Location: Choose a site that gets at least six hours of full sun per day and is protected from wind. In cool or foggy climates, locate the tree where it gets reflected heat from a sunny south-facing wall or driveway.
Drainage: Before planting citrus in the ground, make sure the soil is well drained. If your soil is compacted or of heavy clay and drains slowly, plant citrus in raised beds or containers. To improve water retention in sandy soils, dig in a 4- to 6- inch layer of compost to a depth of about 1 foot.
Mulch: Citrus tend to have shallow roots; adding a layer of mulch on top of the soil helps keep them cooler and reduces moisture loss from the soil. Apply a 2- to 3-inch layer of compost or other organic amendment under the tree's canopy, keeping it away from the trunk.
Fertilizer: Citrus grown on Flying Dragon rootstock are sensitive to highly alkaline soils (pH 7 and above). As long as the tree is fed regularly, such soils shouldn't be a problem. Use a fertilizer labeled for citrus; besides nitrogen, phosphorous, and potassium, it should contain the minor nutrients iron, manganese, and zinc. Follow package directions for application rate. If the foliage turns chlorotic (indicated by yellow leaves with green veins), spray it with a foliar food containing chelated iron and the minor nutrients listed above. Withhold fertilizer from fall through midwinter.
Watering: Drip irrigation is the most reliable way to keep citrus watered, particularly if it's growing in a container. Run the system often enough to keep the soil moist but not soggy. (For plants in the ground, water when the soil is starting to dry out 5 to 6 inches below the surface; in containers, when it's starting to dry 1 to 2 inches down.) Whether you water by drip or by hose, water long enough to moisten the entire root area. Consistent irrigation is key; fluctuating soil moisture can cause fruit splitting, especially of navel oranges.
Frost protection: When frost is predicted, make sure trees are well watered ahead of time. Wrap the tree with a string of Christmas lights, then cover it with a frost blanket or floating row cover (available from Harmony Farm Supply & Nursery, www.harmonyfarm.com or 707/823-9125).
Pruning: Prune in late spring or summer to shape trees. Remove suckers that form below the graft union (denoted by a scar on the lower trunk). In the desert, fruit and trunks are susceptible to sunburn; allow tree canopies to grow low and full to provide shade.
Harvest: Most citrus fruits ripen from late fall into winter; exceptions are 'Valencia' orange, which ripens into spring and summer, and lemons and limes, which can bear year-round. Allow fruits to ripen on the tree. To determine whether sweet fruits like oranges are ready to harvest, pluck, slice, and taste one first. Pick lemons and other tart fruits before they become puffy.
Pest control: Aphids, scale, or spider mites may infest a tree, especially if it's weak from poor growing conditions. To control them, spray thoroughly (including cracks and crevices) with horticultural oil or insecticidal soap. (Scale are most susceptible to pesticides at the crawler stage, in midspring and mid- to late summer.)