by H. M. Butterfield
This article first appeared in the California Horticultural Society Journal, April, 1967 Vol. XXVIII, #2. Please do not reproduce without
The principles and practice of seed germination are of great practical importance to plant propagators.
A review of some of the essentials will be given here, and a few definitions that may help others to understand the discussion and the directions.
A seed is a fertilized and ripened ovule, usually in a resting stage. The ovule contains an egg nucleus, in a socalled embryo sac, and other essential parts that ultimately produce a healthy seedling. When viable pollen which has an affinity for the species and variety being pollinated reaches the receptive stigma of the flower, a pollen tube will be formed that pushes its way down to the ovary. The pollen tube contains two sperm nuclei. One of these units with the egg nucleus; and this fertilized egg then divides repeatedly to form an embryo, and seed development follows. The other sperm nucleus fuses with another nucleus in the embryo sac; and the product of this fusion divides to form the cndosperm.
There are many causes of faulty natural pollination in plants, and artificial pollinators may have to be available at the proper time. Insects and wind may help carry viable pollen to the stigmas. In the case of certain selected and cultivated varieties, such as the avocado, the stigma may be receptive at a time when the parent tree is not shedding pollen or the parent plant may be shedding pollen when the flowers are not receptive to pollen. The stigma normally forms a sticky, sugary material in which the pollen grains can germinate and push on down through the style to reach the ovary and bring about fertilization of the egg. There must be a proper affinity between the pollen plant and the seed parent. When listing the parents of artificial crosses, it is customary to list the seed parent first and the pollen parent next. Date of pollination should always be recorded when known.
In the case of citrus seeds and a few other plants, normal pollination which sets up embryo development through the sexual process may be accompanied by the formation of embryos which develope by budding of maternal tissue into the embryosac. Seedlings developed from such embryos are genetically the same as the mother or seed parent. These embryos which are not the direct result of pollination may require the stimulation of normal pollination to start forming but are asexual in nature. This poses a problem when it is desired to combine the characters of two varieties or species.
The plant breeder may also find that pollination has taken place within the flower before the petals open. The breeder gets around this trouble by emasculating the flowers, that is, removing the anthers, well before any pollen has been shed. Prior to and following hand pollination, the flower is “bagged” to prevent any other pollen from reaching the handpollinated flower.
Seeds need to be handled carefully to avoid damage to the embryo. Rough handling at threshing time can result in a lower percentage of germination. Rains or excessive moisture at the time of harvest cause trouble, whereas a long dry season at time of harvest favors a much higher grade of seed. Keeping seed in a plastic bag while there is still too much moisture can result in mildewing and rapid deterioration.
Consequently seeds should be dried before storage in dryair containers. It may help some to keep the seed cool at 35 to 40’ F. during storage, or else plant promptly, as in the case of rose seed, oak acorns, or maple seeds, and various other seeds that have a short life. In’ all cases there must be a mature embryo inside the seed coat if germination is to take place. Cracking a few seeds will usually show if the seed has filled out properly. There are state and federal laws which set germination standards for many field seeds being shipped, so the buyer will know that their germination percentage will be reasonably good. Most reliable dealers destroy seeds that have deteriorated. Actual tests are made at intervals to insure a good percentage of germination.
Germination refers to the planting of seed and the resumption of growth causing seedlings to appear. In some cases it is difficult to break the rest period. The seed may remain alive for months or years until the proper conditions appear and overcome the block. Many seeds will germinate in a wpek or two or three weeks, but some seeds may take weeks or even years until barriers to germination are removed. Methods for avoiding or breaking the blocks will be discussed later.
Kinds of Seeds
Seeds have been classified in various ways with the hope that the proper method for germination can be selected. Seeds may come from a temperate zone or from the tropics, or possibly from the dry desert. Seeds may be starchy, proteinaceous, or fatty. The longevity may be known or estimated. Certainly the propagator would like to know approximately when germination can be expected after planting. If special treatment has been given the seed, then the period for germination is counted from the time of planting to germination. The great majority of popular flower seeds will require only a week or two or a month at most to germinate. Some seeds of woody plants take more time. Some of the seeds that may take 60 days for germination... include: Species Arbutus, Berberis, Carpenteria, some Ceanothus, Cercis, Cornus, Dendromecon, Rhamnus crocea, and Matilija poppy Romneya coulteri. Seeds of Garrya elliptica and Platanus may need as much as 135 days to germinate.
