Originally written by Jeff Marcus (Floribunda Palms and Exotics) and Ken Banks for April 1999 issue of Palms.
With most palms, propagation from seed is not difficult as long as a few basic requirements are met. Among the most important are fresh seed, good sanitation, proper medium, proper hydration, and adequate heat. Each of these points will be discussed separately, although they are inter-related.
The fresher the seeds are, the better the results will be. To check the freshness of your seeds, cut open a sample seed and inspect the endosperm and embryo. The embryo should be fresh, firm, and not discolored. If the interior of the seed is rotten or has an unpleasant odor, it is unlikely to germinate. The endosperm is of two types, homogeneous or ruminate, and may be hard, oily, or even hollow. If the inside of a homogeneous seed is off-color, such as brown or gray, or if it smells bad, the seed is old or was harvested before maturity. Such seeds are also unlikely to germinate. In a ruminate seed, the seed coat is infolded, creating dark, tangled streaks in the endosperm. Ruminate seed is more difficult to assess because of its more complex appearance.
Removing the Fruit Pulp
The fleshy or fibrous fruit pulp frequently contains growth inhibitors. Removing it before planting will improve results. Methods for doing this vary with the quantity and type of seeds, but most begin with a preliminary 48-72-hour soak in water. Soaking causes the pulp to ferment, which weakens it for easier removal. Change the water daily during the soak. Fruit that is slightly immature should be placed in a tightly closed plastic bag and kept in a warm spot for a week or so. This promotes ripening and softens the outer flesh for cleaning. Sometimes the seeds need to be soaked further to soften the pulp, sometimes not.
There are several ways to remove the seed coat. With small quantities of seeds, simply rub them by hand against a fine-meshed screen and wash away the pulp with water. Another way that works well with small amounts of seed is to shake them by hand in a closed container with water and small, rough-edged rocks. Pour off the water and pulp occasionally, add more water and shake again, until the seeds are completely clean. Seeds can also be cleaned with a knife or other sharp tool, but this is slow and a little dangerous.
Motorized cleaning devices make the job easier and are a necessity for commercial operations. For smaller quantities, use a rock tumbler. Put rocks and water inside with the seeds. Larger seed-cleaning machines can be purchased or fabricated. Some large-scale growers and seed dealers use cement mixers to do the job. The seeds are rotated in the drum for 10-45 minutes with water and rough-edged rocks of 7-10 cm. The time will vary with the machine and the type of seed and rocks. Some seeds are brittle, and without proper care may be damaged by power cleaning. Among large-seeded palms, Actinorhytis is particularly brittle and prone to damage, and many smaller seeds, such as Pinanga, must also be handled with care. When cleaning seeds, remember that the flesh of some types contain crystals of calcium oxalate, a skin irritant that can cause severe pain on contact, depending on the individual’s sensitivity. For this reason, Ptychosperma, Arenga, Caryota, and Wallichia should be handled with care.
|Fig 1. Right, a seedling of Phoenicophorium borsigianum showing adjacent germination. Left, Phoenix sp. showing remote germination. (Illustration by Marion Ruff Sheehan from Genera Palmarum, A classification of palms based on the work of Harold E. Moore Jr.)||
Damaging insects such as seed-boring beetles may arrive with seeds. They may reduce germination and spread to other seed batches. To minimize these risks, seeds collected from the ground, whether in the wild or from cultivated plants, and seeds collected under unknown conditions should be soaked in a contact insecticide solution once the fruit pulp has been removed. The insecticide solution should be prepared at the same concentration you would use to spray for pests. Soak small, thinner-shelled seed, such as Pinanga, for 15 minutes. Soak larger, harder and less permeable seeds longer, from 20 to 45 minutes. Examples of these latter seeds are Mauritia flexuosa, Bismarckia nobilis, Parajubaea cocoides, and Jubaea child. After the insecticide soak, rinse the seeds in clean water for 20 minutes.
After cleaning the seeds, hydrate them by soaking them in water for 24 hours, especially if you did not soak them to help remove the pulp. Within 24 hours most fresh, viable seeds will sink. There are exceptions such as Manicaria saccifera and Metroxylon vitiense, whose viable seed will float even after cleaning and soaking.
