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How to choose and grow citrus

Evergreen, Edible fruit, Trees

<strong>Heat requirements. Generally, sweet-fruited varieties need moderate to high heat to form sugars, sour types less heat. Lemons and limes require the least warmth and will produce usable fruit in cool-summer areas (as long as winter temperatures are not too low). Valencia orange has a lower heat requirement and greater frost tolerance; it is adapted to Florida and areas near the Southern California coast. Navel oranges demand even more warmth and are better suited to inland regions; their fruit development period is shorter than that of Valencia, so trees will produce palatable fruit between winter frosts if summer heat is high. Mandarins (tangerines) require high heat for top flavor and are best adapted to inland areas. Grapefruit develops full flavor only where trees receive prolonged high heat, as in the low desert. (In cooler areas, you’re better off growing grapefruit-pummelo hybrids ‘Oroblanco’ and ‘Melogold’, which produce sweet fruit in more moderate temperatures.)

Citrus plants of one type or another are grown outdoors all year round in regions with warm to hot summers and mild winters. Lemons, limes, and citrons are most sensitive to freezes. Sweet oranges, ‘Improved Meyer’ lemon, grapefruit, and most mandarins and their hybrids are intermediate in cold resistance. Kumquats, satsuma mandarins, sour oranges, and calamondin are the most cold resistant, with kumquats the hardiest of all, withstanding temperatures in the high teens.

Other factors affecting a tree’s cold tolerance include preconditioning to cold (it has greater endurance if exposed to cold slowly and if first freeze comes late), type of rootstock, and location in the garden. Prolonged exposure to freezing weather is more damaging than a brief plunge in temperature. All citrus fruit is damaged at several degrees below freezing, hence the importance of choosing early-ripening varieties in freeze-prone areas.

Growing citrus in Hawaii. Home gardeners in Hawaii can grow many of the varieties grown on the mainland, plus some specialty types adapted only to the islands. However, citrus grows differently in Hawaii, and the fruit often looks and tastes quite a bit different from the same variety grown in California, Arizona, or along the Gulf Coast. The trees tend to bloom almost year-round, so harvest is nearly continual. The lack of cool nights results in a rind that’s thinner and more greenish than the thicker, brightly colored skin of fruit grown in California. Lack of cool nights also reduces acid content, which is why Hawaiian citrus tastes sweeter. The fruit is very juicy as well.

Standard or dwarf. Practically all citrus plants sold have been budded or grafted on an understock. Grafted trees begin bearing fruit in just a few years, contrasted to 10 to 15 years for seedling trees. Standard trees (20–30 ft. tall and as wide) are grown on a variety of understocks. Dwarf trees are grown on understocks of trifoliate orange (Poncirus trifoliata) or ‘Flying Dragon’; the latter is a naturally dwarf, contorted, spiny form of trifoliate orange. Ordinary trifoliate orange understocks produce trees 8–10 ft. tall (some may reach 15–20 ft.). ‘Flying Dragon’ produces even smaller trees (5–7 ft. at 13 years). Check citrus trees periodically for suckers (branches that arise below the graft line) and remove them before they compete with (or overwhelm) the desired variety. Some tender citrus, such as limes, are grown from cuttings; if such trees are killed to the ground by severe cold, they will resprout from the roots and remain true to type.

Harvest periods. Most citrus varieties ripen their fruit from late fall into winter, but some varieties, such as Valencia orange, ripen into spring and summer. In addition, many types can hold their fruit on the tree for long periods without loss of quality. By growing both Washington navel and Valencia oranges, for example, you can have fresh fruit almost 10 months of the year. Everbearing citrus like lemons and limes can produce throughout the year, but they fruit most heavily in winter and spring.

Citrus fruit ripens only on the tree.
The best way to judge its ripeness is to pick a fruit and taste it; rind color is a poor indicator, since many varieties are fully colored before they are edible.

Fast drainage is essential. If soil drains slowly, don’t try to plant citrus in it regardless of how you condition it. Instead, plant above the soil level in raised beds or in a mound. Drainage in average soil (and water retention in sandy soil) will be improved by digging in a 4–6-in. layer of organic matter (such as garden compost or ground bark) to a depth of about 1 ft.

Citrus trees need moist soil, but never standing water. They also need air in the soil. Danger from overwatering is greatest in clay soil where air spaces are minute. In soil with proper drainage, water newly planted trees almost as frequently as trees in containers twice a week in normal summer weather, more frequently during hot spells. Water established trees every other week during summer. In clay soils, space watering intervals so the top 4–6 in. of soil dries between irrigations. Don’t let tree reach the wilting point. Be sure to water consistently. Fluctuating soil moisture can aggravate fruit splitting, a problem that can affect all citrus, but especially navel oranges (typically in autumn). If you build basins, make them wider than the spread of branches. Citrus roots extend out twice as far as the distance from the trunk to branch ends. Keep trunk dry by starting basin 6 in. or more from trunk. When you water, be sure to wet the entire root zone (that is, soak the soil to a depth of 3–4 ft.).

Mulching. Since citrus roots grow near the surface as well as deeper, a mulch over the soil is beneficial. Use a 2–3-in. layer of compost or other organic matter to help maintain soil moisture. In mild-summer areas, large pebbles or gravel will increase reflected heat and hasten ripening. Don’t allow grass to grow near the trunks of citrus trees. Instead, maintain a grass-free, mulched area under the canopy.

