By Susanne von Rosenberg, UC Master Gardener of Napa County
Citrus trees are a familiar sight in Napa County, and many of us grow them. In general, citrus is pretty easy to care for: it just needs consistent water and fertilizer and perhaps frost protection in the winter.
Citrus is also easy to prune. You only need to prune it for shape. However, if you remove the trailing skirts that some varieties develop, you will reduce snail and slug damage.
Citrus is believed to have originated in the foothills of the Himalayas. Citrus was brought to California by Spanish missionary Junipero Serra and first planted at Mission San Diego in 1769, but the first commercial grove of orange trees wasn't planted until 1841, in Los Angeles.
The first citrus trees in Northern California were planted in in 1856. One of the two original orange trees, called the “Mother Tree,” is still alive in Oroville and is the oldest citrus tree in California. It's at 400 Glen Drive, and it is a historical landmark. I'm definitely going to take a drive sometime soon to visit it.
California also has a Citrus State Historic Park. It is an open-air museum describing the citrus industry's role in the history and development of Southern California through the experiences of the migrant and immigrant groups who made it possible.
If you want to know even more about citrus, check out the UC Riverside citrus variety collection website. The Citrus Research Center and Agricultural Experiment Station that is part of UC Riverside maintains a collection of over 1,000 varieties of citrus.
Citrus botany is complicated because plants in this genus readily interbreed and produce fertile offspring. The ancestral varieties of today's common citrus are the pummelo (also spelled pomelo), mandarin and citron. All other varieties are hybrids of these three.
For example, botanists believe that a grapefruit may be a cross of a pummelo and a sweet orange, and that a sweet orange originated as a cross between a mandarin and a pummelo. More recently, plant breeders crossed grapefruits with mandarins to create tangelos.
Although we consider them citrus, kumquats are part of a related genus, Fortunella. They are in the same family as citrus, and they, too, can interbreed with citrus.
Citrus trees are also unusual in that they can produce two types of embryos in the same seed: one from sexual reproduction and one from asexual reproduction. Citrus fruits can develop seeds as long as the flowers are pollinated; the flowers do not actually have to be fertilized by the pollen. Embryos resulting from asexual reproduction are duplicates of the parent. Embryos resulting from sexual reproduction have different traits.
If you sprout a citrus seed and more than one seedling develops, one is from a sexual embryo and the others from asexual embryos. Usually the weakest one is from the sexual embryo.
If you're tempted to grow citrus from seed, remember that the fruiting part of the citrus tree (the scion) is almost always grafted onto rootstock. The rootstock provides desirable traits such as improved disease resistance, better fruit quality and cold hardiness. Also, commercially grown trees are grafted from mature wood and yield fruit in a few years. It can take 10 to 15 years before a citrus tree grown from seed is mature enough to produce fruit.
Citrus needs heat to ripen, and the lower the temperatures where it is growing, the longer it takes to ripen the fruit. It usually takes 8 to 16 months from flowering for citrus to reach eating quality. Some citrus varieties flower more than once a year.
That's why, in our relatively cool climate, our trees tend to bear lemons and limes year-round. The trees produce several rounds of flowers, and then it takes a long time for the fruit to ripen. Most other citrus trees flower only in the spring and then produce their crop sometime between the fall and the following summer.
We tend to think that citrus fruits are ripe when they color up. The color change happens because the rind is losing chlorophyll and gaining carotenoids (yellow, orange and red pigments). This transformation happens as the fruit matures, but the same process also occurs in the fall when temperatures decrease. The only way to really know if your citrus is sweet enough is to taste one.
The UC Master Gardeners of Napa County are volunteers who provide University of California research-based information on home gardening. To find out more about home gardening or upcoming programs, visit the Master Gardener website (napamg.ucanr.edu). Our office is temporarily closed but we are answering questions remotely and by email. Send your gardening questions to mastergardeners@countyofnapa.org or leave a phone message at 707-253-4143 and a Master Gardener will respond shortly.
Enjoy the nearly infinite variety of citrus from UC Riverside's comprehensive website:
By Susanne von Rosenberg, U. C. Master Gardener of Napa County
Regular readers of this column know that it is possible to grow vegetables year-round in Napa County. But did you know that with some strategic planting you can also have fresh fruit almost all year?