Most seeds of ornamental plants remain viable for at least three to five years, and some longer. Tests for seedlongevity were conducted by the Seed Laboratory of the California State Department of Agriculture over a period of ten years. In most cases seed should be fresh to germinate well, but some extremely long lives have been reported by the Gardener’s Chronicle of London and by the Royal Botanical Gardens at Kew. Albizzia julibrissin seed germinated after 149 years. Seeds of Cytisus and some other legumes germinated after 80 years or more. Lotus Nelumbo nucifera seeds from Manchurian peat deposits germinated after 850 to 1250 years, according to a test by the residual carbon 14 isotope method. But the “mummy” seed, of wheat and peas, from Egyptian tombs was dead in all authentic tests and soon disintegrated. Generally, there is little use in trying to hold seeds for a long time.
Special Germination Habits
Occasionally a species may have some peculiar germination habits. The seeds of tree peonies when planted will form a root and then become dormant before the shoot epicotyl starts to grow. After a rest period, the shoot will start to grow and reach the surface. A failure to know about such a peculiarity might lead to the assumption that the seed was not viable.
Seeds of some hybrid irises and lilies have failed to germinate when the endosperm remained. By separating the embryo from the endosperm and culturing the embryo in the Randolph Cox culture medium, breeders have been successful in germinating such seedlings. Even seeds of some common bearded irises may take more than a year to germinate if not planted when fresh and not kept too dry.
Light and Dark
Most popular flowers seeds germinate well at a temperature of 65 to 70’ especially if there is some alternation of day and night temperatures. But some seeds need to be exposed to light to break the rest period. In some seeds investigators have found a pigment which affects the germination according to the wavelength at which it is illuminated. A propagator handling lettuce seed should know that red light is most effective in germinating this seed. Other seeds may respond to farred light. Often, only a very low light intensity is needed to affect seed germination; however, it has been observed that birch seed has failed to germinate under a canopy of leaves in the forest, whereas, the same seed germinated well in an open area.
Many wildflower lovers have noticed that desert flowers appear at their best in seasons with copious, early rains. The seeds remain dormant for several years or until a season with heavy rains comes along. In some way the heavy rains wash away the inhibitor and then germination takes place. Whatever the inhibitor is, it is not much affected by scanty rain. Usually, however, there will be enough seed that does not germinate left in the soil to maintain the stand of the wildflowers, even if the new crop of plants should fail to mature.
Special Kinds of Seed Treatment
Propagators have tried to find out what kinds of seed treatment will help break the dormancy of seeds, especially seeds with hard or oily seed coats. The treatments include seed stratification, hot water treatment, acid bath, and scarification, or a combination of these. But readers should be reminded that there may be very complex organic substances and unknown processes that affect germination. We do know something about the subject, but further study is needed to learn more about the causes of delayed germination.
Seeds in the temperate zones of the world often remain covered with humus for weeks or months and are subjected to chilling temperatures which help break the rest period. The propagator may likewise use artificial stratification to hasten seed germination. Sand, peat, sphagnum moss, and other materials have been used in which to stratify the seeds for chilling. Layers of nuts or seeds may be placed in a container and chilled to hasten seed germination. The initial temperature for chilling is about 35 to 40’ F. The stratification medium is kept barely moist during the period of chilling. After about two and onehalf to three months the seed with container may be removed to room temperature 65 to 70’ F. ; and then, germination will often take place in a few days or weeks, depending on how old the seed is or on the hardness of the seed coat. Once the seeds start to germinate, they can be lined out in a nursery row. On a small scale, the seeds may be planted in the usual way in a pot, and then the pot held in cold storage as suggested above. When removed to room temperature, germination should take place without having to disturb the seed.
Hot Water Treatment
Seeds with a hard or oily seed coat will sometimes germinate more readily after treating with hot water. The water is heated in a pan to 180’ F., then removed from the fire, and the seeds dropped in so they can soak for 12 hours or overnight. Some legume seeds respond very well to this hot water treatment.
Acid Bath Treatment
Seeds with a very hard or oily seed coat may be treated in an acid bath to hasten germination. Sulfuric acid, technical grade, with a specific gravity of 1.84 has been used. A glass jar with perforated lid is used to hold the acid. The user should protect his eyes with goggles and hands with rubber gloves, because the acid can spatter when it comes in contact with water. Mixing of the acid with water should be done very gradually by pouring the acid very slowly into the water not viceversa!. The volume of acid should be about twice that of the seed to be treated. The seed should be dry when covered with the acid. Allow the seed to remain in the acid for one to four hours or more, according to the kind of seed and how old it is. At the end of the acid bath, pour off the acid and save it for future use. Introduce a small rubber hose through theperforated lid, and gently run water on the seed for about two hours. Finally, after about two hours of washing with water, drain the seed and add a teaspoon of washingsoda for each quart of contents, to neutralize any remaining acid. When bubbling in the water stops, wash well and drain. The seed is then ready for planting.
To give some idea of how seeds respond, it may be reported that Rhus ovata seed has to be treated for one to six hours. Rhus integrifolia has been kept in the acid bath for four hours. Cornus nuttallii has been kept in the acid bath for four hours and then stratified for three months. Symphoricarpus albus seed germinated best after being in the acid bath one hour ‘ then stratified for six months. These tests reported by N. Mirov show to what extremes a propagator will go to find a suitable method for germinating seeds.
