Answers to frequently asked questions about Citrus

Ben Faber
Farm Advisor
U.C. Cooperative Extension
Ventura County

Recognizing freeze-damaged citrus

Citrus leaves appear wilted or flaccid during periods of low temperature. This is a natural protective response to freezing temperatures and does not mean the leaves have been frozen. Leaves will be firm and brittle and often curled when frozen. Leaves become flaccid after thawing, and if the injury is not too great, they gradually regain turgor and recover, leaving however, dark flecks on the leaves. Seriously frozen leaves collapse, dry out, and remain on the tree. Foliage form recent flushes are most susceptible to this damage. If twigs or wood have been seriously damaged, the frozen leaves may remain on the tree for several weeks. If the twigs and wood have not been damaged severely, the leaves are rapidly shed. Trees losing their leaves rapidly is often a good sign and is not, as many growers believe a sign of extensive damage.

Cold damage to the twigs appears as water soaking or discoloration. In older branches and trunks it appears as splitting or loosening of bark where the cambium has been killed. Bark may curl and dry with many small cracks. Dead patches of bark may occur in various locations on limbs and trunk. Sensitivity to frost is dependent upon many variables. In general, mandarins are the most cold hardy followed by sweet orange and grapefruit. Lemons are very frost sensitive with Eureka decidedly more sensitive than Lisbon. Limes are the least cold hardy. Healthy trees are more tolerant than stressed ones. The rootstock also imparts sensitivity onto the scion. Injury to the foliage and to young trees may be immediately recognizable but the true extent of the damage to larger branches, trunks, and rootstocks may not appear for on to four months following the freeze. No attempt should be made to prune or even assess damage from the frost until spring when new growth appears.

Rehabilitation of freeze-damaged citrus

Whitewashing

The only treatment that should be done rapidly after a freeze is whitewashing. Often the most severe damage following a freeze results from sunburn of exposed twigs and branches after defoliation. Temperatures do not have to be extremely high to cause sunburn. A white latex paint that has been diluted with water so that it can be sprayed is the easiest way to whitewash. The whitewash needs to be white on the tree, so don’t add too much water.

Pruning

Pruning should be carried out to prevent secondary pathogens and wood decay organisms from slowing tree recovery. Again, however, there should be no rush to prune. Premature pruning, at the very least, may have to be repeated and, at the worst, it can slow tree rehabilitation. It should be remembered that when pruning, all cuts should be made into living wood. Try to cut flush with existing branches at crotches. Do not leave branch stubs or uneven surfaces. Tools should be disinfected in bleach or other fungicide before moving on to the next tree.

Irrigation

Irrigate carefully! Remember that when leaves are lost, obviously evaporation from leaves is greatly reduced, and, therefore the amount of water required is also greatly reduced. A frost-damaged tree will use the same amount of water as a much younger or smaller tree. Over irrigation will not result in rapid recovery. Instead, it may induce root damage and encourage growth of root rotting organisms.  Irrigation should be less frequent, and smaller amounts of water should be applied until trees have regained their normal foliage development.

Fertilization

Fertilization of freeze-damaged trees should be carefully considered. There is no evidence to indicate that frozen trees respond to any special fertilizer that is supposed to stimulate growth. If trees are severely injured-with large limbs or even parts of the trunk killed-nitrogen fertilizer applications should be greatly reduced, until the structure and balance of the tree become re-established.

Trees should be watched for evidence of deficiencies of minor elements. Deficiencies of zinc, manganese, copper, and iron are most likely to develop. For citrus, these materials should be applied as sprays, and they should be used as often as symptoms are observed. Two or more applications may be required the first year.

Heat and its importance in citrus

Heat affects different types and varieties of citrus differently. Heat determines when fruit ripens and how sweet it will be. Grapefruit has one of the highest heat requirements of all citrus. Grown along the coast it will be sour, but in the Central Valley it can be decidedly sweet. A Pixie mandarin along the coast will be 6-8 weeks later in ripening than the Valley and will hang on the tree much longer. Acid fruits like lemon and Bearss lime have low heat requirements and are well adapted to the coast. The everblooming characteristics of lemons and limes are accentuated along the coast where there may be continuous cropping with lemon blooms year round.

High temperatures can have a negative effect on citrus. Coastal citrus may suddenly drop fruit when temperatures swing from the cool 60’s to the 90’s as often happens with Santa Ana conditions. Sudden warm weather can cause fruit to split, induce flower and leaf drop, and cause sudden burn to both the fruit and tree. These problems are compounded by dry soil moisture and problems can be reduced if there is adequate moisture present during the heat wave. In hot environments, some citrus like navels produce less fruit.

