Have you ever cut into a beautiful lemon, expecting a tart spray as the knife slices through the fruit, only to discover dried, hardened fibers and no juice? Or watched your green navel oranges start to size up, only to spontaneously split open, ruining your long-awaited harvest?

Many citrus varieties do well in Napa County, but problems do crop up.

Napa is blessed with a variety of microclimates, including “banana belts” where it rarely freezes. These warmer zones can support citrus, avocados and other semitropical fruit trees easily. Napa areas that collect cold are better suited for apricots and other chill-loving fruit trees. But if you have a site that gets sun most of the day, has well-drained soil, and is protected from strong wind and the harshest weather, you should be able to grow the hardier citrus.

In general, mandarin oranges are the most cold tolerant, followed by sweet oranges and grapefruit. Lemons are frost-sensitive, with ‘Eureka’ decidedly more sensitive than ‘Lisbon’. Among popular citrus, limes are the least cold hardy. Healthy, irrigated trees tolerate more cold than stressed trees. The rootstock also affects sensitivity or hardiness.

Citrus fruit that is disappointingly dry and tasteless is usually the result of frost damage. Prolonged and excessive cold freezes the interior, bursting the cells, which then lose the very juice that makes them luscious.

To protect citrus trees from frost, water regularly. A well-hydrated plant is less susceptible to damage from cold. Draping a fabric row cover (available at local nurseries) over the tree can help, as does wrapping the trunk in material that insulates it.

Some people use small tree lights strung through the branches to provide extra warmth. Make sure any lights you use are approved for outdoor use. Another option, especially for those who live in the coldest regions, is to grow citrus in pots and bring them inside for the winter.

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In times past, Europeans often built buildings specifically to house their citrus, bringing them in before the harsh weather commenced. When the danger of frost had passed, they would throw open the doors of the orangeries and move the pots of lemons, oranges, limes and kumquats back outside to enjoy the sunshine.

Mary Helen Seeger is co-owner of Four Winds Growers, a wholesale citrus nursery in Fremont. In her catalog, she writes, “Potential cold damage is a combination of temperature (how cold) and time (for how long). Brief dips to the mid 20s will not damage most citrus. Prolonged temperatures in the teens caused much damage in 1990 and 1998. Limes and lemons are the most sensitive, needing some winter protection in colder locales. Other citrus are fairly hardy once established. Most citrus fruit will be damaged at about 26 degrees F; lemons and limes at 30 degrees F. Don't pick them unless you have to, because the fruit won't get any sweeter once it is off the tree. Lemons and limes can be juiced and frozen in ice cube trays. The fruit on the outside branches of the tree will be most vulnerable. Tender new growth will be killed, but this isn't very harmful to the tree.”

From September to early December, gardeners may notice some of their navel oranges splitting. While this problem can affect other citrus, navels seem particularly susceptible. The split may be short and shallow or deep and wide, exposing the segments and the juice vesicles, usually starting at the navel end of the fruit.

Researchers at UC Davis say the cause is probably distress from fluctuation in growing conditions, such as temperature, moisture and humidity. Splits often occur when warm weather is followed by rain, and the rind is unable to expand quickly enough to incorporate the added water transported by the roots. Also, rinds that have been drought stressed or sunburned may not be as elastic as well-hydrated fruit and thus more apt to split.

Consistent watering through the dry season can help mitigate the splitting. When hot winds are expected, irrigate lightly a few days ahead. 

You can still eat split oranges, if you can find one ripe enough. But the split makes them vulnerable and they break down and spoil quickly. Pick them off the tree or gather them from the ground as they fall.  Discard them in your yard-waste bin as they are the perfect hosts for fungal diseases and can lure unwanted pests.

Napa County Master Gardeners (cenapa.ucdavis.edu) answer gardening questions Monday, Wednesday and Friday, 9 a.m. to noon, at the UC Cooperative Extension office, 1710 Soscol Ave., Suite 4, Napa, (707) 253-4221, or (877) 279-3065.

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