A variety of germination containers can be used for palms seeds, including pots and flats. Pots are better than flats du to the deeper soil column and better drainage. For very deep-rooted species, and especially those that bury the seedling axis, tree tubes, lengths of PVC pipe, or other improvised containers have been used to provide the extra depth that these seedlings require during early development. Some growers prefer to sow seed in large raised beds constructed from wood or cinder blocks. The most important consideration for any germination container is that it allows adequate drainage of excess water from the medium.
Palm seed germination media must be well-drained, yet have some moisture-holding capacity. A pattern of alternat extremes of dryness and wetness is detrimental to palm seeds during germination. Particle size in the medium should not be excessively large nor prone to separation with repeated irrigation. A 1:1 mixture by volume of peat moss and perlite has been successfully used under a wide range of nursery conditions. The mix in a germination medium should be adjusted depending on the conditions to which the seed will be exposed. For example, seed germinated in full sun will require a medium with higher water holding capacity than seed germinated under shade, all other conditions being equal.
The depth at which palm seed will be sown varies with size of the seed and the species being grown. More importantly, environmental conditions dictate depth of planting. If seed will be germinated in full sun, it is usually necessary to cover the seed with medium so that it will not dry out. A rule of thumb to follow is to cover the seed to a level equal to the diameter of the seed. For example, seed 1/4 inch in diameter would be planted at a depth of 1/4 inch below the surface. However, if the seed is to be germinated under shade, it is usually better to sow it shallowly. In the case of larger seeds, this means merely pressing them into the soil so that the top of the seed is exposed. Frequency of irrigation will also influence the planting depth. Seed germinated in full sun can be planted more shallowly if irrigation will be frequent enough so that the medium does not dry out.
The initial planting density depends on the ultimate use of the germinated seedlings as well as how quickly the nursery operator anticipates transplanting the seedlings. Many growers broadcast small (1/4 inch diameter or less) and medium-sized (1/2 to 1 inch diameter) palm seed very thickly in the germinating container, in some cases completely covering the surface of the medium with seed. This works fine (and saves space and labor) if the transplanted liner will consist of all or a number of the seedlings potted together (for example, areca palms,
Figure 9, and some
Chamaedorea species) or if the seedlings will be separated and transplanted before a great deal of root development has taken place. Otherwise, it is best to sow the seed with some space between adjacent seeds. Large seeds, especially those of difficult to transplant species such as
Bismarckia, are often sown one per container.
Figure 9. Areca palm (Dypsis lutescens) seed sown densely in a 4-inch pot. The germinated seedlings will be transplanted together to make a dense specimen plant.
Virtually all palms require high temperatures for the most rapid and uniform germination of their seed. Seventy to 100°F is the accepted range, and 85-95°F probably yields the best results. Seed of paurotis palm (
Acoelorraphe wrightii) has been reported to germinate best at 92-102°F, with only 11% germination below 86°F (Carpenter, 1988a). The native Keys thatch palm (Thrinax morrisii) and silver palm (
Cocothrinax argentata) germinated best at 91-97°F, with few seeds germinating below 77°F (Carpenter, 1988a; Carpenter & Gilman, 1988). Seed of pindo palm (
Butia capitata) germinated best with 2-3 weeks at 102°F, followed by 86°F for the duration of the germination period (Carpenter, 1988b). Some research has suggested that fluctuating temperatures at 12 hour intervals may increase total germination for certain species (Carpenter, 1987, 1989), but this is not practical for most growers.
Since palm seeds require high germination temperatures, it is best to sow seed during the warmer months of the year. If availability of fresh seed makes this difficult, soil temperatures can be increased by using bottom heat below the germination containers or by covering the containers with clear plastic. Placing the containers on a heat-retaining surface can also increase temperatures by several degrees.
