Yucca Species, Joshua Tree

There are specimans in Salt Lake City, Reno, and even Boise. They typically need excellent drainage, up on at least 2 - 3 foot berms, and...Read More can survive and thrive in desert climates with temps much below zero. There are many large Joshua trees in the area of Caliente, NV, where the temps regularly go into the below zero range. And I personally have several growing here in the Boise area --- some for more than 20 years.

Per Jan Emming owner of the Destination:Forever Ranch and Gardens, a 40 acre desert botanical garden and sustainable living homestead in ...Read Morethe Arizona desert with a nursery:

COMMENTARY ON MY PHOTOS;
1. http://davesgarden.com/guides/pf/showimage/386096/
Joshua tree flower clusters are about 18 inches / 45 cm long. They are scented with a musky, yet lemony, fragrance that appeals to the yucca moths they must attract to do the pollination required.

2. http://davesgarden.com/guides/pf/showimage/386097
Joshua trees flowering moderately in late February 2015, after no flowering at all in 2014.

3. http://davesgarden.com/guides/pf/showimage/386098
This close up of Joshua tree flowers shows the stamens arranged at the base of the pistil above the petals. At the tip of each stamen is a glob of sticky yellow pollen. The yucca moth females take this wad of pollen and purposefully pack it into a groove at the apex of the ridged, greenish pistil at the center of each flower. This completes the pollination of the flower, and the female then lays one or two eggs on the pistil. The developing larvae feed upon some, but never all, of the seeds of the growing pods. The fact that yucca pollen is too sticky to easily be transferred to the pistils by random accident and wandering insects other than the yucca moth means that the deliberate actions of the female moth are a critical link in the chain of reproduction for the species. By ensuring that the flowers get fertilized however, the female moth guarantees that her young will have the only food supply they can eat. This is one of the best examples of symbiosis, where each partner benefits from the arrangement of mutual interdependency.

4. http://davesgarden.com/guides/pf/showimage/386099
This scene and all of the others were taken on my 40-acre parcel at D:F Ranch. The narrow band of land where the Sonoran Desert and the Mojave Desert interface between Yucca, Wikieup, and Wickenburg AZ are the only places on earth where you can see saguaros and Joshua trees growing together naturally.

5. http://davesgarden.com/guides/pf/showimage/386100
Another scene of a wild Joshua tree and a saguaro growing only feet apart on my property near Yucca, Arizona. The town was named for the Joshua trees that grow in the vicinity.


GENERAL COMMENTS
After skipping virtually all flowering in the springtime of 2014, the Joshua trees (Yucca brevifolia) have decided to bloom again in 2015 (at Destination:Forever Ranch and Gardens, a 40 acre desert botanical garden/sustainable living homestead in Arizona), although not quite as abundantly as they did in 2013.

This cyclical flowering pattern, with good years interspersed with poor ones, appears to be a function of at least a couple of factors. One is that Joshua trees can only flower once out of a given "head", since the process eventually kills the terminal meristem as the cells do their final division from leaf and stem cells into reproductive ones.

This does not mean that the branch dies after flowering, just that new side-buds need to activate, grow for a few years, and then they too can rebloom. This feature is the one primarily responsible for ramification, aka new branch formation, in Joshua trees and other yuccas. If a given tree has three heads, and all of them flower one year, then it will be at minimum several more years before the new replacement branches are large enough to also bloom. But if each of the three heads that terminated is replaced by two side-shoots, then the plant may have 6 branches after flowering, which equates to greater reproductive potential. If trees bloomed heavily two or three years ago then it will simply be impossible for them to flower as much for awhile until new branches are big enough to do so.

Weather also plays an important role in how much the trees can bloom. Dry winters are obviously not conducive to much blooming, even if they are otherwise ready. The heavy bloom of 2013 followed by the dry winter and spring of 2014 resulted in a near-total absence of flowers last year. A winter of at least average precipitation, especially if it receives a cold snap sometime in December or January, appears to stimulate flowering as well. I assume that the several inches of snow this area received on New Year's Eve and Day helped spur the plants to flower, especially since they entirely skipped last year.

One more intriguing question is what happens to the yucca moths (Tegeticula sp) during years of poor or absent flowering? Yucca moths are the only insects capable of reliably pollinating the Joshua tree and other yuccas, and the moth requires the yucca plants in order to survive as well. Each species would soon become extinct without the other. But when you are a tiny white moth and your food supply is not present at all one year, how do you manage? What do you do for the entire year or more when yuccas are not blooming for one reason or another? How do the plants communicate to the moths, if in fact they do? Do they send pheromone signals to dormant moth pupae to emerge when they are flowering, and stay dormant if they are not? I have not seen any research into this important matter myself, but if anyone out there has I would be interested to find out more.

Regardless, this year (2015) is a decent one for Joshua tree flowers. Enjoy the pictures since it might be two more years before we get to see them again.

This plant is so beautiful! And I see a lot of them here in Utah! They can be grown in basically every single part of Utah besides the ve...Read Morery tops of the mountains. They are also my favorite Utah native plant! They grow amazingly here and can reach heights of 30’ (i’ve personally seen a few that big in the Salt Lake City area in zone 7b)

I have a Joshua tree that I raised from seed that is about 14 years old. I keep it in a tall pot (3") as I live in western Oregon and am...Read More afraid it would be too wet if planted in the silty loam soil with a high water table in the wet season. I also keep it under the eave of the house on the east side so that it doesn't get a lot of rain falling into the pot. It is about 18" tall. It has damaged me and my wife with the sharp leaves many times!