Orange

Put down the first and largest nitrogen application in late winter, just ahead of bloom, and split the year's total into two or three doses with the next around mid to late spring. Work up from about a tablespoon per tree three times in year one to a quarter pound of actual nitrogen in year two, a half pound in year three, three quarters in year four, and about one pound a year from year five on. Spread it across the root zone, not against the trunk. Bloom and fruit set are the year's biggest draw on the tree's nitrogen reserves, so the supply has to already be in the root zone when the flowers open rather than arriving after the fruit has already dropped.

Sweet and navel oranges take temperatures down to about 20F without significant damage, but plan protection below that. When frost is forecast, cover small trees with blankets, quilts, or burlap so the cover traps the heat the soil accumulated during the day. Water beforehand: moist soil radiates far more heat overnight than dry soil, and this works best where the ground under the tree is bare. A string of old-style incandescent lights in the canopy adds heat under a cover. If a hard freeze is coming, mound soil up over the bud union so the graft survives even if the top is killed and the tree can be regrown. Most cold events are radiation frosts on calm, clear nights, where the ground gives up its heat to the sky and the tree cools below air temperature. Covering and moist soil work by keeping that radiated heat under the canopy. Windborne arctic freezes are a different animal and protection often fails.

Citrus is one of the few fruit trees where mulch is not recommended up against the tree. If your orange sits in a mulched bed, pull the mulch back so there is at least a foot of clear ground on every side of the trunk. Bare, open soil at the base is also what lets the ground soak up heat by day and radiate it back on a frosty night. Mulch held against citrus bark keeps the trunk damp and invites foot rot, a Phytophthora infection at the soil line that girdles and kills the tree - a real risk that outweighs the moisture the mulch would save.

Do the year's structural pruning once freeze risk has passed but well before summer heat arrives. Take out dead, damaged, and crossing wood, thin the canopy until you can see dappled sunlight on the ground at midday, cut water sprouts (gourmands) off at the base, remove any shoot growing below the bud union, and skirt the lowest branches 18 to 24 inches off the ground. Keep winter touch-ups to twigs under a quarter inch. If the tree was frozen, wait until the spring flush shows exactly what is dead before cutting. Pruning early in spring pushes a tender growth flush straight into the last frosts, and cutting in summer exposes thin citrus bark to sunburn. A thinned canopy also dries faster and holds fewer scale and mealybugs, which thrive in still, humid interiors.

Do not let the tree run dry from the time the flowers open until the young fruit have sized past marble stage. Water deeply and regularly through this window, especially in any spell where daytime highs reach the 90s. Navel oranges, and early sweet oranges to a lesser degree, shed flowers and small fruit under heat stress at bloom and set. Heat alone will cost you some crop and nothing prevents that entirely, but water stress stacked on top of the heat is what turns normal drop into an unacceptable one - adequate irrigation is the only lever you actually control.

Apply the final nitrogen of the season in early summer and then stop - no nitrogen through the fruit ripening months. If your tree has granulated in past years, finish all fertilizing by about the start of July. Going into fall, if you feed at all, use a blend higher in phosphorus and potassium with reduced nitrogen. Late nitrogen crystallizes the juice sacs before harvest (granulation), roughens the rind, and delays ripening. It also pushes a soft growth flush that will not harden off, and succulent new leaves are the first tissue a frost kills - and the growth citrus leafminer lays into.

Expect silvery serpentine mines and curled, distorted new leaves on the summer and fall growth flushes. On an established backyard tree, do nothing about it. Manage instead by limiting the succulent flush the moth needs: avoid pruning live branches more than once a year, and do not put nitrogen down when leafminer numbers are high in summer and fall. Damage tends to be worst the first year or two after the pest arrives in an area and then eases off. The larva feeds inside the leaf, so most homeowner insecticides simply cannot reach it. Tiny parasitic wasps such as Cirrospilus and Pnigalio species find the larvae in the mines and do the work for free, and mature trees tolerate the leaf damage without meaningful loss - even young trees with heavy populations are unlikely to die.

Do not judge ripeness by rind color - taste a fruit. Oranges color up in response to cool temperatures, not ripeness, so in a warm fall the fruit can be greenish outside and perfectly sweet inside, and a late Valencia can regreen while the flesh stays sweet. Once the fruit is good, the tree is the best storage there is: pick as you eat over the weeks and months of that variety's window. Navels hold into roughly late winter, Valencia far longer. The sugar and acid balance inside the fruit is set by accumulated fall heat - the more heat, the faster the acids fall and the sugars rise - while the peel's green-to-orange change is a separate reaction triggered by cool temperature. The two are driven by different things, which is why the outside and the inside routinely disagree and why tasting is the only reliable test.