Whether or not to discard a batch of heavily infested, damaged seeds depends on their rarity and your ability to get more. For very rare seeds, when even a single germination could be valuable, plant it. Remember, however, with heavily infested seeds, especially in large quantities, there is the danger of introducing pests into your nursery. Balance this risk against the desirability of propagating the seeds and follow the treatment procedures described above.
Fungi flourish in the heat and humidity necessary for good germination, so equipment, fixtures, seeds and growing medium must be kept clean to prevent damping-off and other disease problems. You may want to soak seed in a fungicide before planting.
Germinate the easy varieties in a commercial mix of peat moss or sterile sphagnum moss mixed with an equal amount of perlite or vermiculite. You may also use commercially prepared, finely cut coconut coir to which the same fast-draining material has been added. Sand, wood chips, screened rock or volcanic cinder screened to a maximum size of 9 mm can substitute for vermiculite or perlite. Whatever you use, the medium should be very porous and drain extremely well. All containers should have plenty of holes in the bottom to ensure quick and thorough drainage.
When containers and planting medium are ready, lay out the seeds on the surface, and before covering them, dust with a commercial insecticide. Bury the seeds in the medium to a depth of half the seed diameter and then cover everything with finely screened cinder (3-6 mm particle size), thick enough so it will not wash away during watering. This top-dressing dries out quickly and discourages the moss that grows on peat. Sand or finely crushed rock would work just as well. When planting is complete, place the containers on clean benches, 60-90 cm above the ground. Be sure to label your containers with a waterproof and fade-proof marker.
Palm seeds known as remote germinators may require special treatment and a little extra patience. Remote germinators push a shoot downwards as much as 20-25 cm before sending up the first leaf (Fig. 1). The larger ones such as Voanioala and Borassodendron, should be planted in deep containers such as citrus bags or large tubs, or be transferred to such containers soon after germination. If seeds and seedlings can be protected, the collector may want to plant large remote-germinators directly in the ground.
Hydration and Heat
At this point, the most important factor in seed germination is proper hydration, followed by constant high heat. Maintaining proper hydration is the trickiest of the two. Water your containers thoroughly, but just as important, let them dry out thoroughly before watering again. Over-hydration can drastically reduce the germination percentage. Once seeds begin to germinate, the containers will require more frequent watering. Seeds should be kept at 26-35’C. Some growers provide constant bottom heat by means of electric pads on their benches.
For difficult seeds and rare seeds, the most reliable method of germination is the Plastic Bag Method. For this method, seeds are blanketed in damp sphagnum moss and germinated in zipper-type, re-sealable plastic bags. Thoroughly saturate the sphagnum moss with water and wring it until no more can be expressed. Place the seeds and the sphagnum moss inside the plastic bags (along with a label) and keep the bags at 26-35’C (Fig. 2, top). Check inside the bags periodically to ensure that the sphagnum has not dried out. Once seeds have germinated (Fig. 3, bottom), place them in community or individual pots containing the potting mix described above and the quick-drying top-dressing. When transferring germinating seeds from the relatively sanitary conditions inside the bags to pots containing ordinary medium, treat them to a precautionary fungicide drench. Germination setups can also be improvised from plastic foam boxes with tight fitting lids, such as are used to pack fish or fruit. Fill the boxes ‘/3 full of fine perlite pieces and lay the seeds on top. Use a hand mister to dampen thoroughly the seeds and perlite, replace the lid and place the box in a warm location. These germination boxes are space-savers, because they can be stacked. The tight-fitting lids help keep out fungus and insects, but the boxes should be checked periodically for hydration and germination.
A final method (if it can be called a method) is simply to germinate the seed on the ground in an out-of-the-way part of the greenhouse or garden. Growers have had good results this way with Pelagodoxa henryana, Jubaea chilensis, and some Acrocomia species. Discarded seed has also been found germinating in many a surprised grower’s compost pile.
Fig 2. Germination bags suspended on a string near a metal roof where temperatures stay at 26-35°C (78-98° F)
Fig 3. Seedlings germinated in plastic germination bags and sphagnum moss. They are ready to be transferred to pots. Photos: B. Langer