Brassica tournefortii, Sahara Mustard

Sahara Mustard Germination Experiment

In order to see how many Sahara mustard seeds might have survived three years of drought, I nabbed some soil samples from road right of ways from the Borrego Springs Area, to water heavily to see what comes up. A number of us would like to know if there are Sahara mustard seeds lurking in the sandy soil of the Borrego Springs area, just waiting to germinate the next time we get a heavy rain year like 2005. This experiment to find out was suggested by Craig Dremann, who also advised me to add fertilizer to break the mustard seed dormancy (but see below; this was not needed for the seeds germinated in a moist paper towel).

Ted Caragozian kindly also gave me some seeds that he had gathered some time ago and stored in a sealed container. Ted estimates he collected those seeds 7-8 years ago (which would be 2006 to 2007), making them a perfect sample to see if they can still germinate after that time.

Note that the longevity of seeds kept in a jar is a different question than the longevity of seeds in the ground! Seeds in the ground are exposed to significant environmental variation as well as predators. Further, the lifetime of the seed in soil may be different in the Borrego Desert than in the environment in which Sahara Mustard evolved, since the pattern and variation of rainfall is different in those locations. In particular, light rainfall here followed by a significant period of no rain may germinate the seeds, which then die because they aren't accustomed to such a long interval between rains in their native environment. This is a significant possibility, since Sahara Mustard will germinate after rainfall that is insufficient to germinate our native annuals.

The timing of this germination experiment is good, since in several recent years we only had germination in the desert in February. So it is not too late in the desert season to expect germination.

Soil samples (actually sand samples, since that is the "soil" on the desert floor where Sahara mustard dominates) were gathered on 5 February 2015 from the top ~5 cm of the ground, and placed in plastic bags. The contents of each bag were placed in small basins in my fenced vegetable garden beds on the morning of 6 February 2015, which will be called Day 0, and watered heavily, equivalent to perhaps an inch of rain in the basins. On 7 February 2015, Day 1, I gave the plots a dosage of fertilizer, which is said to "break the dormancy" of mustard seeds (although this probably was not needed; see below). The dosage was 1.5 tablespoons of Miracle-Gro fertilizer in 1.5 gallons of water, the dosage recommended by Craig Dremann, which is also the dosage recommended on the Miracle-Gro box. This amount of liquid was equivalent to about 1/4 inch of "rainfall" for the basins.

The soil in that garden bed is good soil that has been enriched with compost repeatedly over 20 years, and has grown a number of crops. It has been fallow since a summer zinnia crop was taken out in early December. I weeded the area of the small number of small weeds prior to creating the small basins. Although it has been over a month since the last significant rainfall on 12/31/14, the soil was still moist beginning just an inch below the surface. Thus the inch of watering on Day 0 was enough to create a fairly-deep moist soil profile.

See photo of Sahara Mustard seeds spread on the ground surface before being covered by a thin layer of compost. Ted's seeds were also mixed with the soil in one of the same small bermed areas that the sand samples were placed in.

Since the initial waterings, I have kept the upper soil surface moist so that any germinating seedlings would not dry out.

The Sahara Mustard Germination Experiment using a Paper Towel

Thanks to a suggestion by James Dillane, I also placed some of Ted's seeds in a wet paper towel at 2 p.m. on Day 0, which I then folded in half and rolled up. I put it in a transparent Ziploc bag, which I placed in a bright window that receives no direct sunlight.

The paper towel test gave the quickest answer: just 30 hours later, a number of seeds had cracked their seed coat and started to grow roots. See Fig. 1 for pictures of the seeds in the paper towel.

Day 1

Day 2

Day 3

Fig. 1. Left: 7 to 8 year old Sahara Mustard Seed 30 hours after being placed in a wet paper towel. The root has emerged from the seed and is growing. Middle: Same seed 48 hours after being placed in a wet paper towel. The root has lengthened considerably (to ~10 mm) and the cotyledons are emerging. Right: Same seed 72 hours after being placed in a wet paper towel; the cotyledons are ready to receive sunlight. Click on the pictures for larger versions.

Nearly all the seeds germinated 48 hours after being placed in the wet paper towel: see a random shot of the seeds; cotyledons emerging from two seeds; and the 10 mm roots of some seeds. A quick examination of the seeds on Day 2 showed that at most just ~10 of the ~125 seeds had not obviously germinated, and even those seeds may have produced a root hidden by the seed.

On Day 3, I examined the seeds more carefully. I counted 116 seeds that had germinated, and 11 that had not yet germinated. See roots growing through the paper towel and some of the germinated seeds.

On Day 8, I examined the 11 that had not yet germinated. Of those, 4 had germinated. I squeezed the remaining 7 seeds with a tweezers. One remained firm; for six their contents gushed out. See seeds on Day 8 that had sprouted by Day 3; seeds that germinated between Day 3 and Day 8; and a closeup of the unsprouted seeds which shows the gelatinous coating of the seeds.

The final count on day 8 is that 120 seeds out of 127 had germinated, an amazing 94%. The reader can decide how to count the seven seeds that had not germinated by day 8. It is possible that six of those seven seeds are just severe laggards, with one seed remaining dormant. It is also possible that something went wrong in their germination process, and they would never have germinated.

Thus I quickly obtained one answer, thanks to Ted's 7 to 8 year old seeds: Sahara mustard seeds preserved in a jar can germinate within eight days at a 94% rate after seven years, with 90% of the seeds germinating within three days. Furthermore, nothing other than moisture and oxygen was needed to break their dormancy (i.e., they did not need to be exposed to nitrate in fertilizer).

Field survival of seeds is a different question, which may be answered by the soil samples. For example, if there is a germination rate of 90% in the next year after the "year zero" seeds were mature, that only leaves at most 10% of the original "year zero" seeds to germinate in the second year (there might be some seeds that will never germinate), and probably 1% or fewer of the "year zero" seeds to germinate in the third year or later. Hence field survival of seeds might well be much less than seven years.

The Sahara Mustard Germination Experiment using Soil Samples

Photos of the field plots on Day 0 are shown in Fig. 2.

Day 0, before watering

Day 0, after watering

Fig. 2. View of seven bermed basins. The six closest to the camera have sand soil samples from the Borrego Springs Area; the farthest basin has 7-8 year old Sahara Mustard seeds that had been preserved in a jar mixed with the garden soil. Left: Before watering, with the quart-size Ziploc bags that held each sample. Right: After watering. Click on the pictures to get more close-up views.

The first Sahara mustard seedlings emerged on Day 5. By Day 8, a number of seedlings had cotyledons spread above ground. The seedlings came up much more densely in the small basin where I had spread the seed and mixed it into the dirt, than in the area around the basin where I had spread the seed on the ground and lightly covered with compost. Roughly equal densities of seeds were spread in both places. The difference may be due to the larger amount of water that went into the basin, and / or possibly the longer time the water spent in contact with the seed.

As of Day 8, nothing had sprouted in the Borrego Springs area soil samples, making for a stark contrast with the basin that had been seeded with 7-8 year old viable Sahara Mustard seed; see Fig. 3.

Fig. 3. View on Day 8 of the farthest two basins shown in Fig. 2. Nothing has germinated in the Borrego Springs soil sample, whereas abundant Sahara Mustard has germinated in the mustard-seeded basin. Click on the picture for a larger unlabeled version.


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Copyright © 2015 by Tom Chester.
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Comments and feedback: Tom Chester
Updated 14 February 2014.