Germinating Red Flowering Gum Seeds (Corymbia ficifolia)

We planted 48 seeds in each of three different  soils from Wheeler Zamaroni, to test which soil best encouraged germination.  For each soil type, we planted half in non-sterile soil and half in sterilized soil. We also measured the pH of each of the soil types, 5.9, 6.2, and 6.9 for Nursery Mixes #1, #4, and #5, respectively. Soil temperature was not controlled, and ranged from 18–32°C. Lights were on for 12 hours per day.

Germination occurred within 10 days, with sprouts continuing to appear for three weeks. The soil with the lowest pH had the highest germination percentage, although this might have been an artifact of seed selection. At the end of this experiment, 25 seedlings were healthy enough to transplant into larger pots to continue growing. Sterilization didn’t seem to have much of an effect, although the microbes present in the soil can certainly vary from batch to batch of soil.

On November 27, 2016, we planted two new sets of Corymbia ficifolia seeds. One set was planted in rock wool, a sponge-like fibrous material having no soil and no nutrients of any sort. We chose the best-looking seeds, with clear bulges. We created four subsets of 24 seeds, each subset having water with a different level of acidity, with pH = 5.5, 6.0, 6.5, 7.0. The desired pH was achieved using phosphoric acid, which also supplies some phosphate to the sprouting seeds.  We employed a heating pad with a thermostat in the soil at 25°C, held constant 24 hours a day. Lights were on for 16 hours per day. We also put clear plastic domes over the trays, to keep heat and moisture in.  This experiment allows us to judge the effect of pH from 5.5 to 6.9 on germination, and it also gives the seeds steady temperature 24 hours a day and humidity all the time.

Germination in the rock wool set (at all pH values) is hindered somewhat by the stiffness of the medium. Sprouts were clearly trying to emerge for slightly less than half of the seeds at 13 days, but many were having a difficult time pushing through the rock wool. Orientation of the seed appears to be key for success with this method. For best results, the thick side of the seed (where the root emerges) should be pointing down when the seed is planted.

The second set of Corymbia ficifolia seeds were planted in ordinary potting soil in a 148-well tray, with a heating mat at 25°C for 24 hours per day and covered with a humidity-containing dome. The bulk soil had a pH of 6.9 when we started, but we sprayed  the surface of the soil with a pH 5.7 solution of phosphoric acid to keep it moist and slightly acidic. This set of seeds has done the best of any of our trials to date. After 14 days, 69 percent of the seeds had sprouted. All look healthy, and more are coming up every day. The soil is soft enough that the seed can easily reorient if necessary to allow the root to go down and the leaves to come up.

Both the rock wool and soil sprouts have good healthy roots. On transplanting at the stage where the first true leaves are just starting to open, the roots are 2-3 inches long and nicely branched (see right).

Learn more about Corymbia ficifolia at:

Australian Native Plants Society

Garden Drum

Dave’s Garden