DIvisI o n of Agricultural Science \ UNIVERSITY OF CALIFORNIA J. C. JOHNSTON ED^A^ARD F, FROLICH CALIFORNIA AGRICULTURAL Experiment Station Extension Service CIRCULAR 463 THE PROPAGATION of Avocado trees for commercial production is a process requiring attention to detail and extreme care at each step. The Avocado is a tree sensitive to weather and soil condition and susceptible to certain pests but the careful grower can successfully propagate trees with minimum trouble by following the directions in this pub- lication. FROM SEED Selecting the seed . . . planting the seed . . . care of the seedbed . . . the nursery . . . digging the seedling . . . core of the nursery Page 3 BUDS and GRAFTS Budding the seedling . . . selecting the bud . . . budding methods . . . digging the trees ... tip grafting . . . graft- ing the seedlings Page 7 TOPWORKING Reasons for topworking . . . best time for topworking , . . the bark graft . . . the notch graft . . . selecting the scion . . . tools for topworking . . . care of the grafted tree. Page 13 THE AUTHORS J. C. Johnston is former Extension Subtropical Horticulturist, Riverside (retired). Edward F. Frolich is Principal Laboratory Technician, Department of Subtropical Horticulture, Riverside. NOVEMBER, 1957 AVOCADO PROPAGATION J. C. JOHNSTON EDWARD F. FROLICH ./VvocADOS MAY BE propagated in a number of different ways. Nursery trees are produced by budding or grafting the various varieties on seedling rootstocks. Trees can be grown from cuttings but the performance of such trees is not known and most varieties are difficult to root. Older trees are topworked to new varieties by various types of grafts or by budding into vigorous new shoots. This circular describes the more common procedures, together with methods which are used under special con- ditions for avocado tree production. Avocado trees grown from seed are usually slow to come into bearing. The tree is extremely variable in yield and fruit type. The fruit is rarely of acceptable quality. Even if the fruit were good it would be diflBcult to market because each tree would bear fruit differing in size, color, shape and quality. Seedling trees are occasionally used for a windbreak. In cold locations where it is difficult to establish nursery trees, growers sometimes plant hardy seedlings directly in the orchard. After the seedlings have grown two or more seasons they can be grafted to the desired variety. THE SEED and SEEDLING ROOTSTOCKS In California seedlings of the hardy Mexican varieties are preferred for root- stocks. The use of seedlings of the Guate- malan varieties is not advised because although they make strong growth and produce vigorous trees when budded or grafted the stocks are more susceptible to disease and injury from cold. Mexican seedlings are more resistant to lime-induced chlorosis, Dothiorella canker and Verticillium wilt than those of the Guatemalan race. Selecting the seed Select seed from trees of a hardy Mexi- can variety known to produce seedlings that make vigorous, disease-free trees. If possible, the tree or trees should be iso- lated so they will not receive pollen from less desirable varieties. The varieties most commonly used as a source of seed are Topa Topa, Mexicola, Ganter, and [3] Duke. There are a number of other varieties and individual trees equally good as seed sources. It will be best to discard seed that is small for the variety. The virus disease sun blotch is some- times carried by seed. Since some trees carry the virus without visible symptoms, it is important to know that seedlings from trees used as a seed source produce disease-free trees when budded or grafted. The avocado root rot disease can also be carried by seed. Fruit that has been permitted to lie on infected soil may be invaded by the fungus and many of the seeds will carry the disease and infect the nursery. For this reason it is wise to take seed from sound, mature fruit and avoid the use of windfalls. Your University of California Farm Advisor can assist you in locating reliable seed. Plant seed soon after it is removed from the fruit. It should also be protected from drying. Since fruit of the Mexican varieties matures in the fall it will, in some cases, be necessary to store the seed until spring if the nursery is to be located out of doors. Seed may be kept for several months if packed in dry moss, sand or sawdust and placed in a cool location. Temperatures between 40 and 45 degrees F are desirable. Planting the seed for standard nursery trees Avocado seedlings should be grown in a soil that has never been planted to avocados or other crops, that carry the root-rotting cinnamon fungus or Verticil- lium wilt fungus. To insure freedom from disease, it is best to fumigate the soil with a recognized soil fumigant. Methyl bromide and vapam are currently recom- mended. Sandy soil that has not recently been manured is best. A mixture of half sand and half loam is preferred by some growers. A convenient and economical way to grow seedlings for field planting is in