Growing Citrus in Planters

Photo: John J. Kehoe

You don't have to reside in the Sunbelt to grow citrus. Dwarf varieties are well-suited to containers, allowing gardeners everywhere to enjoy the benefits of homegrown citrus trees, including glossy evergreen foliage, intoxicating floral fragrance and the ultimate payoff: plucking fresh fruit from your tree.

Citrus trees thrive with eight hours of sunlight daily. Outdoors, choose a southern location sheltered from drying wind. Indoors, if you don't have a sufficiently sunny south or southwest exposure, add full-spectrum grow lights, leaving them on for up to 12 hours per day.

Cold sensitivity

Like snowbirds fleeing northern climes in winter, citrus trees don't like cold weather and must be protected or moved indoors when the mercury drops. If you garden in a relatively mild climate, it may be possible to leave your tree outdoors most of the year, covering it with frost cloth (like the GardenQuilt Cover) or moving it to a protected location for short periods, as needed. Otherwise, bring the container inside for the winter.

Depending on the citrus variety, foliage and fruit suffer damage at about 32 degrees F (lime) to 20 degrees F (kumquat), with mandarin, orange, grapefruit and lemon falling in between that range. Some citrus varieties can tolerate temperatures in the teens for a couple of hours. Determine hardiness from the nursery plant tag or catalog description.

Choosing Containers

If you live in a mild climate, you can leave citrus trees outdoors year-round. Kumquats, such as this one, are some of the hardiest citrus, tolerating temperatures as low as 20 degrees F.

Non-porous materials, such as plastic or polypropylene, are good choices. They retain moisture and are lightweight, making it easier to move pots indoors and out. The porosity of unglazed ceramic, terra cotta and wood containers requires extra diligence in monitoring soil moisture to make sure your plant doesn't dry out, and they are heavier to move. Don't use black plastic nursery pots, which absorb and retain heat from the sun, creating triple-digit soil temperatures that "cook" roots.

Pots should have multiple drainage holes because good soil drainage and root aeration are key to successfully growing containerized citrus. If a pot has only one hole in the center, you may want to drill four to six more spaced evenly around the circumference.

If you plan to move your container indoors and outdoors with the seasons, consider a planter with built-in casters or set it on a plant caddy. Easy maneuverability helps you shift outdoor locations to take advantage of sun angles that change with the seasons.

Soil mixes

Use a good-quality, lightweight planting mix formulated for containers. Look for inorganic ingredients, such as perlite and vermiculite, that will provide long-term drainage and aeration. Larger-sized particles, such as pine or other wood chips and shavings, peat moss, coir, coconut husk chips and/or coarse sand also help promote drainage and aeration around the root system. Mixes that contain all organic matter or have uniformly fine particles, on the other hand, will decompose and compact quickly, reducing aeration, which is bad for roots. If necessary, amend such mixes by 1/3 to 1/2 with the above-mentioned materials. Avoid mixes that contain chemical wetting agents, which may encourage soil conditions to remain overly moist.

Planting

Locate the graft union. This is a slight bump or scar where the fruit variety was grafted to the rootstock, about 4" to 8" above the root ball. When transplanting, be sure the graft union remains above the soil line. Remove any young green shoots that arise from below this spot as soon as they appear. These suckers from the rootstock will expend the tree's resources without producing the fruit you expect from the variety.

Watering and fertilizing

Unlike roots that spread freely through the garden to absorb water and nutrients, containerized roots are at your mercy for food and drink. Citrus trees in containers are particular about water, which is also essential for fruit development. Provide consistent soil moisture, keeping it just a bit on the dry side. However, don't allow roots to dry out completely. Consistently wet soil is bad news because citrus are susceptible to root rot.

"How often do I water?" must be the most frequently asked question in gardening. "It depends" is the most accurate, although frustrating, answer. Water use depends on many interrelated factors, such as soil type, plant size, pot type, growing season, weather (sun, wind, rain) and if the pot is inside or outside. Be careful about accepting watering "rules" that may not be applicable to your conditions. Your best bet is to monitor soil moisture regularly. Follow a consistent schedule, watering as infrequently as possible, but allowing water to soak through the root zone and out the drainage holes each time. Make sure the pot sits above any salty drainage water, so it won't be reabsorbed and damage roots.

