As of May 17, twelve plants still survived, and most of those have grown several times larger than when they were planted. Figure 8 shows three of the largest, all of which were under water only a few days.
Causes of mortality
The largest cause of mortality was desiccation.
The second cause was inundation of the creek. The creek flooded over its banks during a mid-February storm, and water continued to flow for about a month. Seedlings were under water for durations ranging from around a week to a month. Survival was better for seedlings that were immersed less, but a few survived the long immersion. Some were killed by the immersion, and some were eroded away.
A deer browsed a mature plant and several small seedlings in Spring Canyon. One or two small seedlings succumbed, but others sprouted new leaves and survived. The mature plant has also recovered.
Propagation and planting
Willowy Mint flowers in May, and seeds can be collected in July or August. 2018 was so dry that the plants produced few seeds. We only plant willowy mints at each site grown from seeds collected near the site.
Seeds are relatively easy to germinate and to grow into young plants. However, my propagation results have been mixed, which has limited what we have been able to do. I intend to put more attention to improving propagation prior to next fall.
Our preferred method for planting now is to grow as many plants as possible in containers and to plant them soon after they grow their first true leaves. Our containers are for food storage, with drainage holes punched using a hot metal rod. Teasing seedlings apart for planting leaves most of the seedlings bare root, but that seems not to affect their growth. The time from seed to seedling is about two months, so the process begins in August or September.
This approach has some advantages. It is easy to carry a lot of seedlings in a backpack. Planting seedlings bare root means they immediately establish themselves in their native soil. Planting is quick. We make a small slice in the soil with a putty knife, drop the plants in, then gently push the soil back around the roots. These small holes are less disruptive to the soil than digging in pots. It is not clear to us that potted plants have a better rate of success than these small seedlings.
The plan for the future is to plant as many seedlings as we can as soon as the soil gets wet. It may help to plant seedlings multiple times during the rainy season. Seedlings several months old should have considerable advantages compared with seeds that germinate naturally in the rain.
Once planted, we will not irrigate them. I expect all seedlings to succumb in dry years, but enough to survive wet years to produce satisfactory results overall. Plants that survive one dry season without supplemental irrigation should generally be considered established.
Our big challenge now is to improve our propagation methods.
Appendix - Tests behind my house
Behind my house, there is a habitat that approximates a natural willowy mint habitat. It is a small stream fed by runoff from about 14 houses in the neighborhood above me at the top of a hill. The stream often runs when it rains. The soil is cobbles and sand, and it gets no supplemental irrigation. This site is about 3/4 mile west of the San Clemente Canyon site.