beds 12 inches deep, 3 to 4 feet wide, and long enough to accommodate the number of seeds desired. It is often desirable to build a frame of 1'' x 12" boards on the soil surface and fill with soil specially prepared or selected for the seedbed. If S rodents are likely to be a problem, place a wire netting under the frame before filling. It is sometimes necessary to screen the top of the beds until the seedlings are well established to prevent their removal or damage by rats and squirrels. Fumi- gate the bed before planting to avoid soil- borne diseases. Seed can be planted directly in the field in nursery rows. This requires special care and does not permit culling weak and offtype seedlings without leav- ing blank spaces. In addition, sprouting will be slow with some seeds, causing an irregular stand. Some growers place a paper cup or hot cap over each seed to protect it from drying and to facilitate weed control. When seed is covered in this way contact weed-killing sprays can be used in the nursery. Seed should be planted with the large flat end down, leaving the tip flush with the soil surface. Be sure the soil makes firm contact with the seed. Spread a layer of clean sand, sawdust, or ground peat moss over the beds, deep enough to cover the seeds % to % inch. This will prevent the formation of a crust over the soil after irrigation and will protect the seeds from drying. To avoid burning tender sprouts as they emerge during hot weather shade should be provided. Germination can be hastened by cutting a thin slice off the tip and base of the seed. This is not considered neces- sary when seed is planted immediately after removal from the fruit. Seedlings that are to be transplanted to the nursery bare root are planted at 2-inch intervals in the bed. If they are to be removed with soil around their roots they are planted at 4-inch intervals to permit easy digging. Some growers pre- fer to plant in 2x2x8 inch paper pots [4] to facilitate moving the seedlings into the nursery. If a hothouse or hot bed is available, seed can be planted as it be- comes available. Seedlings grown in this way from fall or early winter plantings will be ready to line out in the nursery by spring. In warmer locations seed planted in the fall out of doors or in a lath or cloth shelter will also be ready to transplant by spring. Seedlings lined out in spring should be ready to bud by summer. Under favorable conditions seedlings from fall plantings made directly in nursery rows can also be budded during summer. In cooler locations field plant- ings are made in March or April and budding can begin in the fall. Care of the seed bed The seed bed should be kept moist at all times. This will require sprinkling every two or three days. In cool locations germination will be hastened if the beds are covered with black paper. This will warm the soil, decrease moisture loss, and discourage weed growth. The paper must be removed at the first sign of germination to avoid injury to the tender shoots. Germination will require about 30 days when seed is planted in a hothouse or out of doors in spring. Most of the seedlings will be ready to transplant 60 to 90 days after the seed is planted. Seed planted out of doors in fall or winter will usually require 60 or more days to ger- minate and the seedlings will be ready to transplant 30 to 60 days after germina- tion. Seedlings are ready to transplant when they are 5 or 6 inches high. It is not necessary or desirable to fer- tilize avocado seedlings at this stage. The seeds provide an adequate supply of nutrients. The nursery The nursery site should be as free as possible of frost, strong wind and ocean breezes. The soil should be free of rocks, and well drained but containing enough clay to form a good ball when the trees are dug. As with the seed bed, the soil should not have been used for avocados or other crops that may have carried the cinnamon or Verticillium fungus. Avoid locations below established avocado or- chards or land formerly planted to avocados, where drainage water may carry disease to the nursery. It will save expense and trouble if the site is also free of noxious weeds such as nut grass, Bermuda grass or morning glory. It is important to have an ample supply of good water available at all times. Before planting, the soil should be moist and worked to form a good plant- ing bed. If furrows are to be used for irrigation they should also be established and tested for grades before planting. Furrows should rarely be more than 100 feet long. Planting distances should be adjusted for equipment. The usual practice is to space the rows 3 feet apart and plant the seedlings 14 to 16 inches apart in the rows. Digging the seedlings When the seedlings are ready to trans- plant, moisten the soil in the seedbed. Begin at one end of the bed, draw the soil away from the seedlings and remove them with as many small roots as pos- sible. Use special care to leave the seed attached to the seedlings, otherwise they will not make satisfactory growth. Plant