Propagation Of Lilies
Few genera of plants are as amenable as Lilium to varied means of propagation. Besides the obvious means, seed, lilies can be increased from daughter bulbs that offset from the main bulb. In scaling, portions of the bulb are detached and induced to form new bulbs. Many kinds of lilies form underground bulb lets on the stem just above the main bulb. Some produce stem bulbils in their leaf axils, and the production of bulbils on the stem can be induced in most lilies.
The tissue culture laboratory has become an excellent tool for the propagation of new and desirable clones for the commercial market. A clone can be increased very rapidly by this method, with a single choice seedling yielding as many as 2 million tissue-culture bulb lets in two years. Embryo culture is a form of tissue culture with the specific purpose of preventing the premature death of hybrids derived from unusual, incompatible crosses.
Propagation by bulb scales
Vegetative propagation by scaling is the most cost-efficient and rapid method to increase a clone. Commercial growers use this system extensively, believing that it rejuvenates their stocks. It also has a cleaning effect if carried out correctly, and diseases such as basal rot (Fusarium) can be controlled in the process. This technique is often used to increase a group of outstanding individuals selected from within a seed strain. Most lilies can be propagated readily from bulb scales.
Healthy bulbs with no traces of disease should be selected for multiplication by scaling. This method affords no protection against the transfer of virus disease, or any other disease for that matter. It is sheer folly to propagate material with serious disease symptoms of any kind.
The bulbs selected for scaling must be as clean as possible. Any excess soil adhering to the bulb should be washed off. The best results are obtained by selecting the largest bulbs from the stock; strength and vigor will then be optimally maintained.
For maximum bulblet formation, the scales must be broken off cleanly right at the basal plate. Because the scales are arranged spirally, they are removed around the bulb-as when dismembering an artichoke. The time of year does not seem to be important for scaling; it depends on local conditions and individual preference. Bulblets can form on scales at any time if the correct stimulus is applied. It is well to remember, however, that lily scales decay rapidly if moisture and temperature levels are not satisfactory.
If the scales are to be planted outdoors, they are best taken very soon after flowering. This will give them enough time to form a callus and new bulblets before severely cold weather sets in. Growing climates vary greatly, but a good rule to follow is to allow at least two to three months of good growing weather to succeed with planting scales directly into the ground. Therefore, this method is never recommended for the later-flowering Orientals, although they can be grown similarly if the scale beds are covered with frames or other protective devices. It is very important to keep the soil evenly moist always after the scales are planted, a critical condition for bulblet development.
The direct planting method, although excellent for the amateur grower, is seldom used in commercial operations. Lilium candidum (Madonna lily) is an exception; because it forms leaves immediately after the bulblets develop, it lends itself only to this method of scaling. Excellent crops of this magnificent lily have been produced by scaling the bulbs in late June or early July and planting the scales in rows or beds. Adequate irrigation is essential after planting. With proper conditions, the scales produce large bulblets followed by overwintering leaves; these plants will flower beautifully the following June.
The older commercial method of scaling involves lifting the bulbs in late summer and fall. The scales are removed and subjected to an 8- to 12-week incubation period, followed by 8 to 12 weeks of cold storage (vernalization), then planted the following spring. This may still be the easiest method for the amateur grower. At present, commercial operations often scale bulbs after harvest, in late fall or early winter, in which case the following steps must be observed with special care.
Scale only clean, large, disease-free bulbs from stocks that are verified true to name. Break the scales off cleanly at the basal plate to permit good bulblet formation. Dust or dip the scales in a suitable fungicide; thiabendazole, also known as TBZ (e.g., Mertect), has been satisfactory as a dip for many years. Pack the scales thinly in layers in a moist medium in trays or boxes; sphagnum peat and vermiculite are both excellent media and should completely cover the scales. The containers should be lined with a perforated plastic liner to conserve moisture, and vented to allow good aeration, a critical factor for satisfactory bulblet and root development.
Incubate the scales in a well-ventilated room at 15°C to 21°C (60°F to 70°F) until the bulblets and roots are fully formed. The duration of incubation depends on the variety. Asiatic hybrids require 6 to 8 weeks, trumpet species and hybrids 8 to 10 weeks, and Oriental species and hybrids 12 to 14 weeks. Trumpet hybrids do not require a cold period for shoots to develop; they must be watched closely, removing them from the incubator immediately when the bulblets are fully formed. Trumpet bulbs can also be scaled in early spring and the scales planted out directly into rows, where they will form bulblets when the soil becomes warm enough; leaves emerge following the development of the bulblet and excellent growth can be achieved the same season.