Test the soil moisture levels in the root zone. Use an inexpensive soil moisture meter or make a low-tech version from a wooden chopstick. Push it into the soil, pull it out, "feel" if it is dry or moist, or observe if moist soil sticks to it. Keep a simple journal of water use through the seasons to help you fine-tune and understand your citrus tree's requirements. If won't be long before you just know when it's time to water.

Feed containerized citrus regularly with a complete fertilizer, containing nitrogen, phosphorous and potassium (NPK), as well as other macronutrients and micronutrients, including iron, magnesium, manganese and zinc. Citrus fertilizers are an easy option as they contain the extra nitrogen and micronutrients the plants need to thrive. Complete all-purpose fertilizers can also do the trick if you supplement with an occasional foliar spray of a nitrogen-rich fertilizer that contains micronutrients.

Follow package instructions for amounts and frequencies. As a general guideline, apply fertilizer just as new growth starts in late winter/early spring and continue through the prime growing season into late summer/early fall. Citrus trees don't go completely dormant, so you may continue feeding after you move them indoors, although you may choose to dilute the dosage by half. Most gardeners who leave citrus pots outdoors all year stop feeding in late summer/early fall because tender new growth is susceptible to frost damage.

Moving citrus indoors and out

Over a period of two to three weeks, "harden off" citrus trees by gradually acclimating them to the different growing conditions between indoors and outdoors. This reduces shock, which appears as yellowing or dropping leaves and fruit, a fairly common reaction with citrus. In spring, move the tree outdoors into a sheltered location - such as a shady spot out of direct wind - for 7 to 10 days before moving it into filtered light, and then ever-increasing direct sun.

In fall, reverse the process. Before the final move inside, examine for insects and hose off dusty foliage. Increase indoor humidity levels around the trees with a humidifier or by elevating the pot on pebbles in a saucer of water, high enough so that water can't be absorbed into the soil. Keep an eye on the weather forecast for early or late frosts during these transitions.

The Potlifter

Citrus varieties, pollination and harvest

Bees are primary pollinators of outdoor citrus, producing delicious honey as a byproduct if you are lucky enough to be a beekeeper. However, the majority of citrus trees get the job done themselves, which is known as being self-fruitful or self-fertile. A handful of varieties, such as Minneola and Orlando tangelos, require another nearby tree to cross-pollinate and set fruit.

Nothing comes close to the heady fragrance of a citrus in full bloom. This honeybee loves it, too. Photo: Forest and Kim Starr

Depending on variety, most citrus trees bloom and set fruit in spring. Fruit "sizes up" through the summer and is ready to harvest in fall and winter. Some varieties have different schedules or may flower or bear fruit off and on through the year, such as lemons, limes and kumquats, which make fun-to-grow container plants. If you have space for multiple trees, extend your harvest season by growing varieties that mature in different months.

It usually takes two to three years, and as many as five or six, for most trees to bear a large consistent crop. In the first year or two after transplant, some growers recommend pinching off any fruits that set when they are still tiny. This allows the tree to expend its resources on root and branch development, ultimately creating a stronger, healthier tree that will produce more fruit for years to come.

Taste-test to determine if fruit is sweet because rind color does not indicate ripeness. Citrus may be harvested over several months, so the best storage method is to leave fruit on the tree until you are ready to use it.

Choosing and sourcing trees

Because of USDA regulations to control the spread of citrus pests and diseases that devastate orchards, online or catalog sources may not ship citrus trees to the major citrus-producing regions in the U.S, including Arizona, California, Florida, or Texas. If you live in these states, buy citrus trees that have already passed required inspections at your favorite local nursery or order from in-state growers.

Check out these sites for citrus variety characteristics, hardiness and harvest seasons:

• University of California Riverside Citrus Variety Collection
• Four Winds Growers
• McKenzie Farms
• Greenfield Citrus Nurseries

Troubleshooting

• Yellowing leaves may indicate overwatering or a nutrient problem. Review photos of citrus foliage with nutrient deficiencies at the University of California Agriculture and Natural Resources.
• The Citrus Problem Diagnosis Guide from Texas A&M University Dept. of Horticultural Sciences contains photos of foliage and fruit affected by citrus pests as well as cultural problems.
• Frost Protection from the University of Arizona Cooperative Extension explains how cold affects plants and describes methods to protect citrus.