Willowy Mint Population Expansion Project

By Lee Gordon, CNPS-San Diego Garden Committee

This report summarizes progress on a project begun in 2016 to find and implement inexpensive methods to expand populations of San Diego Willowy Mint, Monardella viminea. Willowy Mint is a federally listed endangered species, partly because its natural range and habitat are so limited, and because some of its known populations have been in decline.

The purpose of this project is to study and develop methods, encompassing low cost and minimal labor, to expand existing populations and to establish new populations of willowy mint. Our intent is to maximize the number of mature new plants that survive at least 3 years, while minimizing the expenditure of resources.

I began work in November 2016, planting 5 plants grown in pots. We watered the plants when we first put them into the ground, but they got no water after that. One of these plants still survives.

Al Field has since joined me, and in January 2018, we planted 25 small seedlings, all grown in one small, rectangular container. We backpacked water to three, which survived until 2019. We did this because 2018 was a dry year. One of these three was washed away by high flow in a February, 2019 storm. The remaining two appear to be established now and should survive and grow.

On the basis of what we have learned thus far, we will grow and plant as many small seedlings as we can each year, with the expectation that there will be reasonable survival only in good years.

Figure 1. Willowy Mint sites. We get to the three Spring Canyon sites through Mission Trails Regional Park, and the San Clemente Canyon (MCAS) site through the military base. My home (Appendix) is below the T in "Fairbrook Estates".

Figure 1. Willowy Mint sites. We get to the three Spring Canyon sites through Mission Trails Regional Park, and the San Clemente Canyon (MCAS) site through the military base. My home (Appendix) is below the T in "Fairbrook Estates".

Project sites

Figure 1 shows the locations of four sites, three in Spring Canyon and one in San Clemente Canyon, on San Diego City Property that is accessible through the Miramar Marine Corps Air Station.

The three Spring Canyon sites are on property owned by the Sycamore Landfill (Republic Services). Two of these sites (Spring South, Spring North D) have nearby mature Willowy Mints. Spring North A is about 500’ from Spring North D.

The San Clemente Canyon site is about 200' upstream from a population of a dozen or so mature Willowy Mint plants.

Figure 2. Willowy mint planted in 2017 has survived two years without irrigation. This photo and the next two were taken May 8, 2019.

Figure 2. Willowy mint planted in 2017 has survived two years without irrigation. This photo and the next two were taken May 8, 2019.

Spring Canyon

In December, 2016, I planted five Willowy Mints at Spring South. These plants were grown in 4"x4"x9" pots, and their foliage was 4"-6" high. I watered them enough to get them to the rains, and they were on their own after that. One plant survived and is still alive after two summers (Figure 2). Three desiccated and did not recover. One plant placed in the creek bed was washed away in a torrential flow during heavy rains in winter 2017. The flow was unusually heavy, and it also washed away a mature plant that was a few feet from the seedling that disappeared.

In January 2018, Al and I planted 24 small seedlings and one in a pot. The small seedlings were all grown in one 5"x6"x4" plastic container. When planted, they were an inch or so tall and each had one or two pairs of true leaves. We planted them in 8 groups of 3 each, distributed among the Spring Canyon sites. The potted plant died quickly.

Figure 3. Willowy mint planted in 2018 and irrigated during the dry season.

Figure 3. Willowy mint planted in 2018 and irrigated during the dry season.

Figure 4. Willowy mint planted in 2018 and irrigated during the 2018 dry season.

Figure 4. Willowy mint planted in 2018 and irrigated during the 2018 dry season.

Because 2018 was a dry year, we backpacked water to the small seedlings during the dry season. We watered only the most vigorous plants, and the end result was that three survived to the rainy season at the end of 2018. All three were growing vigorously by February 2019, but one was then washed away during a heavy storm. The remaining two are still growing vigorously (Figures 3 and 4).

We planted no seedlings in Spring Canyon for the 2019 season because none of my Spring Canyon seedlings survived to be planted. Given the great rainy season, this was a missed opportunity.

San Clemente Canyon

In January 2019, we planted 25 small seedlings at San Clemente Creek. As before, each seedling had just a few true leaves. The seedlings were planted on the creek bed, at the side of the creek, and on the adjacent bench.

Figure 5. San Clemente Creek flooded by heavy rain (2/14/2019). All of the Willowy Mints planted at this site were under water at this time.

Figure 5. San Clemente Creek flooded by heavy rain (2/14/2019). All of the Willowy Mints planted at this site were under water at this time.

Figure 6. Willowy Mint planted in late December, 2018 and photographed February 27, 2019. This mint had been under water for about a week.

Figure 6. Willowy Mint planted in late December, 2018 and photographed February 27, 2019. This mint had been under water for about a week.

Figure 7 left. Willowy Mint that survived a month under water. Figure 7 right. Willowy Mint that survived three weeks under water. Photos taken May 17, 2019.

Figure 7 left. Willowy Mint that survived a month under water. Figure 7 right. Willowy Mint that survived three weeks under water. Photos taken May 17, 2019.

The creek was flooded by heavy rains starting in mid February (Figure 5). All of the seedlings were inundated, some for a few days, and others as long as a month. Figure 6 shows one seedling a month after planting. This seedling had been under water for about a week, and it had no signs of harm. Figure 7 shows two seedlings that survived inundation for three or four weeks.

Figure 8. Three seedlings that were under water for a few days or a week. Photos taken May 17, 2019.

Figure 8. Three seedlings that were under water for a few days or a week. Photos taken May 17, 2019.

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.

Looking forward

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.

Figure 9. Four year old Willowy Mint behind my house (April 2019). This plant has grown to a diameter of 3.5'.

Figure 9. Four year old Willowy Mint behind my house (April 2019). This plant has grown to a diameter of 3.5'.

I began planting mints at this site four years ago, and eight plants have survived at least three years with no mortality (Figure 9). Every summer, the plants turn crispy brown, and fresh green then leaves appear with the rains.

Figure 10. This Willowy Mint was flattened by stream flow during a January 2017 storm. It recovered and it now extends 2.5' along the stream bed, elongated by the action of the water.

Figure 10. This Willowy Mint was flattened by stream flow during a January 2017 storm. It recovered and it now extends 2.5' along the stream bed, elongated by the action of the water.

An interesting observation is that the plants withstand abuse from running water. Figure 10 shows a plant that was flattened by the stream in 2017. Another plant was completely covered by sand. Within a few weeks, leaves began to appear, pushing up through the sand. Both of these plants are doing well now.

Another observation is that the mints are perfectly capable of procreating, but they are not very good at it in their natural habitat. I have never observed immature seedlings in either the mint's natural habitat or near the small stream behind my house. However, about 30' from this stream, I planted one plant in an organic clay topsoil that I irrigate monthly with 1" of water. The plant is partially shaded by a tree. This plant has at least six offspring.