After removing the trays of scales with bulblets from the incubator, store them at an intermediate temperature (4° to 10°C/40° to 50°F) for three to four weeks, then place them in cold storage (1°C/34°F). The minimum vernalization period required to break dormancy varies; Asiatic lilies generally require a minimum of 6 weeks and Oriental lilies a minimum of 12 to 14. A longer period of cold storage is not harmful. Trumpets and their hybrids do not require cold to break dormancy and should be held at a slightly higher temperature (2°C/36°F), but one that is sufficiently low to deter sprouting, until planting time. When soil conditions and temperatures are favorable, the scales are planted in outdoor rows or beds. The bulblets have now been conditioned to sprout readily, so it is important not to plant them too early and expose them to frost damage.
Scale plantings are usually allowed to remain in the ground for two growing seasons. Many Asiatic varieties can produce flowering stems the same season they are planted, an indication of superior vigor in the clone.
Propagation by bulb division
The typical round lily bulb sends its flowering stem from its basal plate through the centre of the bulb. Around the bottom of the stem one or more embryonic growing points will emerge. In some lilies, new bulbs are produced from buds around the rim of the bulb's basal plate, which will develop into replacement bulbs for the following season. Each species or hybrid reproduces at its own rate -a very small bulb, a weak species, or a struggling bulb may produce just one new bulb; two new bulbs is more normal and, with really happy, robust bulbs, a three- to six fold increase is possible.
When you lift bulbs at the end of summer, you will find the new bulbs still joined at the basal plate, but they can easily be cut or snapped off. If bulbs are left for two seasons before lifting, the original connecting tissue will have perished, and of course last season's bulbs will have produced their own successors.
The rhizomatous L. pardalinum and its relatives are best divided with a sharp knife so that each severed piece has at least one growing point, each with clusters of smaller, developing, white or pale cream scales, unlike the nicotine-colored older ones. Rhizomatous bulbs need lifting with the same care a dedicated archaeologist might use to unearth some precious artefact as their scales are very brittle. Stems are cut to a third of their length and provide useful handles for lifting the bulbs. At this time a new flush of roots is usually evident, just beginning to emerge from the base of scales. Early lifting will not damage these new roots which will grow rapidly once the divided pieces have been replanted and watered.
The types that benefit most from lifting and dividing regularly are the Asiatic hybrids and some of the species that were the founders of these races, such as L. dauricum, L. bulbiferum and L. davidii. These may be lifted in late summer, split and replanted in fresh spots or returned to their former positions, once these have been well dug over and envigorated with generous additions of humus.
The received wisdom about L. martagon, L. hansonii and their hybrids is that, having got bulbs growing nicely, it is best to leave them well alone. It is easy to see how such an attitude has developed: from seed it takes many years to achieve freely-blooming bulbs; bulbs purchased from dealers having perhaps spent quite a while in store before reaching the customers may 'not grow' above ground at all for a complete year although they will be forming a root system below and the following year will send up a stem.
Propagation by stem bulblets
Bulblets may be formed on the underground part of the stem of stem-rooting kinds. The number and size depends on the species or cultivar and the strength of the individual bulb. The stem of a vigorous one can generate up to a dozen or more bulblets -perfectly formed small bulbs with scales and roots ready for independent life once the stem dies back. A few bulblets can be the size of a small walnut, most are more or less marble size, but they can be graded down to tiny ones with two or three wispy, minuscule scales. Even these very tiny bulblets can be potted up and, with care, be persuaded to fatten up.
The number of these bulblets to be harvested from the buried part of the stem is positively correlated to the amount and vigor of the stem rooting. To increase the harvest by number and, even more significantly, by weight and volume, stem rooting needs to be encouraged. This can be done initially by providing an open, gritty, humus-rich soil and planting the bulbs with at least 10-15cm (4-6in) above their noses, kept moist but not over-wet. Stem rooting activity can be dramatically increased by humus mulches from late spring onwards.
Outstanding bulblet producers are L. lancifolium, L. henryi and L. regale, together with hybrids having these lilies in their breeding. Others that may behave this way include L. auratum, L. speciosum, L. davidii and L. longiflorum. Sterns of L. dauricum may meander a little having emerged from the bulb, and will produce bulblets along this length.
Species with stoloniferous stems, such as L. duchartrei, will form two or three bulbs along the wandering underground stem. If all goes well, these bulbs will repeat the process without any fussing so that in a very few years a colony of beautiful flowers can be established.
