I live in New Mexico, and I’d like to harvest Yucca seed to share with the local cactus and succulent society. Last time I tried, I didn’t find any seeds at all. I want to go about this the right way–can you tell me what I should do?
In searching for an answer to your question about how and when to collect seed from Yuccas, I came across several articles on the interdependence of Yuccas and Yucca moths. Here is one example (now archived), from Emporia State University:
Excerpt:
“… the yucca plant and yucca moth are the textbook case of coevolution. First, the yucca plant has no ability to reproduce seeds without the moth. Yuccas can propagate small rosettes around the parent plant, but these vegetative sprouts are copies of the parent. Over decades, the plant cannot move but a few feet, and there is no possibility for genetic variation. Without the moth, the whole flowering effort (expensive to the plant in energy terms) is a total waste. The only pollinator of the plant is the yucca moth; bees are not attracted and neither wind nor bees can pick up the sticky pollen.
The yucca moth is likewise dependent upon the yucca plant. There are no alternate host plants known for the yucca moth; the yucca moth caterpillars must eat yucca seeds or starve. Without the plant, the moths die off in one generation. Without the moth, the plant cannot reproduce variation or disperse; given any major climate changes, it too will go extinct. The system is therefore tightly coevolved.[…] You can watch yucca moths pollinate flowers between dusk and midnight. The female gathers pollen from the flower anthers by using her specially adapted mouthparts, called palps. She forms the sticky pollen into a ball which she carries between her tentacles and her thorax (under her “chin” so to speak). The pollen ball is then “stuffed” or “combed” into the stigma of the various flowers she visits. The stigma is the receptive tip of the female pistil. Without this process, the yucca flower will not develop into the fruit or pod with seeds.”
The Arizona-Sonora Desert Museum states that Yucca may not set seed every year:
“Biologists have only recently determined that almost every species of yucca has its own species of yucca moth; some yuccas have two moth species. Such a tight mutualism has risks for both partners. Emergence of adult moths must coincide with yucca flowering for the reproductive needs of both species to be met. However, the synchronization of moth emergence with flowering is frequently poor and seed set and moth reproduction in such years are low. Furthermore, yucca populations may flower sparsely or not at all in dry years. Yuccas don’t have to set seed every year because they flower many times in their long lives.”
Regarding seed collecting and preparation, a question similar to yours was answered by the Lady Bird Johnson Wildflower Center:
“Gather capsules as they begin to dry but before they split. Allow to dry, then crush to remove seeds. Overwinter, keep seeds in moist sand in the refrigerator. For longer storage periods, keep in sealed, refrigerated containers.
At the Lady Bird Johnson Wildflower Center, we have collected and prepared yucca seeds for the Millennium Seed Bank. Crushing the pod to remove seeds is not as easy as it sounds. We found that pliers worked about as well as anything, but it was a struggle, either way. Inside the broken pod, you will find channels of seeds. They are flat, black wafers, very thin. As you pull out a stack of them, you may find a neat, round hole drilled up the center. This is the nursery for the larvae of the yucca moth, who have been munching on the seeds. However, the yucca moth is essential for the blooming of the plant.”
Late summer to early fall (September/October) seems to be the time when some Yucca seeds ripen. Several places I looked suggested this is the case, including Plants for a Future database, which describes propagation for several species of Yucca.