Where the seed coat is hard and moisture cannot penetrate to the embryo, the seed coat may be chipped with a knife, scarified with sand, or rubbed with sandpaper. Sometimes the seed has to be cracked, but great care is needed to avoid any damage to the embryo within. Some legumes lend themselves nicely to scarification on a commercial scale. Thus, clovers used in field culture have been scarified to improve germination. Cytisus scoparius seed may also be scarified to hasten germination. Two hours in a good sand scarifier drum should be adequate for largescale treatment.
Control of Damping Off
The propagator may be frustrated in getting seed to germinate without damage from certain soil fungi, commonly referred to as “damping off” fungi. Fungi may kill the seedling before it emerges preemergence dampingoff or may destroy the seedling tissues near the ground level soon after the seedling emerges postemergence dampingoff. If seedlings are planted too closely, or the soil surface is kept extremely wet, or the temperature is too low, a fungus may ruin a whole planting. By partial sterilization of the planting medium and regulation of the soil moisture and soil temperature, this danger can be greatly reduced. There are soil drenches which are effective in preventing and arresting this fungus.
The writer has successfully used boiling water to treat the planting medium used for seeding, also sand covering. This is only partial soil sterilization but it helps.
Chopped sphagnum moss, run through a sieve with three meshes to the inch, has helped prevent dampingoff fungus from growing around seedlings. This chopped sphagnum moss may be used to fill a planting container or to top the soil in the container. Seed is planted directly in this moss and watered without any sterilization.
Nutrient Agar as a Planting Medium
Sterilized nutrient agar has been used to germinate orchid seeds. The Knudsen formula is one commonly used for germinating orchid seeds. A similar agar formula has been used to germinate small seeds, such as those of azaleas and rhododendrons. The RandolphCox agar formula has already been mentioned for germinating iris embryos. A nutrient solution has been used to float fern spores for germination.
Germinating Unusual or Unidentified Seeds
When a propagator is asked to germinate unidentified seed or seeds with unknown requirements, he does the best he can, having to guess at the best methods to use. Recently the writer was asked to germinate seeds of what may be Echeveria bicolor from Venezuela. The collector found the plants growing near orchids, mosses, and ferns at an elevation of about 5,000 feet on Mount Avila, five miles from Caracas in a rain forest area. With this bit of information and with over 25 years of experience in germinating Echeveria seeds, I made a good potting soil in a clay pot, and the top inch or so was screened to remove any lumps or hard materials. The planting medium was firmed and made level, then covered with a paper towel. Boiling water was poured over the surface and allowed to run through the drainage hole. The paper towel was used to keep the surface level and smooth. As soon as the soil had cooled, the seed was planted and barely covered with No. 4 sand, then the planted seed was drenched with a mist spray of water. The pot was covered with glass and kept warm at about 65 to 70’ F. Daily application of a mist spray of water was given.
Thus, the growing conditions were similar to those existing on the tropical mountain. The small seedlings began to show on the eighth day and many more appeared on the ninth day. Evidently this home method worked very well for germinating the very small seeds.
On another occasion the writer was handed a large seed from a tropical plant in Honolulu and asked to try to germinate it. This seed had been found on the ground in a park. Was it from a tropical shrub, tree, or vine? The seed had been carried around for about 11 years. Would it germinate? I looked at the seed and believed it was an oily one, and so treated it with hot water at 180* F. The seed was planted at a depth of about onequarter inch. In about three weeks a sprout began to show above the ground surface. It proved to be a vine from the tropics, but what vine? I checked with a friend familiar with tropical plants. He had an uncle who used to visit Brazil and the Amazon where this vine was native. The seeds floated down the Amazon and far out into the Atlantic Ocean. The vine has been called the “Itch Vine” because it causes a slight dermatitis on some persons. This is a case of working with an unknown.
At the present time the writer is working with another unknown plant seed collected by Michael Deems, an artist and teacher, in Venezuela, and still does not know what it isshrub, tree, or vine. The very small seeds were covered with a pulp, and before planting the pulp was removed as much as possible to avoid any chemical block that might be present in the pulp. After about four weeks of careful daily sprinkling, the first seedlings showed. At the end of three months about 15 seedlings appeared. Daily attention for three months takes time and patience, but here were seedlings from some unknown “pretty” tropical ornament. It got tropical care and results show that the care was about right. Only time will tell what the plant is. The care given seeds from a temperate zone, such as stratification, might kill the tropical seeds or seedlings. Methods have to be adapted to the kind of seed being germinated.
It is hoped that some of the techniques here discussed will help many others to become successful in germination work.