Propagation by stem bulbils
The tiger lily, L. lancifolium, (L. tigrinum), is the leading exponent of the stem-bulbil method of increase. Small, dark buds appear in the leaf axils; they swell to become purple-black mini-bulbs, sometimes starting to produce leaves and roots before they are fully ripe, and fall to the ground. While the majority of leaf-axils are happy to bear one bulbil, it is possible to find two or three in some. If left to their own devices they will eventually fall to the ground and pull themselves down into the soil. If you time your arrival before they are ready to fall you may be able to harvest as many as a hundred or so from a single stem.
Other species noted for similar tiger-lily behavior include L. bulbiferum, in which some clones are clearly more adept and prolific than others. The lovely, rare, yellow trumpet L. sulphureum and the white L. sargentiae also produce bulbils, sometimes very prolifically. L. davidii may produce some bulbils of its own accord or can be induced to do so. Hybrids of these species may give bulbils freely or can be persuaded to do so by cutting their flowering tops off before they get beyond being small buds. If bulbils are harvested and potted up before late summer a good proportion of reasonably sized ones can be got into such energetic growth as to bear one or more flowers the very next summer.
To achieve the greatest possible increase by bulbils for a particular type of lily, a semi-intensive regime can be started. Having made sure the chosen clone is one that can produce bulbils, we try to propagate as large a number of small bulbs as possible by harvesting bulbils and by raising small bulbs from scales. Under cool glass such small bulbs will produce wiry stems the following season and, if grown strongly, usually seem able to bear large numbers of bulbils, especially if any flowering buds or growing stems' tips are nipped off early. Once the summer crop of bulbils is harvested, the production process may be easily maintained until the required number of plants is obtained.
Propagation by tissue culture
Tissue culture is a method of propagating plants in the laboratory. Tiny amounts of plant tissue are placed in a sterile nutrient medium. Plant growth hormones are added to the medium to induce these cells to multiply and differentiate until they form tiny bulblets. This technique has revolutionized the horticultural industry by permitting the production of vast numbers of offspring from an individual plant. It is possible to select a seedling and within two years produce several hundred thousand bulblets, which can be marketed in another two years.
Tissue culture is commonly cited as a method of producing virus-free plants, but the method of doing this is much more complex than most people assume. When plant tissue is initiated into culture, it is tested with an enzyme-linked immunosorbent assay test for the presence of common lily viruses. If virus is present, it can be eliminated by "meristemming," a process in which tiny pieces of rapidly growing meristem tissue are removed, cultured, and repeatedly tested. Obtaining truly virus-free tissue in this way can take more than a year of repeated procedures. When the resulting plants leave the laboratory, of course, they can quickly become reinfected, so breeding virus-tolerant lilies remains a high priority.
The material used to initiate a lily tissue culture usually comes from bulb scales, but stem segments with an internode, or flower buds, can also be used. To avoid sacrificing a precious plant, the bulb may be carefully dug, leaving the stem intact in the ground, where it usually produces stem bulblets. Tissue cultures of endangered wild species have been produced from unopened flower buds.
The donor plant should be one that is unlikely to have been exposed to virus. The material to be cultured should be kept fresh and packed dry to prevent fungal growth.
Once the material reaches the laboratory, it is surface sterilized in a 10 percent solution of household bleach for 20 to 30 minutes. It is then rinsed and cut into segments (called "explants") approximately 5 millimeters (0.25 inch) square; the bottom half of the scale is used for this, the portion closest to the basal plate forming bulblets most readily. The explants are then placed in their culture vessels in a medium, wrapped in plastic, and stored at 21°C (70°F). Often a considerable percentage of the new cultures fails because of contamination by soil organisms. Those that survive produce visible bulblets within four to six weeks.
The medium in which the explants grow is a solution of basic fertilizer nutrients and a few complex vitamin-like compounds. Often, synthetic hormones are added; most lily media contain 0.03 milligrams per liter of naphthalene acetic acid (NAA) , a synthetic auxin. Sucrose (table sugar) supplies the energy for growth. The medium is gelled with agar or a proprietary agent such as Gelrite. The most important factor in the medium is the concentration of sugar, which supplies all the energy that a normally growing plant would derive from photosynthesis. (Tissue culture takes place in the dark.) Lilies need higher levels of sugar than many other tissue-cultured plants. Sucrose is added at the rate of 45 to 60 grams per liter of medium; during the latter stages, when bulblets are being produced, this may be increased to as much as 90 grams per liter.
The cultures are grown in complete darkness at a temperature of 20° to 24°C (68° to 75°F). They produce few if any leaves, which maximize production of new bulblets as the energy is directed to that end. After the culture has been initiated, it is repeatedly tested for virus until the technicians are reasonably sure it is virus-free. Then the multiplication stage starts, during which cultures are maintained in very active growth. Under optimal conditions, these cultures can be "subcultured" or divided into multiple new cultures every 8 to 10 weeks.