You callem tangerines, but they are mandarins

Seediness and citrus

Seediness in citrus is often unpredictable. Officially, a fruit can have up to 5 seeds and still be considered seedless. Some varieties such as ‘Washington’ navel, ‘Pixie’ mandarin and ‘Armstrong’ satsuma are  consistently seedless. Some varieties, such as the Valencias are consistanly seedy. Others,such as ‘Clementin’ and ‘W. Murcott’ mandarin only produce seed if a pollinizer nearby. The fruit number and size may be reduced without seeds, though. There is no precise list of compatible pollinizers and varieties may perform differently depending on the region and the weather that year. In the spring time the trees are alive with bee pollinators (notice the difference, the tree is a pollinizer and the insect is the pollinator).

What size plant to buy

The longer the plant has been in the container, the longer it takes the plant to adjust to the bull soil after it has been planted. The smaller the plant is that goes in the ground that can survive, the more rapid the growth. A 5 gallon container grown lemon will have outgrown the 15 gallon container in three years. This has been shown consistently with all manner of container grown plant……..and they are cheaper.

Pruning

Leaving a citrus unpruned is not as critical as it is for deciduous trees. They form a blob with leaves extending to the ground. Pruning however, improves air circulation (reduces fruit disease), increases fruit size, reduces alternate bearing (especially in mandarins), reduces limb breakage and controls trees size. Light pruning to open up the centers in late winter at flowering is the best time. It helps even out flowering, allows for regrowth during the summer, avoids spread of disease to cuts during the rainy season and reduces the likelihood of sunburn which can be a problem when done in the summer time. Late fall/winter pruning stimulates growth that can easily freeze.

Severe pruning can rejuvenate an overgrown tree, but expect yield reduction. Also expect to whitewash the tree (dilute latex paint), to reduce sun burn.

Suckering

All citrus is sold as grafted trees. The tree is a combination of a rootstock (used because it consistently propagates well for the nursery) and the scion (a known variety that consistently reproduces the same fruit). Early on and even later the rootstock growth (suckers) may be more vigorous than the scion and out grow it. Rootstock growth is often more thorny than the scion. Know where the graft union in on your tree. It can usually be seen as a diagonal scar between 6 and 12 inches from the soil. Remove all shoot growth below the graft. Remove suckers as soon as they are observed.

Rootstocks

There are many different rootstocks available to growers. A certain rootstock will be chosen because it has greater nematode resistance, salt resistance, disease resistance etc. The retail nursery typically sells whatever rootstock the wholesale nursery propagates. Wholesale nurseries do not all use the same rootstocks, but use those that they feel grow best for them. In some cases, a retail nursery may be able to special order a rootstock for a special situation. You can always ask.

There is one choice that the buyer can make, though, whether it is dwarfing or not. The ‘Flying Dragon’ rootstock creates small tree, under 6 feet and it is very slow growing. It especially lends itself to container culture.

Container grown citrus

Citrus grows well in containers, especially if you choose varieties like ‘Meyer’ lemon which is a less aggressive tree or use ‘Flying Dragon’ dwarfing rootstock on one of the other citrus varieties. There is a long history of orangeries in Europe, where full sized trees were grown outside in containers in the warm weather and then moved into large greenhouses when it got cold. Half barrels and terra cotta pots can be used, but if a large container is used and you want to be able move it, put the container on some wheels first. Fill the container with a good quality potting mix and plant your tree. Containers dry out much faster the soil grown trees, so stay on top of the irrigation. When irrigating, make sure water comes out of the bottom of the pot to avoid salt accumulation in the root zone. Prune as necessary to keep the canopy in balance with the pot or pot up to the next size.

Asian citrus psyllid and greening disease

Currently in much of the southeast there is a pest-disease complex. A small insect about the size of an aphid can carry a bacteria that causes fruit to be distorted and bitter, causes a mottled color of the leaves and eventually kills the tree in five to eight years. This disease is in Louisiana, Florida, the Caribbean, Mexico and Brazil. Massive amounts of energy and pesticides are being used to keep in under control. The psyllid is now in California along the California border, but the psyllid at this point is not carrying the bacterial pathogen. The insect is being monitored and tested for the bacteria at this time and it is hoped that control practices in that area and in Mexico will prevent the introduction of psyllids carrying the bacteria.

