At the University of Hawaii in Manoa, we operate a fungal disease laboratory and confirm the pathogenicity of new fungi on local plants. Healthy disease free plants are needed for these tests and plants are propagated by employing clean seeds or cuttings. These healthy plants are required to test the infectivity of new fungi, to document early symptoms and to record disease progress.
For ohia or Metrosideros polymorpha, we commonly use seeds for propagation. However, it takes many years to produce a plant ready for pathogenicity testing, as well as for retail or for out planting for commercial operations. Alternatively, by using vegetative cuttings, propagation is faster and clones can be made from valuable plants. The following describes basic procedures used to propagate ohia from cuttings.
Gathering cuttings: When going into the field to obtain cuttings of ohia plants, it is important to keep the cuttings hydrated and vigorous. Thus bring a bucket of water to place the cut ends of the cuttings in it. If specific trees are selected, bring newspaper to the field, place cuttings on the newspaper with the stem ends together, wrap cuttings in a bundle, tie or tape, label, and place cut ends in the water.
Cuttings are taken from the tips of plants and are cut into pieces about 5 to 10 inches (7 to 12 cm) in length. Avoid tips with flowers or young growth. Tips with expanded mature leaves are the best. If tips with flowers, buds, or young leaves are gathered, then remove them, at the time when cuttings are prepared for planting (Figs. 1 and 2)
Preparing cuttings for planting: Take each cutting, gathered from the field and cut into smaller pieces, each with about 8 to 12 leaves. Seven to 10 leaves per cutting will be common. The lowest leaves on the cuttings are removed, as well as any buds, flowers or young shoots. The remaining leaves are cut in half to reduce transpiration from the cutting. The cut stem ends are dipped into Hormex Rooting Powder that contains indole-3-butyric acid at 0.3%. Cuttings are then inserted in Sunshine Mix Blend #4. For the potting mix, placed it in a clean, large container, add water, and mixed until the medium is wet. Place the mix into pots (e. g. 4” in diameter), filling the pots then pressing the medium into a firm layer. The medium should fill about ¾ of the pot. Insert the cuttings into these pots. Place numerous cuttings in each pot (about 10-15). The greater the number of cuttings, the higher the humidity will be. This will discourage water loss and help the cuttings to retain water and initiate new roots (Fig. 3).
Location: Place cuttings in a cool location, with light, and preferably on a completely clean bench at least 24 to 30 inches above the ground. We grow the ohia cutting and plants in a glass greenhouse with a solid roof and screen walls located in Manoa valley. Fans keep the temperature under 30 C during the hottest periods but overall, 25 C would be better. Cuttings should be placed in a shade house or under solid cover. Gently water the cuttings at least once a day. Avoid areas that are windy as this will increase water loss from the cuttings. Some growers will also place the pots of cuttings into a tray then place the tray into a large clear plastic bag that is cut about 4 to 6” above the plant. The walls of the bag, keep humidity high although, the top is open. The bag is clear and does not interfere with light transmission. Drainage should be excellent and holes should be made on the bottom of the plastic bag to allow for good drainage.
Sanitation: The bench should be clean with no algal or moss growth. If there is such growth then scrub off the material and be sure to also scrub the under surface of the bench. Drench the bench with 10% bleach (one cup bleach and 9 cups of water). Following the bleach treatment, allow the bench to dry then rinse if needed. Clean the legs of the bench also. Clean the ground to remove algae, moss, fern and any weeds. Cover with new gravel, cinders, or weed mat.
Avoid pests: Be sure that there are no insects (ants, millipedes, fungal gnats) or snails or slugs. These pests will move pathogens into the pots and the cuttings will be contaminated. Even weak pathogens will cause problems for these tender cuttings. Thus, use new potting mix, clean pots, and clean benches. Environments that are slug and snail free are highly recommended. Growers can also use mist benches. However, be sure that there is no moss or algal growth on the bench or in the surrounding area. Adequate light is also needed. Clean the area that will be used for the ohia cuttings. Again keep slugs and insects out.
Transplanting: Cuttings are frequently rooted in 3 to 4 months. Remove the entire pot of cuttings, by laying the pot in its side, holding the cuttings and move the entire mass outwards. Pull all cuttings out, side-wards. Do not pull upwards or roots will be lost. Proceed slowly and avoid breaking or injuring any of the roots. Healthy roots are white. If any are brown, discard all the cuttings in that pot. Use Sunshine Blend 4 for transplanting the cuttings and pre-moisten the medium as before. Add about ¼ pot of moist medium to bottom of each pot and hold the cutting in the center while filling the edges of the pot. Fill each pot to ¾ filled with one cutting per pot. Add a small amount of fertilizer to a corner surface of the pot. The fertilizer used is Osmocote Slow Release fertilizer at triple 14. Water and return to an environment with at least 65% shade. As cuttings are established in 2-3 weeks, move them to an environment with more light. Once the root system expands, growth will be rapid. After a month or two, plants can be grown in higher levels of light. After a few months if they are getting too large for the 4” pot (Fig. 4), transplant to 6” pots, and eventually into 10” pots.
Janice Y. Uchida, PhD and Chris Y. Kadooka, Research Associate, both are with the Department of Plant and Environmental Protection Sciences at the University of Hawaii College of Tropical Agriculture and Human Resources.