This paper will focus on the techniques used in the production of the eastern redbud. The methods of production that will be examined are seed, cutting, grafting, and tissue culture.
The eastern redbud is considered to be one of the more beautiful species native to North America. It is a small flowering tree, occasionally reaching heights as tall as forty feet, although twenty-five feet is average. The species has purple pea-like flowers in the early spring, emerging before the first leaves. The leaves are heart-shaped and turn yellow in the fall.
The eastern redbud is an especially challenging plant to propagate, as we will discover. A member of the pea family, the eastern redbud exhibits double dormancy characteristics which make seed germination challenging.
The eastern redbud has a large natural range, and to ensure hardiness, seed should be collected from a local source (Dirr 1998). Seed is collected in late summer when the pods turn dark and in the eastern areas of the country, may continue into November (Young and Young 1992). To avoid insect damage, seed is best collected as soon as it is ripe (Young and Young 1990). After collection, spread in the sun or hang in coarse-fiber fabric bags to dry. Once dry, the seed can be stored in sealed containers at three to five degrees Celsius (Young and Young 1992).
Ripened seed not completely dry can be sown directly into the field in the fall with good results (Young and Young 1992). Once dried, both scarification and stratification must be implemented to produce good germination.
Scarification is used to break down the seed coat. This is necessary to allow water and gas exchange within the seed (Hamilton and Carpenter 1975). Acid scarification, in concentrated sulfuric acid for thirty minutes, is standard (Dirr 1998), although this may vary by seed source (Young and Young 1992). Another method of scarification is hot water treatment.
There are two methods of hot water treatment. One is to place the seeds in boiling water for one minute; the other is to place the seeds in water heated to eighty-two degrees Celsius and allow to cool overnight (Young and Young 1992).
Table 1. Effects of Scarification and
Stratification in Germination (Frett and Dirr, 1978).
(Minutes in Acid)
|0 30 60|
Table 1 demonstrates the importance of both scarification and stratification treatment techniques in obtaining good germination results. Stratification is done at three to five degrees Celsius for five to eight weeks (Young and Young 1992).
While successful results have been reported, questions exist as to their reliability. The cultivars available in the trade are rarely if ever found on their own roots and an Oklahoma nursery reports that the roots system on those plant propagated by cuttings are not as vigorous as on those plants propagated by budding (Dirr and Heuser 1987).
According to one report, untreated softwood cuttings rooted 75-90% in four weeks when placed under mist and bottom heat at 72 degrees Fahrenheit. The age of the parent plants was unreported (Dirr and Heuser 1978).
Another report dipped softwood cuttings in 8000 ppm IBA-talc and placed under mist. When the air temperature was 75 degrees Fahrenheit, 75% of the cuttings successfully rooted. When the air temperature was reduced to 65 degrees Fahrenheit, the percentage of cuttings successfully rooted was reduced to zero (Dirr and Heuser 1978).
Grafting on redbud is difficult and results are unreliable, varying from year to year (Trigiano et. al. 1988). Grafting techniques implemented in the propagation of redbud are T-budding and pot-grafting.
T-budding, done from late July to early August, requires plump budwood with large buds to be successful and two different techniques exist. Rebudding—the process of placing another bud directly above the other—is done two to three weeks later. The understock is cut back in March; suckering is common and such growth is removed. There is a 70-95% success rate with plants propagated in this manner (Dirr and Heuser 1987).
In the second technique, second-year
seedlings are lined out in the spring, side dressed with nitrogen, cut
back to the ground. One shoot from the new growth is retained and a T-bud
is placed in it. The understock is cut back to the bud in three weeks and
12-15" of new growth occurs from the bud the same season If not cut back,
the plants will be dead in the spring. There is a 90% success rate with
plants propagated in this manner (Dirr and Heuser 1987).
|Table 2. Order of budding
ease of selected cultivars of eastern redbud, from easiest to hardest:
|Source: Dirr and Heuser 1987.|
Pot-grafting, using a side graft, can be done during the winter (Dirr and Heuser 1987).
Tissue culture techniques show great promise. Due to the difficulties described in the above methods, tissue culture and especially somatic embryogenesis may be more efficient and provide greater uniformity (Trigiano et. al.1988).
It has been demonstrated that successive sub-cultures have been related to increased rooting potential in a number of species. In eastern redbud, there is a significant increase in shoot proliferation with each sub-culture with the highest occurrence after the third sub-culture. After the third sub-culture, many developing roots showed terminal necrosis (Yusnita et. al.1990).
On a bio-chemical level, TDZ has proven ineffective. TDZ treated explants are characterized by numerous axillary and adventitious shoot production followed by failure to elongate. Many are also distorted and fasciated. BA has proven effective in promoting axillary bud break and shoot development. NAA and IBA have proven effective in root initiation and development. NAA is used at 55.9 ppm and IBA at 60.9 ppm. The roots produced on NAA treated explants are thick with no secondary branching. IBA treated explants produced a more finely textured root system with greater secondary branching (Yusnita et. al. 1990).
Seed propagation, while it has been refined and is quite successful, is only of limited value because the cultivars must be vegetatively propagated to maintain their unique characters.
Grafting is a difficult, labor-intensive process with results that are variable from year to year.
There are techniques reported for successful
propagation by cuttings, and the development of these techniques would
be a distinct advantage. The same is true of tissue culture. More work
is required, especially as to the relative age of the parent plant and
how to produce more vigorous root systems.