Growing citrus in a lawn

Trees don’t belong in lawns. In California we irrigate. Do you irrigate to the needs of the lawn or to the tree? Frequently, lawns are irrigated by timers, putting a short burst of water on. Trees like a deep watering. Shallow watering leads to an accumulation of salts in the tree’s root zone and salt burn results. If possible, keep a 6 foot turf free area around the trunk. And best of all irrigate the tree separately from the turf and make sure the lawn sprinklers do not wet the trunk which can lead to crown rot in the tree.

Budding and grafting of avocado and citrus

By: Pam Elam

It is often tempting, after eating a particularly good orange or avocado, to plant the seed and grow our own tree full of these delicious fruit. Trees grown from these seed, however, may produce fruit that are not edible at all, or the trees may not bear fruit for many years. The best way to produce good-quality fruit is to grow seedlings from them and then attach, by budding or grafting, material from trees that are known to be good producers. Budding and grafting can also be used to change or add varieties to mature citrus or avocado trees, a process known as top working.

This publication is a brief introduction to budding and grafting for the home gardener. For more information, consult the materials listed at the end of this publication or contact your local Cooperative Extension office.

Establishing Seedlings

The best time of year to start citrus or avocado seedlings is in early spring. To germinate citrus or avocado seed, plant them in a shallow container such as a nursery flat or a pan with drainage holes in well-drained commercial potting mix. Plant the seed two to three times deeper than their length. For example, a citrus seed about ¼ inch (6 mm) long should be planted about ½ to ¾ inch (12 to 18 mm) deep. Keep the seed in a warm place-between 70° and 80°F (21° to 27°C)-and keep the soil moist. Covering the nursery flats with clear glass or plastic will help maintain the proper humidity. Avocado seed can also be germinated by suspending them in water. Place toothpicks horizontally into the seed near the top. Suspend the wide end of the seed in a small container of water with the toothpicks resting on the edge of the container. Place it in indirect light and refresh the water at least weekly.

After germination (usually 12 to 15 days), replant the seedlings into a larger container of good-quality commercial potting mix. (If all danger of frost has passed, the seedlings may be planted directly into the ground where you want the tree to grow instead of replanted into containers.) Good choices for containers include a cardboard milk carton cut horizontally in half or a one-gallon can. Punch drain holes in the bottom of the container. The seedling will be ready for budding or grafting when it has grown to 24 to 30 inches (60 to 75 cm) tall.

Keys to Budding and Grafting

Budding and grafting are vegetative propagation techniques in which a single bud or stem (scion) of a desired plant (cultivar) is attached to a rootstock plant. In budding, a single bud with its accompanying bark (often referred to as budwood) is used as the scion. In grafting, part of a stem or branch is used as the scion. One of the most important keys to successful budding and grafting is properly positioning the scion on the rootstock. In order for the scion and rootstock to grow together, the thin greenish plant layer (cambium) just under the bark of the scion and rootstock must be aligned so that they touch each other. If they do not touch each other, the bud or graft will fail. Within 10 to 15 days, a successful bud or graft forms a hard whitish tissue (callus) where the two cambium layers grow together.

Always use sharp cutting or grafting instruments and make clean, even cuts. Options include a budding knife, a sharp kitchen knife, or a single-sided razor blade. Do not allow the cut surfaces of the scion or rootstock to dry out. Immerse cut scions in a pail of water, wrap them in plastic, or graft them immediately after cutting. Also, remove any leaves from scions after cutting to help keep the scions from losing water. Keep the scions in a cool place during the work.

When to Bud or Graft

Budding and grafting are best done in the spring or fall when the bark is easily separated from the wood. It should be timed to be early enough so that warm weather will help ensure a good bud union, yet late enough so that the bud will not begin to grow and callus will not grow over the bud itself. Citrus budded or grafted in the fall must be protected from frost. Avocados are best grafted in the spring when the bark is easily separated from the wood.

Budding

Budding is the standard method used to propagate citrus. Aside from being the easiest method, it allows a large number of plants to be propagated from a small amount of scion wood and is suitable for trees, rootstocks, or branches from 1 /4 to 1 inch (0.6 to 2.5 cm) in diameter.

Budwood should be taken only from high-producing, disease-free trees (see Warning at end of this article). The best citrus budwood is located just below the most recent flush of new growth; the best avocado budwood is located near the terminal end of shoots that have fully matured, leathery leaves.