Even if you don't intend to have home-grown fresh fruit year-round, it's still a good idea to give some consideration to spreading out your harvest. So many types of fruit ripen from August through October that it's easy to end up with a glut in late summer and early fall. Why not spread out your harvest instead?
With a little planning you can harvest fruit over a six-month period, and with more careful planning and a bit of adventurousness you can have fresh fruit all year. Take inventory of what you're growing now, and then fill gaps in your harvest calendar by selecting the right varieties of plants. Now is a great time to consider what to add.
You can think of fruits as belonging to one of four primary categories: deciduous tree fruit, citrus, perennial vine and cane fruit, or annual fruit. Some gardeners rely mostly on tree fruit; however, annual fruits and perennial vine and cane fruits can extend your harvest. Growing fruit from these different categories will give you the greatest flexibility.
Fruits that store relatively well, like apples and citrus, can extend your fresh-eating season. When you research varieties, look for information on storage quality as well as harvest period.
Deciduous fruit trees can provide a crop from June through late November or early December. These trees include stone fruits (such as peaches, nectarines and plums) and pome fruits (apples, pears and quince). If you plant early, mid-season, and late varieties of some of these types of fruits, you can usually get fresh fruit from June through October.
Figs are also an option. Some types set an early crop (referred to as a breva crop) in June as well as a main crop later in the summer. Pomegranates and persimmons round out the deciduous fruit season. They ripen late in the year, typically late October through early December. Apples and some varieties of persimmons have proven to be my best bet for fruit that stores well.
The period from mid-December through January presents the greatest challenge for the year-round fruit gardener. You've used up your deciduous tree fruit and are still waiting for your citrus to ripen. But by late January or February, depending on where you live, mandarins, navel oranges and kumquats start to ripen. In most areas of Napa County, citrus can be harvested into March and possibly April.
Check the ripening period for the citrus variety you're considering, because some mandarins and sweet oranges ripen in the summer. All sweet citrus needs heat to develop its sweetness; and all citrus is frost sensitive, so make sure you understand the temperature tolerance of the variety you're planting. Kumquats and some mandarins are fairly hardy.
Once your citrus is harvested, there may be another short gap until May when you begin to enjoy strawberries from plants planted in late winter. Some of the newer varieties of raspberries also bear fruit starting in May. Loquat trees are another source of relatively early fruit; in our climate loquats ripen in May. Loquat trees flower in the winter and the flowers are frost sensitive. They may not be suitable for the colder areas of Napa County, but they grow all over the city of Napa.
What if you prefer not to plant fruit trees or don't have much space? Start with everbearing strawberries. They produce fruit from May through October, although the fruit quality declines as the weather cools. If you also plant raspberries, blackberries and boysenberries you'll also be able to harvest fruit from June through October.
Kiwis are good late-season option. They typically ripen in November and December. Kiwi vines are dioecious, meaning there are male and female vines. You need at least one male plant per 10 female plants. A mature kiwi vine can produce up to 50 pounds of kiwis per year, and be up to 30 feet long, so a trellis or other support is essential.
To extend your fruit harvest, it pays to be adventurous. Cape gooseberries (also known as golden berries) are a tender perennial in the tomato family that produces small yellow “berries” in husks. They look a bit like miniature tomatillos and have a sweet-sour taste. Cape gooseberries have approximately the same harvest period as tomatoes and store very well. I still have some on my kitchen counter that I harvested in October.
Or you could grow late-ripening melons, such as Valencia, that can be stored for up to four months after harvest without losing their sweetness and taste. Heirloom seed catalogs can steer you to these and other unusual fruits to try.
It's easy and fun to extend your fruit harvest. Simply start with one new plant. Or give your creativity full reign and plan a mix of different fruit-bearing plants to have a harvest all year.
Next workshop: “Citrus: Preserve It, Serve It” on Thursday, January 16, from 1 p.m. to 4 p.m., at the University of California Cooperative Extension office, 1710 Soscol Avenue, Napa. Presented by UC Master Food Preservers. For more details, contact Robin at 530-621-5538 or rkcleveland@ucanr.edu. REGISTER ONLINE with credit card or check.