Many palms germinate in the understory of a forest canopy in their native habitats, even if they eventually grow up into full sun (royal palm,
Roystonea spp., for example). Seedlings of these species can be germinated in full sun but their leaves may bleach to some extent under those conditions. Many growers feel that, despite the bleaching, root growth and overall seedling development are enhanced in full sun. Under shade, seedlings will generally have a deeper green color. Some species grow best in the shade (
Licuala spp., for example). Seed of the latter group should be germinated under shade. Seedlings of such species, if exposed to full sun, usually bleach severely, burn and may even die. Species native to open habitats show no ill effects when germinated in full sun. It is generally necessary to adjust seed planting depth according to the light levels to which the seed will be exposed (see previous section).
Palm seeds require uniform moisture during the first critical stages of germination when the cotyledonary petiole (in remote germinators) or button (in adjacent germinators) first emerges from the seed. Alternating periods of extreme wet and dry during this time period will usually have deleterious effects on total germination percentages. If the germination medium does not receive some type of automatic irrigation, it may be necessary to cover the containers with clear plastic to retain adequate soil moisture. Overwatering can be equally deleterious. At no time should standing water be visible on the surface of the germinating medium.
Palm seedlings do not require supplementary fertilization for the first two months after germination. The endosperm within the seed provides all the nutrition that the seedling needs during this period. Supplemental fertilization during the first two months not only wastes fertilizer but can injure the young seedling.
The rate at which palm seed germinates, the uniformity of germination, and the percentage of total germination can vary tremendously from species to species, from seed lots collected from different plants of the same species, and even from seed lots collected in different years from the same plant. Seed of Mexican fan palm (
Washingtonia robusta) may begin to germinate in less than 2 weeks, seed of areca palm (
Dypsis lutescens) in 3-4 weeks, while seed of parlor palm (
Chamaedorea elegans) may not begin to germinate for several months and then continue sporadically for over a year. When planting palm seed of species with which one has no previous experience, or for which no germination information can be found, one should remain patient as long as the seed appears in good condition. A fairly comprehensive list of germination data across a wide variety of palm species can be found in Wagner (1982).
Figure 10. Palm seedlings in the germination container ready for transplanting.
Palm seedlings may be transplanted either immediately after germination or after 1-4 leaves have formed. The ive is to lessen the degree of root disturbance to the seedlings; thus it is best to transplant before roots begin to circle the container or roots of adjacent seedlings become entangled (
Figure 10). Transplant in the warmer months of the year, when root growth will be rapid. Delay transplanting until at least one leaf has appeared. Seedlings will usually have one long root at the time of first transplanting. Seedlings should be first transferred from the germination container to a small liner pot that just accommodates the root system and allows some subsequent root growth. Deep liner pots with essentially open bottoms are being used by a increasing number of growers. Palm seedlings benefit from the deeper root run, and long roots emerging through the bottom opening are "air pruned" and cease growth, thus significantly eliminating the circling of roots around the inside walls of the pot. Two strategies are then possible for subsequent transplanting of the seedlings. They can be shifted successively to slightly larger containers as they grow (frequent small shifts), or they can be transplanted to larger containers than their size might seem to warrant (fewer and larger shifts). Frequent small shifts lessen the chance of loss due to over-watering, but increase labor costs. Transplanting into large containers lowers labor costs and provides for more unrestricted root growth, but may promote increased loss due to root rots when the seedlings are small. Thus, larger, less frequent shifts will require careful irrigation monitoring while the transplants establish in the new containers.
Palms are very intolerant of being planted too deeply, regardless of age or size. For palm seedlings, planting as little as 1/2 inch too deeply can result in severe production setbacks and ultimately death of the seedlings. Palm seedlings should be transplanted so that the point on the seedling stem just above where the root system appears to begin lies at the soil surface (
Figure 11). This point is sometimes marked by a noticeable swelling, particularly on older seedlings. On palms with adjacent germination, it is the point at the base of the button. Do not sever the connection of the seed to the seedling palm. If the seed is still attached to the plant by the cotyledonary petiole (remote germination), drape the seed over the edge of the pot or allow it to sit on the soil surface.
Figure 11. Palm seedlings should not be transplanted more deeply than the point indicated by arrow.