Fertilizing. Nitrogen is the main nutrient that must be supplied in all regions. If you garden in sandy soil, choose a complete fertilizer containing a full range of nutrients. Apply 2 oz. of actual nitrogen the first year after a newly planted tree puts on new growth; then increase the amount by 4 oz. each year for the next few years. After the fifth year, apply 1–1 1/2 lbs. yearly. (Depending on tree size, give plants growing in raised beds or with restricted root space as well as trees grafted onto ‘Flying Dragon’ rootstock about a third to half the recommended amount after the fifth year.) To determine weight of actual nitrogen, multiply percentage of total nitrogen (as stated on fertilizer label) by total weight of fertilizer.

Divide total fertilizer into several feedings throughout the growing season. In freeze-prone areas, start feeding in late winter and stop in late summer. Make sure trees are well watered before feeding. Spread the fertilizer beneath and a foot or two outside the branch spread of the tree; then water it in deeply.

Citrus trees that receive too much or too little nitrogen show the evidence in leaf color. Dark green, lush leaves with burned tips or edges indicate too much nitrogen; yellowish leaves are a sign of nitrogen deficiency.

Citrus may suffer from chlorosis due to iron, manganese, or zinc deficiency. In iron chlorosis, leaves turn yellow from edges inward; veins remain dark green. (The same symptom may be caused by overwatering, so check your irrigation practices.) Manganese deficiency shows up as fine mottling, usually on young leaves, and as pale or yellowish areas between dark green veins. Signs of zinc deficiency are yellowish blotching or mottling between leaf veins. Manganese and zinc deficiencies may occur together and be difficult to distinguish from each other. Commercial products containing chelates of all three nutrients are available as foliar sprays.

Pests and diseases.
Citrus can get aphids, mites, scale insects, and mealybugs. If these pests’ natural enemies fail to handle the infestations, and if jets of water fail to keep the pests in check, spray with appropriate chemicals. If scale remains troublesome, spray with horticultural oil in early spring. Control snails and slugs whenever necessary, especially during warm-night spells in winter and spring.

Copper bands, available in some areas, will keep snails out of trees. Where it is legal to do so (in Southern California), colonize citrus groves with decollate snails, which prey on the garden snail.

The citrus bud mite causes weirdly deformed fruit (especially lemons). Control with horticultural oil spray in spring and in fall; spray only in fall in hot-summer areas. Reduce harmful insect populations by keeping ants out of trees with sticky bands on trunks. (Ants prey on natural insect predators of the mites.)

The few fungal ailments of citrus occur in poorly drained soil. Water molds, causing root rot, show up in yellowing and dropping foliage. Best control is to correct your watering schedule.

Brown rot gummosis usually occurs in older trees at base of trunk. Keep base of trunk dry; trim and clean the oozing wounds, removing decayed bark to a point where discolored wood does not show. Paint areas with Bordeaux paste mixture.

The bark of newly planted citrus trees is subject to sunburn in hot-sun areas. Trunks should be wrapped (paper trunk bands are available commercially). When heavy pruning exposes trunks or limbs, protect bark with whitewash or latex paint diluted by half with water.

Commercial trees are allowed to carry branches right to the ground. Production is heaviest on lower branches. Growers prune only to remove twiggy growth and weak branches or, in young plants, to nip back wild growth and balance the plant. You can prune garden trees to shape as desired; espaliering is traditional, though espaliered citrus is not very productive. Lemons and sour oranges are often planted close and pruned as hedges. ‘Lisbon’ and ‘Eureka’ lemons should be pruned to keep the trees within bounds and the fruit easily reachable. Many citrus are thorny, so wear gloves and a long-sleeved shirt when picking fruit or pruning. In freeze-prone areas, don’t prune in fall or winter. Wait until late spring or summer to prune frost-damaged trees new growth will make it clear which wood is dead. If you’re growing a multiple-variety citrus tree, you must continually cut back the vigorous growers (lemon, lime, pummelo, grapefruit) so the weaker ones (sweet orange, mandarin) can survive. On all citrus, remove fruit from newly planted trees so that the trees’ energy will be channeled into new growth rather than fruiting.

Citrus in containers.
In general, containers should have a diameter of at least 1 1/2 ft., though calamondin and Chinotto sour orange can stay in 8–10-in. pots for years. Plant in a light, well-drained soil mix. Daily watering may be necessary in hot weather. Fertilize monthly from midwinter to mid-autumn with high-nitrogen liquid fertilizer containing chelated zinc, iron, and manganese.

Potted citrus can stay outdoors all year in mild-winter climates, but plants should be moved to a protected location if a freeze is predicted. In cold-winter regions, shelter plants in winter; a cool greenhouse is best, but a basement area or garage with good bright light is satisfactory. Most container-grown citrus will need to be root-pruned and repotted in fresh soil every 4 or 5 years to remain healthy.

Citrus as houseplants. There s no guarantee of flowering or fruiting indoors, though plants are still appealing. ‘Improved Meyer’ and ‘Ponderosa’ lemons, ‘Bearss’ lime, kumquats, calamondin, and ‘Rangpur’ sour-acid mandarin are most likely to produce good fruit. Locate no farther than 6 ft. from a sunny window, away from radiators or other heat sources. Ideal humidity level is 50 percent. Increase moisture by misting tree; also ring tree with pebble-filled trays of water. Water sparingly in winter.

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