The UC Master Gardeners of Napa County are volunteers who provide UC research-based information on home gardening and answer your questions. To find out more about upcoming programs or to ask a garden question, visit the Master Gardener website (http://napamg.ucanr.edu) or call (707) 253-4221 between 9 a.m. and noon on Mondays, Wednesdays or Fridays.
Citrus trees are evergreen, not deciduous, and never go fully dormant. As a result, they cannot withstand extremely low temperatures as well as deciduous trees, which are better prepared to handle cold weather.
I thought I had followed good preventive measures for my own trees by covering them with tarps and watering them deeply before the first freeze, but they still suffered significant damage. With near perfect hindsight, I realize that I should also have strung the trees with lights of at least 15watts to provide a little warmth.
After a brief mourning period, it was time to move on. The old English proverb that “necessity is the mother of invention” came to mind as I surveyed the ripe fruit on the trees.
A quick Internet search revealed that citrus juice can be canned and the process is easy. I already had canning jars and lids, a water-bath canner and a juicer so I was ready to go. I sterilized the jars and lids, squeezed dozens of lemons and limes and then heated the juice to 190°F for five minutes. I poured the hot juice into the sterilized jars, sealed the jars with lids and bands and processed them in the boiling- water bath for 15 minutes. Now I have enough canned lemon and lime juice to get me through the coming year. If you want to juice your frost-compromised citrus, do it sooner rather than later as the damaged fruit will dry out over time.
Today, when I look at my citrus trees, my first reaction is to do something to help them recover. However, the wise gardener waits until the extent of the damage becomes apparent. Prune too soon and you may remove parts of the tree that could recover if left alone.
Pruning too early may also encourage new growth that is susceptible to cold weather. It is also best to delay watering and fertilizing until spring. Resist watering until new growth appears, and avoid fertilizing until you know the full extent of the damage and have pruned the dead parts. The pruned tree, being smaller, will not need as much fertilizer as before. For now, just kick back and wait until spring or even summer.
Wait to prune until trees show signs of new growth. Identify the branches damaged by frost. Branches that aren’t generating new leaves need to be removed. In some cases, the bark will have a different color than the rest of the tree, or even begin to fall away.
Make all pruning cuts into living wood. Clean pruning tools in a mixture of 10 percent bleach and 90 percent water to prevent spreading disease between trees. If you are pruning more than one citrus tree, clean the tools in between.
Wait to water until the soil has dried out and there is new leaf growth. With fewer leaves to evaporate the water, damaged trees are less thirsty.
When new growth appears in spring, begin fertilizing. Several light feedings are better than one heavy application. Choose a fertilizer formulated specifically for citrus trees. These products are high in the nitrogen and chelated iron that citrus trees need.Apply a two-inch-thick layer of mulch around each tree, extending the mulch beyond the drip line but keeping it at least six inches away from the trunk.
Spring begins on March 20, so I will have to ignore my freeze-damaged trees for a few months before I can pick up my pruning shears and get rid of the unsightly leaves, twigs and branches. Fortunately, now is the time to start seeds for late-spring planting so I won’t be without something to do.
Workshop: Join Napa County Master Gardeners for a workshop on “Rose Pruning” on Saturday, January 18, from 10 a.m. to noon, at the University of California Cooperative Extension (address below). January is the best time to prune your roses. Come learn pruning techniques from a certified Rosarian. Bring your rose questions. Online registration (credit card only)
Mail-in registration (cash or check only).
Napa County Master Gardeners welcome the public to visit their demonstration garden at Connolly Ranch on Thursdays, from 11:00 a.m. until 1 p.m., except the last Thursday of the month. Connolly Ranch is at 3141 Browns Valley Road at Thompson Avenue in Napa. Enter on Thompson Avenue.
Master Gardeners are volunteers who help the University of California reach the gardening public with home gardening information. Napa County Master Gardeners ( http://ucanr.org/ucmgnapa/) are available to answer gardening questions in person or by phone, Monday, Wednesday and Friday, 9 a.m. to Noon, at the U. C. Cooperative Extension office, 1710 Soscol Avenue, Suite 4, Napa, 707-253-4143, or from outside City of Napa toll-free at 877-279-3065. Or e-mail your garden questions by following the guidelines on our web site. Click on Napa, then on Have Garden Questions? Find us on Facebook under UC Master Gardeners of Napa County.