Caring for Citrus in the Low Desert
January
Cover frost sensitive plants; always remove covering during the day. Do not use plastic. Most citrus fruit will not freeze unless temperatures drop into the mid-20's or lower for at least a couple of hours. Lemons, limes and other thin-skinned fruit on the upper and outer edges of the tree may be injured at about 28 degrees. Lights can be used to warm the tree. Shine the light on the large limbs or trunk to warm the most mass. Remember the higher the wattage of the bulb, the more heat is produced.
February
Protect plants from frost damage if the temperature is projected to drop down into the 20's for more than an hour. (See January for details).
Starting in February thrips feed on the new flush of citrus leaves, green stems and small fruit, causing leaves to curl and some external scarring of fruit. Generally the damage has been done by the time the thrips are noticed. Thrips are slender yellowish insects about 1/16 inch long. Shake citrus blossoms over a piece of white paper and look for tiny yellow insects that resemble wood splinters.
Damage is largely cosmetic and does not affect the internal quality of the fruit so no control is necessary. Furthermore, insecticide treatments can kill natural pollinators, drastically reducing fruit yield.
Apply fertilizer to established citrus per recommendations. Refer to University of Arizona publication MC91 for information on the amount for your tree. Do not fertilize newly planted citrus for one year.
March
Prune frost sensitive citrus in late February or early March, after they begin to leaf out with new spring growth. Wait until all chance of frost has past but allow time for regrowth to shade exposed bark, protecting it from sun scald as the days get longer and hotter.
Plant Citrus Trees - Young two to five-year-old trees transplant most successfully. Larger, older trees are more costly, harder to transplant without injury (to yourself and the tree), and suffer more from transplant shock. It will generally be three years after transplant before fruit production and that is the same whether you plant a 2-year-old tree or a 10-year-old tree. Go small! Refer to planting guidelines contained in publication AZ1022 Planting Guidelines: Container Trees & Shrubs. Or to AZ 1001 Low Desert Citrus Varieties for tree choices.
April
You may begin to see citrus fruit drop. Water moderately during this season. Adjust your watering schedule frequency as temperatures warm. Refer to AZ 1151 Irrigating Citrus Trees.
Continue planting citrus. Protect bark from sunburn and mechanical injury with a sturdy wrap of cardboard or newspaper.
May
The shed of newly set fruit is a natural thinning worsened by hot weather and dry winds. Navel oranges drop more than other varieties. There is no cause for concern.
Fertilize citrus after natural fruit thinning. If you fertilize three times a year, apply 1/3 of the yearly total this month. (Use only one-half the amount for grapefruit.) For best results, broadcast the fertilizer over the entire root zone and water in well. Refer to MC91 Fertilizing Citrus.
Watch for giant swallowtail butterflies. Females lay single eggs on citrus leaves that develop into larvae disguised as bird droppings. Mature citrus can withstand the munching of a few leaves from these “orange dog” caterpillars. Enjoy the soon-to-develop magnificent adult butterfly stage.
June
June is the driest month. Increase water application as the weather warms. Watering deeply with each application is beneficial. Citrus benefit from deep (to three feet), infrequent irrigation over the entire root zone. For young citrus water every 5 – 7 days, for citrus 3 years and older every 14 days.
Apply mulch to the ground in the root zone around citrus to keep the roots cooler and prevent evaporation. Mulch can be organic such as wood chips or compost or inorganic like decomposed granite.
July
Hot winds may cause citrus fruit drop. This is a natural thinning process.
Do not expose citrus and other sun sensitive plants to sunburn by pruning during the summer.
Water citrus deeply, but infrequently allowing the soil to dry slightly between applications. Moisture should penetrate to a depth of three feet for best results. Overly wet soils that keep oxygen levels low can contribute to root rots and iron chlorosis. Refer to AZ 1151 Irrigating Citrus Trees.
August
Late summer application of nitrogen fertilizer probably helps fruit sizing. This is more significant for fall ripening (navels & tangerines) than spring ripening (Grapefruit and Valencia orange) varieties.
Build a dike around the trunk of citrus trees one foot out from the base of the tree to keep standing water from touching the trunk of the tree when you irrigate. This will help to prevent the spread of Phytophthora, a water borne fungal disease which causes gummosis on citrus.
September
Citrus fruit split - Inappropriate water management leads to sunburn, or tough rind that is then unable to expand as fruit enlarges in size and matures. There is nothing to be done for this season, monitor irrigation carefully next summer.
Peeling bark on branches is usually a sign of sunburn. Painting the branches with white latex-based paint may prevent further damage. Never prune citrus during the summer months as they are very vulnerable to sudden exposure to our intense sun.
If you fertilize three times per year, apply final 1/3 of the yearly total this month.
October
Cut back watering frequencies as plant needs decrease with shortening, cooling days.
As temperatures cool, the peels of citrus fruits begin to show color. This may not necessarily be a true indication of ripeness. It is usually necessary to taste a fruit to determine if it is truly ripe.
You may notice a hole in the side of a few of your ripening citrus, especially oranges and tangelos. This is caused by birds looking for a tasty sip of juice. They always seem to know which fruit is the sweetest. The hole they create attracts fruit flies and other insects. Usually only a few fruits are affected. Hanging reflective strips in your tree may help repel the birds.
November
Be prepared to protect citrus from early winter frosts.
Wrap the trunks of young citrus and other cold, tender trees with cloth, cardboard or several layers of newspaper (NOT plastic) to protect them from the winter freeze. Leave them wrapped until the threat of frost has passed in the Spring. Refer to AZ 1222 Protecting a Citrus Tree from Cold.
Chlorosis and leaf drop of citrus can occur. Short days, winter weather, drought or over watering may be responsible.
Winter watering schedules should be adjusted to about half of the summer frequency. For young citrus water every 14 - 21 days, for mature citrus every 21 - 30 days. The key is to water deeply each time.
December
Be prepared to protect citrus from winter frosts.
Cover frost sensitive plants; always remove covering during the day. Do not use plastic. Most citrus fruit will not freeze unless temperatures drop into the mid-20's or lower for at least a couple of hours. Lemons, limes and other thin-skinned fruit on the upper and outer edges of the tree may be injured at about 28 degrees. Lights can be used to warm the tree. Shine the light on the large limbs or trunk to warm the most mass. Remember the higher the wattage of the bulb, the more heat is produced. Refer to publication AZ1002 Frost Protection for details on frost protection.
Peel color is not a good indicator of maturity or taste in citrus. Give it the taste test. If it is not sweet enough for your liking, leave it on the tree. Up to a point, the longer the fruit is left on the tree the sweeter.
Small fruit size - This may be related to a heavy fruit set or weather. Lack of normal rainfall during the midsummer period can ave a direct influence on fruit size. Citrus fruit growth is always greater as a response to rain vs. a similar amount of irrigation water.
A complete list of citrus publications are available at Horticulture Publications for Maricopa County.