How to make a T-bud

T-budding (see fig. 1) is generally the best budding method for citrus and avocados. To make a T-bud, make a T-shaped cut on the rootstock about 8 to 12 inches (20 to 30 cm) above the ground (fig. 1A). The vertical part of the T should be about 1 inch (2.5 cm) long and the horizontal part about one-third of the distance around the rootstock. Twist the knife gently to open flaps of bark. Avoid cutting through any buds on the bark of the rootstock.

On the scion (fig. 1B), cut a selected bud beginning about 1 /2 inch (1.2 cm) below the bud and ending about 3 /4 to 1 inch (1.9 to 2.5 cm) beyond the bud. Make a horizontal cut about 3 /4 inch (1.9 cm) above the bud down through the bark and into the wood. Gently remove the shield-shaped piece for budding (fig. 1C).

Slip the budwood down into the T-shaped cut under the two flaps of bark until the horizontal cuts of the bud match up with the horizontal cut of the T (fig. 1D). After inserting the budwood into the rootstock, wrap the bud and rootstock with budding rubber (fig. 1E). Budding rubber is available from agricultural supply or hardware stores; if budding rubber is unavailable, use wide rubber bands, green tie tape, or stretchy tape. Leave the bud exposed while wrapping. Do not coat the area with grafting wax or sealant.

GRAFTING

Whip grafting

The best grafting technique for small-diameter 1/4 to 1 /2 inch [0.6 to 1.2 cm]) rootstocks is whip grafting. Whip grafting should be done in the fall or spring. Although whip grafts use more scion wood than budding does, they allow the grafted plant to develop more rapidly.

To make a whip graft (fig. 3), select as a scion hard and mature green wood. First make a long, sloping cut about 1 to 2½ inches (2.5 to 6.2 cm) long on the rootstock (fig. 3A). Make a matching cut on the scion. Cut a "tongue" on both the scion and rootstock by slicing downward into the wood (figs. 3B-3C). The tongues should allow the scion and rootstock to lock together. Fit the scion to the rootstock (fig. 3D) and secure with budding rubber (fig. 3E). Apply grafting wax to seal the union. To prevent sunburn, new whip grafts should be protected from the sun until they heal. After the scion has begun to grow, remove any growth from the rootstock. If necessary, support new shoots by staking.

Bark grafting

The best grafting technique for large-diameter trees or branches is bark grafting (fig. 4). To make a bark graft, first cut off the rootstock (the trunk or branch to be grafted) just above a crotch where smaller branches sprout out. If possible, try to retain one branch of the original plant as a nurse branch. The nurse branch will provide the scion nutrition and support from wind (the nurse branch will eventually be removed).

Cut vertical slits 21/2 to 3 1/2 inches (6.2 to 8.7 cm) long through the bark of the remaining freshly cut rootstock stubs down to the wood. These slits should be spaced 3 to 5 inches (7.5 to 12.5 cm) apart. Cut the scions 5 to 6 inches (12.5 to 15 cm) long with 4 to 6 buds per scion (figs. 4A-4C). If scions are cut longer than this, they may dry out before healing. When cutting the scions, make a sloping cut about 3 inches (7.5 cm) long at the base of the scion.

Using a grafting knife or other very sharp knife, lift the bark on one side of the slit. Insert the scion into the slit with the long-cut surface of the scion facing the wood of the rootstock and push it down into the slit (fig. 4D). Make sure that the scion fits snugly into the slits in the bark and that the cambiums are properly aligned.

Secure citrus scions by nailing them in place with thin flathead nails or tying them with strong cord or tree tape. Secure avocado scions with plastic nursery tape. Coat all cut surfaces thoroughly, including the tops of the scions, with grafting wax or pruning paint. To protect the graft from sunburn, paint it with white interior water-based paint, either undiluted or mixed 50/50 with water. Paint the entire area around the graft union, including the scions, waxed areas, and the exposed trunk below the graft union. Inspect the grafts frequently and re-wax them if they begin to crack or dry out.

Once the scions begin to grow well, remove all but one scion per branch. Early on, however, prune the scions that will be removed to reduce their vigor but do not prune the scion that will be kept. The one scion you keep will eventually become a main scaffold branch. Any nurse branches should also be removed after all the scions are growing well.

Top working

Top working is the process of changing fruit varieties on a mature tree. Most citrus and avocado are top worked by bark grafting (see above). Top working should be done in the spring or fall.