather- in- southeasttexas/. Broadly, we have eight chill zones. Seemy accompanying chill recommendations for your area.
Bob Randall, Ph. D. is the retired co- founder of Urban Harvest and is author of “Year Round Vegetables, Fruits and Flowers forMetro- Houston.” Contact him at http:// yearroundgardening. me and BobInTheGarden@ urbanharvest. org. Urban Harvest produces this column. Learn about gardening classes, community and school gardens, farmers’ markets, fruit tree sales and more at www. urbanharvest. org.