the seedlings immediately and protect them from drying at all times. They can be kept in moist burlap, moss or shavings for a short time. Discard all weak and offtype seedlings. They rarely produce good trees. Sort the good seedlings into 2 or 3 sizes and plant the different sizes in separate rows. This will give a more uniform and better-appearing nursery. Seedlings with 4 and 5 leaves can be planted without reducing the leaf sur- [5] face. Larger seedlings will do better if half of each leaf is clipped off before planting. Do not clip or damage the growing tip of the seedling. Holes for planting are usually made with a large dibble. They should be deep enough to accommodate the whole root syst-em without bending. If the holes are made deeper than necessary, the root can be inserted to the full depth of the hole and then lifted to the proper planting A dibble is easy to make J\ PfPE — %V ^//e" level. This will straighten the tender root tips and prevent formation of crooked tap roots. Plant the seedlings at the same level as they were in the seedbed and press the soil firmly around the roots and seed. When seedlings are grown in paper pots it is necessary to remove the pot before planting. When seedlings are taken from the seedbed and planted in pots or other containers, it is best to dig them with a ball of soil around their roots. As soon as the seedlings are planted they should be watered to settle the soil. For late plantings when sudden hot spells are expected, provide protection from the sun by placing a shingle on the south side of each plant. The seedlings should be irrigated again the day following plant- ing. Care of the nursery After the seedlings are established, ir- rigation will be needed every week to ten days. The success of the nursery depends more on careful irrigation than on any other operation. The soil should be ex- amined at frequent intervals to make sure it never becomes dry. This will keep the seedlings growing without interrup- tion and produce the most desirable type of seedling for budding or grafting. It is important to keep the nursery free of weeds. They compete with the seed- lings and retard their growth. Be espe- cially careful to keep noxious weeds out of the nursery; otherwise state regula- tions may prevent moving the trees. Fertilization will usually be needed. Use 1 lb. of a chemical nitrogen carrier to 150 ft. of nursery row as needed. This must be determined for each soil by the color and vigor of the seedlings. Except in cool coastal areas, most seedlings will be large enough to bud by fall and the rest will be ready by spring. z" [6] BUDS and GRAFTS Budding the seedlings Budding can be done in the fall or in the spring as soon as the bark will slip. Budding can be done during the summer but it is difficult to get a good stand. There is a tendency for the eye of the buds to fall out. Seedlings should be at least % inch in diameter where the bud is to be inserted before budding. Those which have not made vigorous growth or show diseased or offtype growth should be discarded. They usually pro- duce poor trees. The most difficult part of the job is to select suitable buds. They will be found on mature terminal growth that is not producing new growth. Those nearest the terminal end are usually best. Select plump dormant buds on mature wood. For late summer or fall buds, select new growth with fully expanded mature leaves. For spring budding the best time to obtain buds is in late winter before growth begins. Since varieties differ in growth habits, only experience will enable one to select the best buds for the various varieties. Some varieties bear good buds at most seasons. Others have acceptable buds only during short periods. Buds cut be- fore spring growth begins can be kept in polyethelene bags at temperatures be- tween 40 and 45 degrees for 1 or 2 months. When suitable buds are located, cut the leaves off i/i inch from the bud and keep the sticks in moist burlap till Scions selected for budding. Left Fuerte, right Haas M...y T Budstick with bud removed T-cut Bud inserted Bud tied used or placed in storage. Always pro- tect them from drying. It is often best to employ a professional propagator to bud the trees. This will insure a good set of buds and save time and money. How- ever, with a little practice most growers can bud successfully. When the seedlings are ready to bud, use a sharp, thin-bladed knife with a rounded point. Make a T cut in the bark of the seedling at a smooth place, about 3 or 4 inches above the soil. The T cut may be either upright or inverted. When making the cut crossing the T, hold the knife at an angle so as to raise the edge of the bark slightly. This makes it easier to insert the bud. Next, cut a bud from one of the bud- sticks with a single smooth stroke of the knife. It is important to have the cut sur- face smooth. If the upright T is used, start about % inch below the bud and remove a shield-shaped chip about II/2 to 2 inches long. Cut deep enough to take a sliver of wood under the