Subculturing takes place when the cultures have produced small clumps of bulbs 0.5 to 1 centimeter (0.25 to 0.5 inch) in diameter. These clumps are separated (a process called "singulation") and replated onto fresh medium. After 8 to 10 weeks, each forms a new clump.
Alternatively, the-small bulbs may be dissected into their individual scales. This produces more bulb lets but requires a longer cycle. Some varieties can also be subcultured by chopping the small clumps of bulbs into pieces about 1 millimeter square.
The last phase of tissue culture micro propagation of lilies is the rooting and bulbing stage, during which the subculture is made to produce as large a single bulblet as possible. The bulbing medium contains no growth hormones (because multiplication is not desired) and has an increased concentration of sucrose at 60 to 90 grams per liter. A period of 10 to 16 weeks is required to produce a high-quality bulblet.
Large single bulblets are the easiest to grow after they are removed from culture. Some laboratories produce small clumps of bulbs in the last stage, but the grower must invest extra labor in dividing these before planting, and losses in the first year are usually higher. Bulblets originating in the sterile environment of the tissue culture laboratory must be scrupulously protected from infection, especially by fungi, in early stages of growth. Timing is also critical: varieties requiring a vernalization period before spring planting must not be delivered too late.
When the bulblets are removed from the culture vessels, they must be washed thoroughly in running water, removing all traces of agar. The bulblets are then dipped in a fungicide to protect against soil-borne diseases, such as Fusarium and Cylindrocarpon. A combination of TBZ (Mertect) and captan has been very effective.
The bulblets are then packed in plastic bags, using a well-aerated medium with a low moisture level; a mixture of dry sphagnum peat and moist vermiculite has proven ideal. The bulblets and their roots must be kept under optimal conditions, neither too wet nor too dry. Pack only a limited number in each bag, and ensure that there is good aeration. The bags are stored for a minimum of 8 weeks (for Asiatic lily) to 12 weeks (for Oriental lily) before planting. In the spring they are planted in sterilized greenhouse beds, or in trays of pasteurized soil that are also kept in the greenhouse.
Much enjoyment can be obtained from deliberately crossing two different lilies in order to create a new one that has not existed before. The purpose of raising new forms can be to create new flower colors or to change some other characteristic. For example, a grower may wish to breed disease resistance into a particular form or to develop a form with shorter, more wind-resistant, stems. The parents must be selected carefully and the serious lily breeder should learn something about genetic inheritance. For the less serious breeder, different color forms are the main attraction of hybridization and through experimentation he or she will begin to learn how to gain some control over the crosses.
Lilies are very easy plants to pollinate as all the sexual parts are large and easy to locate. The basic idea is to transfer pollen from a selected plant (the pollen or male parent) on to the plant that is to bear the seed (the seed or female parent).
A bud that is about to open on the plant chosen as the female parent should have its perianth segments (petals and sepals) eased open and all the male anthers should be removed with a pair of sharp, pointed scissors. Care should be taken not to cut off the female stigma. The purpose of removing the anthers is to ensure that the flower does not self-fertilize. The flower-head should then be covered with a cotton bag which will allow in air but not insects and stray pollen. Some growers now use polythene bags with hoIes pricked in them but others avoid them as they can produce a moist atmosphere conducive to introducing rot to the cut stamens. An alternative favored by many growers is to encase just the stigma in a piece of silver foil, leaving the rest of the flower open to the air. After a couple of days the stigma will become shiny and sticky; this indicates that it is now receptive to pollen.
An anther should be removed from the male parent with a pair of tweezers and rubbed all over the stigma so that pollen sticks to the latter. The bag should then be replaced for a few days, in which time fertilization is carried out and the ovary begins to swell as the seeds develop. Once the petals have begun to wither the bag can be removed. The seed pod continues to develop until it is ripe and ready for harvesting.
It is essential to tie a label to the flower-head that has been pollinated. This label should contain information about the two parents plus the date of pollination. All this information should also be recorded in a more permanent form in a notebook along with subsequent details of the results of the cross.
Both hands and tweezers should be sterilized with methylated spirits (wood alcohol) before moving on to the next plant. Hybridization need not be confined to plants in your own garden. Anthers can be dried by packing them in a bottle or tube containing calcium chloride (a desiccator). The powder should be put in the bottom of the tube and kept in place with cotton wool. The anthers are placed on the cotton wool and then held in position with more cotton wool. The tube can then be sent to other growers. Another possibility is to put the tube in the warmest part of the refrigerator where it can be kept for up to two or even three months. It can then be used on plants that, under normal circumstances, could not possibly cross-pollinate with the male parent because they flower at different times.