Several years ago, I bought a small chocolate mimosa tree which was grafted onto the Albizia julibrissin root stock. I generally keep track of the zone of plants, but this particular nursery was known for only selling things that would grow in our area. Unbeknownst to me, our area was being “upgraded” to a zone 6 due to someones’s idea of global warming. The graft was supposed to survive in zone 6. That same year, we had a very hard, deep freeze and the grafted part died; but the root stock remained strong and has grown year after year into a beautiful tropical looking tree. It has now survived several brutal winters, so I take this as a sign that this tree is hardy to zone 5.
It began producing seeds the last couple of years and I wondered about growing some trees from that. Even though it is one of the last trees to put on leaves every year, it provides quite a bit of pizazz in mid to late summer, even into late fall until it freezes hard. I know that in some areas it is considered invasive, but my observations lead me to hypothesize that it doesn’t sprout well outdoors here in southwest Idaho. I have never seen any seedlings in the lawn (which readily grows maples tree and elm seedlings), and I have never seen any other mimosa trees in my area.
After reading a bunch about sprouting tree seeds in general (The New Seed Starter’s Handbook is a very useful book to have around.) and mimosas in particular, I decided to give it a try even though there was some doubt that a single tree could produce viable seed. This last concern made me lean toward methods with which I could observe at least the initial stages of sprouting, but I also wanted to compare different suggestions.
I decided to set up a bit of a comparative experiment using variations of the suggested ways to sprout mimosa tree seeds. But, first I needed to get the seeds out of the pods. The seed pods look like bean pods, but they definitely don’t snap open like bean pods. The only way I was able to get any seeds out was to crumble the papery covering. I was glad I had saved several pods, because not all of the seed looked mature, judging by size. I only saved the seeds that looked the largest. (click on any photo to enlarge)
Next, I divided the seeds into 2 groups. One group had the outer shell slightly sawed at, or “nicked,” by a kitchen knife. The other group was be untouched. I put each group of seeds in a different glass bowl, labeling them according to process, date, and time of day. I ran the tap water until it was the same temperature I use to activate yeast, that is, warm enough to be uncomfortable on the wrist, but not to burn. I put enough of this temperature of water in each bowl to cover the seeds completely and allow for some evaporation. I set the bowls in a warm corner of the kitchen and covered with a square of wax paper, to keep bugs and debris out.
I checked the next day, and maybe the next, but for sure 4 days later, almost to the hour. The seeds that had had the outer shell slightly damaged had obviously plumped more than those that were not scratched at all. Of those that were not nicked or scratched at with the knife, only a couple of them had barely started to absorb water. Some of the really plump seeds had sprouted short, stubby roots. A couple had even nearly shed their outer shell.
It was time to plant some of the seeds. I chose the seeds with the best root development, including the 2 with the outer shells coming off. I gently helped those shells off the rest of the way. I lost track of which pot one of those was planted in, but I labeled one of them. I also planted the 2 un-nicked seeds that were starting to absorb water. I tried to keep any sprouted roots pointing in a downward direction and covered the seeds to about twice their diameter with the potting soil mix I use for all my seed starting.
After misting the pots well enough to settle the soil and make sure the seeds were still going to be pretty moist (but not creating a lake in the seed tray), I covered the whole tray with a clear trash bag and put it in my warming cupboard. I will check it every day and will probably end up removing the plastic after a few days. From what I read, I can expect to see growth above the soil anywhere from 2-4 weeks, so longer than the average vegetable seed.
I drained and rinsed the remaining seeds, then added clean water to the bowls. They are back in the warm corner. Some of the nicked seeds that I didn’t plant have roots just beginning to poke out, so I will have to decide what to do with those soon. I am interested to see if the unnicked seeds sprout at all in the bowl. All in all, it looks hopeful that I have viable seed, which should be particularly suited to our climate since it was produced here. The final test will be if I get normal looking little trees!