bud. As the cut is completed, hold the top of the bud against the knife with the thumb and as quickly as possible insert the bud down- ward under the bark. Wrap all of the cut surface but leave the eye of the bud exposed. Various kinds of budding tape are available but plastic materials are best. The wrapping can be removed from spring buds in about 6 weeks and from summer buds in about 4 weeks. Fall buds may be left 8 weeks or more before the wraps are removed. Always remove wraps before the stock is constricted. With fast-growing seedlings it may be necessary to loosen the wraps in about 3 weeks to prevent constricting the stock. At the time of budding or shortly there- after, remove the tip of the rootstock seedling. This will retard growth and help force the bud. When the wraps are removed, examine the buds to see if they have "taken." Successful buds will be green and the bud or eye will be firmly in place. If the bud has taken, cut the seedling stock back to 10 inches above the bud to force growth. When the bud has grown about 6 in- ches, tie it to the seedling stock for sup- port. Trees are sometimes dug at this stage and planted in the orchards as "tied up buds." This gives good results but requires more attention than standard trees. They must be given the same care in the orchard as described below for the nursery. Staking. When the bud is ready for the second tie, the trees should be staked, using lath or 4-foot long ^^ x ^4 "^ch redwood stakes which are available for this purpose. The buds grow rapidly and will need tying at frequent intervals. When the bud has grown 12 or 14 inches the seedling stock can be cut off close to the bud union. This should be done not less than 2 weeks before the trees are to be dug. Use a coping saw or other fine- toothed saw. Slant the cut down and away from the bud union so the scar will heal [8] quickly. Cover the wound with a good pruning compound, such as emulsified asphalt. Use care to keep all sprouts from developing on the rootstock. Trees can be dug any time after they are 18 inches high. First cut on a budded seedling. This will retard the growth and help force the bud. Digging the trees Standard budded avocado trees are usually dug with a ball of soil about 10 inches in diameter and 14 to 16 inches deep. A special balling spade is used. Second cut on budded seedling. This will give the bud further nour- ishment. [9] Have the soil moist but not wet. Insert the spade 5 inches from the tree trunk to a depth of 14 to 16 inches on all four sides. This will cut the roots on all sides and make a ball 10 inches in diameter. Next, remove the soil from one side of the ball, leaving the balling spade in place on the opposite side of the ball. Then using a shovel cut the bottom of the ball 14 to 16 inches below the soil sur- face. Remove the ball with the balling spade and lay it on a piece of burlap or material suitable for wrapping the ball. Burlap grain sacks are commonly used. One sack will make 2 wraps. Paper can be used if the trees are to be planted immediately. To tie the wrap slip the spade from under the ball and fold the wrap around the ball. Gather the wrap around the trunk of the trees and tie it in place as close to the soil surface as possible. Next, make 3 half hitches around the ball and pass the twine under the ball and up the opposite side to the trunk where it is tied securely. Have the lower half hitch near the bottom of the ball to protect it from spreading when wet. Always lift the ball from the bottom, never by the tree. The tree is then ready to plant. The trees can be handled with less damage to the balls if they are held in a shady place and watered lightly for several days. If trees are to be held for some time after digging, the twine used to tie the ball should be treated to resist decay. Growing tip grafts Many avocado trees are produced by whip grafting seedling rootstocks when they are approximately 14 iiich in diam- eter. This kind of tree is known as a "tip graft." Field-grown seedlings can be tip grafted but this is not common practice. Tip grafts are usually produced in a hot- house where temperatures can be main- tained above 55° or 60° F. Some growers maintain a soil temperature of 70° or 80° by means of bottom heat and pay [10] little attention to air temperature. By these methods trees can be grown more rapidly. Seed is planted as early in the fall as it becomes available and the seed- lings are grafted in late winter or early spring. The trees are usually ready to plant in the orchard by spring or early summer. In cooler areas where fruit matures later and trees develop more slowly it is often desirable to store seed from the previous season for summer planting. This gives the seedling an early start so the trees will be readyfor spring planting. Tip grafts are also grown out of doors or in lath or cloth shelters. Cloth shelters are best because they exclude wind and maintain more favorable temperature. Seed planted in late summer or early fall will usually be ready to graft in