Grapefruit

Mulch the root zone but leave a bare foot around the trunk

Spread organic mulch over the root zone out toward the canopy edge, then pull it back so at least a foot of bare soil rings the trunk. Keep that collar clear when you top the mulch up. Foot rot is a soil-borne fungus that needs a persistently damp trunk base to get established. Mulch piled against the bark holds exactly that moisture there, and the infection can seriously debilitate or outright kill the tree.

⚠ Pull mulch back from the trunk before a forecast freeze if you plan to mound soil over the bud union, and remember that bare moist soil radiates more heat to the tree overnight than mulched soil does.

Split the year's nitrogen: before bloom, then twice while fruit sizesFeb – Jun

Give a mature tree about 1 pound of actual nitrogen a year, roughly 5 pounds of ammonium sulfate, divided into three feedings: the first in January or February just before bloom, the second in May, the third in June. Young trees need far less and ramp up slowly, about a quarter pound of nitrogen in year two, half a pound in year three, three quarters in year four, reaching the full pound by year five. Broadcast it out to the canopy edge and never against the trunk. Citrus sets its whole crop on the spring growth flush, so the nitrogen has to already be in the soil when the tree blooms. Feeding past midsummer pushes leaves at the moment the tree should be pouring sugar into fruit instead.

⚠ Stop nitrogen after June. Late-season fertilization affects fruit quality, delays fruit coloring, and roughens the rind. Do not feed nitrogen in fall: trees should be hardened off going into winter with lower nitrogen, because succulent new growth is the most frost-sensitive tissue on the tree. Resume in late winter just before bloom.

Prune lightly in spring, and rub off rootstock sprouts

In late winter to early spring, as the tree wakes up and starts pushing growth, take out deadwood, crossing branches that rub, and every shoot rising from below the bud union. Rootstock sprouts give themselves away with trifoliate three-part leaves against the scion's single broad leaf, and while they are soft you can rub them off with a thumb instead of cutting. Keep canopy pruning to the minimum the tree actually needs. A mature grapefruit rarely needs canopy pruning to stay productive, and unnecessary cuts only cost you fruit. Rootstock sprouts are the real target: they bear inedible fruit, and left alone they outgrow and eventually replace the variety you paid for.

⚠ Do not prune in fall. Succulent new growth is the most frost-sensitive tissue on a citrus tree, and a fall cut pushes tender shoots straight into freeze season.

Check every new flush for Asian citrus psyllidMar – Oct

Every couple of weeks through the warm months, go over each round of new leaf growth. Adults are aphid-sized and mottled brown and feed in an unmistakable pose, head down with the tail tipped up in the air. Look for tiny yellow-orange almond-shaped eggs tucked into the folded new leaves, and nymphs trailing curly white waxy tubules. Watch the older leaves too for blotchy yellow mottling that is asymmetric across the midrib, green on one side and yellow on the other, which is huanglongbing. Control ants while you are at it, since they defend psyllids from the wasps and beetles that would otherwise eat them. The psyllid can only breed on tender new flush, so the flush is both its nursery and the window in which it picks up and injects the huanglongbing bacterium. There is no cure for huanglongbing and it kills a tree within about five years, which makes catching the insect the only real defense.

⚠ Report suspected huanglongbing to your county agricultural commissioner, and never move citrus plants, budwood, or clippings out of your area. If you treat, do not apply insecticide while the tree is in bloom.

Greasy spot: oil the leaf undersides once the summer flush has expanded

Soon after the major summer flush has pushed and expanded, spray the canopy with horticultural petroleum oil, working for real coverage on the undersides of the leaves. On grapefruit, add a copper fungicide, because grapefruit is among the susceptible varieties where copper is usually required in the spray to control greasy spot rind blotch on the fruit itself. The fungus reproduces by spores on infected and decaying leaves lying on the soil surface, and rain splashes those spores up into the canopy where they germinate and penetrate the stomata on the leaf undersides. The oil works by physically blocking that penetration, so it has to be on the leaf before infection, not after. You will not see the damage for months, then the tree drops its leaves in late winter and the next crop pays for it.

⚠ Do not apply petroleum oil when temperatures exceed 94 F. Copper controls greasy spot rind blotch on grapefruit but may spot the fruit, especially applied with petroleum oil or at high rates in hot, dry weather. Follow the product label. Rake up and remove fallen leaves under the tree, which is the only place the spores are produced.

Before a hard freeze: wet the ground, mound the bud union, cover the tree

Grapefruit is among the hardier citrus and may survive temperatures as low as 20 F without significant damage, but when a hard freeze is forecast, water the soil a day ahead and keep it bare rather than mulched. Mound soil up the trunk to above the bud union. Cover small trees with blankets, quilts, or burlap, and for extra warmth hang a string of old-style incandescent lights inside the canopy under the cover, keeping the bulbs clear of water. Pick any mature fruit first. Soil kept moist does not radiate as much heat away to the atmosphere overnight, and that effect is much stronger where the ground around the tree is bare. The soil mound is the insurance policy: soil almost never freezes, so even if the whole top is killed the tree can be regenerated from the trunk just above the bud union instead of reverting to rootstock.

⚠ Do not prune a frost-damaged tree until the spring growth flush shows the extent of the damage. Live wood is green just under the bark, dead wood is brown. Paint any bark newly exposed to sun with white latex paint or whitewash to prevent sunburn.

Taste one fruit at a time, and let the rest hang

Color is only a rough hint and can mislead you, so pick a single fruit and eat it, then wait and try another in a couple of weeks until the flavor is where you want it. Flavor is always the best indicator of ripeness. Grapefruit holds on the tree for months and gets better while it hangs, which is the whole appeal of the crop: a tree you harvest from all winter rather than all at once. Grapefruit sweetens on the tree as its acidity falls through the winter. Duncan, for example, is edible in November but sweetens and is best in February when the acidity is lower (Univ. of Arizona AZ1925). Flavor is the only honest maturity test.

⚠ Strip the tree ahead of a hard freeze: frozen citrus will not recover and will decay. Cold-damaged fruit goes water-soaked inside, then within one to two weeks the rind separates and the fruit turns soft and puffy, and finally pithy (Univ. of Arizona AZ1222); juice it quickly if it has already been frozen. Do not leave a ripe crop hanging indefinitely either, because held fruit delays future growth.