Some growers prune palm seedling roots when transplanting. This is not recommended, and usually results in growth setbacks or even death of some of the seedlings. If the seedling root is longer than the transplant container, it can be allowed to slightly curve upward or around the inside perimeter of the container. A better solution is use pots large enough to accommodate the full length of the root.
Some palm species (
Bismarckia and
Borassus, for example) bury the seedling axis for some distance below the ground. Seed of these species are usually planted singly in deep containers. A number of palm growers have found it beneficial to remove the soil from the container down to the level of the subterranean stem crown as soon as the first leaf emerges above the soil.
Ideally, newly transplanted seedlings should be placed under light shade (30-50%) for several weeks, or until new growth is apparent. If this is not possible, irrigation frequency must be carefully monitored so that the transplants are not water stressed during establishment.
For further information, see
Container Production of Palms.
Despite the overwhelming reliance on seed propagation for palms, there are several methods of clonal (vegetative) propagation that can be used for a few species.
Clustering palms, that is those that produce new erect shoots from a common base or system of rhizomes, can be divided carefully as a means of increasing stock. Species that produce new shoots at some distance from the parent stems (
Rhapis species, for example), are the most easily divided. Many
Chamaedorea species, Areca and other
Dypsis species, and Paurotis palm (
Acelorraphe wrightii) are amenable to this type of propagation. Stock in containers is generally easiest to divide. For best results in the field or landscape, it is advisable to separate divisions from the parent plant with a sharp spade in the spring, but leave the divisions in place until new growth is evident. At that time the divisions can be carefully lifted, with as much of the root ball as can be managed. Newly separated divisions are best potted and kept shaded and well-watered until established (at least 1 year), after which they can situated in the ground.
A number of
Chamaedorea species produce conspicuous short aerial roots at the stem nodes (leaf scars). These species can be airlayered by applying a swath of moist sphagnum peat moss around one to several nodes and wrapping the area in aluminum foil. The aerial roots will grow into the moss. When sufficient root growth has occurred, the stem can be cut from the parent plant and potted. Newly cut layers should be kept shaded and well-irrigated until established in their containers.
Several date palm species, most notably the commercial date palm,
Phoenix dactylifera, produce offsets or suckers at the base of the trunk. These can be cut from the parent plant and either planted in containers or planted directly in the ground. If no roots are present when the suckers are cut, the leaves should be reduced in number and/or size.
1. Collect palm seeds from source trees when the fruit is ripe.
2. Clean palm seed from the fruit immediately upon collection, air-dry it, dust it with fungicide, and either plant it immediately or store it.
3. Store palm seed dry in sealed plastic bags at temperatures above 65°F. Life in storage will vary from several weeks to over a year.
4. A 1 to 7 day presoak in water is recommended for palm seeds before planting. The water should be changed daily. A soak in gibberelic acid (GA3) is not recommended.
5. Germination containers and media should balance moisture retention and good drainage. A 1:1 by volume mixture of peat moss and perlite is widely used.
6. If seed will be germinated in full sun, it is usually necessary to cover the seed with medium so that it will not dry out. However, if the seed is to be germinated under shade, shallow sowing is best.
7. Palm seeds require high temperatures for best germination. Seventy to 100°F is the accepted range, and 85-95°F probably yields the best results.
8. Seed of most palms can be germinated in full sun, though leaves of some species may bleach slightly. Seed of understory palms are best germinated in shade.
9. Palm seedlings do not require supplementary fertilization for at least two months after germination begins.
10. The seed of many palm species requires several months to begin germination, and the total period of germination may last for over a year.
11. Transplant palm seedlings before the root systems in the germination container become greatly entangled. It is generally best to wait until at least one leaf has appeared. Transplanting is best reserved for the warmer months of the year, when root growth will be rapid.
12. Palms are very intolerant of being planted too deeply. Transplant palm seedlings so that the point on the seedling stem where the root system appears to begin lies at the soil surface.
13. Do not sever the connection of the seed to the seedling palm when transplanting.
14. Root pruning seedlings when transplanting is not recommended.
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Thrinax morrisii. Proc. Fla. State Hort. Soc. 101: 288-290.
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