spring and the trees will be ready to plant the following spring or early summer. Since tip grafts are usually grown in tar paper pots it is possible by careful choice of soil and by using soil fumiga- tion to be more certain the trees do not carry soil-borne diseases. Tip grafts re- quire more care after planting in the orchard but this is offset by the lower initial cost and the reduced hazard of soilborne diseases. Seedlings that are to be tip grafted are usually grown in individual open bottom pots made of 30 lbs. felt roofing paper. Some growers prefer 40-60 lb. roofing paper. The most popular size is 12 inches deep and 5 inches in diameter. They are made by cutting the roll into three 12 inch sections. These strips are then cut into lengths to give the size of pot de- sired. Eighteen-inch pieces make 5-inch pots. The pots are made by rolling the pieces into cylinders, sealing the lap with hot tar or stapling the seams to hold them in place. Some growers, especially those who produce trees without artificial heat, cut 90-lb. felt roofing paper rolls in two and make pots 18 inches deep and 6 inches in diameter to allow for greater root development. Pots this deep tend to buckle near the base unless given sup- port. This support can be provided by cutting the top and bottom out of gallon tin cans, slitting the side, wrapping them around the pots and tying them in place with wire. It is important to choose soil for pot- ting that has not been used to grow avocados or other crops subject to cin- namon root rot or Verticillium wilt. It must drain readily and remain in good condition under frequent irrigation. It is a good idea to pot some of the soil and see how it works before making large scale plantings. A sandy loam is usually preferred. Drainage can be improved by adding peat moss, compost or well-rotted manure. These materials sometimes con- tain excessive amounts of salt and should be checked before use. Clay loams can be used to advantage if treated with a suit- able soil conditioner. Krillium used at 1 lb. to 1500 lbs. of soil has given good results. Avocados are unusually sensitive to salty soils. Be sure the soil has a low content of soluble salts and always irri- gate the pots to get adequate leaching. To fill the pots place about 2 inches of moist soil in the bottom and tamp it firmly so it will not fall out when the pot is handled. Complete filling the pot to within 1% inches of the top, making the soil firm but not too compact. To keep the soil in the pots from being contaminated by the cinnamon fungus, place them on a plank bench. If there is no infected soil in the area, gravel or sand beds are satisfactory. Before planting the seed it is often best to fumigate with a good fungicide after the pots are filled and in place, treating the whole bench. The seed may be planted directly in the pot, leav- ing the tip flush with the soil surface. Because avocado seed does not germinate uniformly some growers prefer to sprout the seed in moist sand, moss or vermi- culite. This practice is especially desir- able with seed of questionable germina- tion. Care must be used in planting sprouted seed to avoid injury to the tender root. This practice insures more uniform development of seedlings and simplifies their care. The seed should be covered with % inch of clean sand, vermiculite or other suitable material to prevent drying and to keep a crust from forming on the soil. After planting the seed, irrigate at once. Examine the soil in the pots at frequent intervals and keep it moist at all times. It is important to keep the seedling growing vigorously without setbacks. With most soils some fertilization will be necessary. A convenient way is to dis- solve % lb. of a chemical nitrogen fer- tilizer in 10 gals, of water and give each pot % pint. One application will usually be enough but it should be repeated if the trees do not retain a good green color. Grafting the seedlings Seedlings are ready to graft when they are about 1/4 inch in diameter. The size must be determined by the scion wood available. The seedling and the scion should have about the same diameter. Slow growing or offtype seedlings should be discarded. Select wood for grafting as described for budding but have in mind the size of the seedlings to be grafted. To maice the graft, cut the seedling off 3 or 4 inches above the soil, making a smooth slanting cut about 1 inch long, depending on the size of the stock. Next, choose a piece of graft wood about 2-3 inches long, with at least one good bud. This is called a scion. Make a sloping cut on the base of the scion to match the cut on the seedling stock. A second cut is sometimes made on both scion and stock. Beginning about % of the way down from the tip of the stock and the base of the scion make a cut about % as long as the first cut. This cut is not made with [11] Scions selected for tip grafting. Scions on the left have been selected for the knuckles. Scions on the right have been selected for the axillary buds. A common splice graft Whip and