I am relatively new to the world of Arum, having started growing them in Fort Worth, Texas only two years ago (the coming growing season will be my third). However, when I lived in Los Angeles, CA, I grew about every aroid you can think of including a number of Arum.
First of all, you are not the only person in Northern California growing Arum purpureospathum!!! I know of several people growing that particular species there, not the least of which is the Botanical Gardens at the University of California at Berkley.
I belong to the Arisaema Enthusiasts Group, who have an annual exchange/distribution of seed. In the last two years, I have "planted" several hundred Arum seed and only had a dozen or so successes (seed germinate and leaf sprouted). So I am still learning, but intend to try again this Fall with seed from 10 - 15 different species.
I am told that the berry may incorporate some kind of anti-germination enzyme, so the seed must be cleaned as much as possible. The second thing I do know for sure is that the seed is going to germinate or not germinate at some time certain and with the exception of some species, there is little you can do about it. I am told that with Arum pictum (the botanists have decided that this is no longer an Arum, also I am told), may germinate at any time of the year given the right temperature and other conditions. I currently have two seedlings of A. pictum, Majorcan form, which have not yet gone dormant (although everything else has).
Some say that certain species need refrigeration prior to germination, but Dr. Peter Boyce (THE EXPERT), advised me that this is not necessary. A recognized US expert in the area of growing arum from seed is Ellen Horning, who tells me that "arums germinate in autumn, as average temperatures fall. This is a common adaptation in Mediterranean plants. They're dormant in summer, when it's too hot and dry to grow, and they germinate as soon as conditions become more comfortable in fall. That way the little seedlings can grow, and their tubers bulk up, for the better part of a year, until hot dry weather returns.
I've grown thousands of arums from seed, and all I ever did was sow them in summer, leave them alone outdoors (natural rainfall is OK, but don't give extra water during the summer), and watch them emerge in autumn. The first year you'll see only one dull-looking leaf; don't try to separate them or push them - just feed them as you would any seedling and leave them in their community pots. When they're dormant the next summer, you can empty the pot and give each little tuber a small pot of its own, if you're so inclined. Remember, though, that the surest way to kill them is to give them too much room and let the mix stay soggy, so it's actually safer to let them stay in their community pots for 2 years unless you've sowed them so thickly that they can't develop."
Finally, I am also told that it may take up to five years from seed to flowering plant. As I am 73 years old now, I am not sure I can wait 5 years. So I have been trying to obtain tubers, although I am more than willing to continue trying seed.
I hope this is helpful.
From: PlantsAndStuff.com <firstname.lastname@example.org>
To: Discussion of aroids <email@example.com>
Sent: Wed, June 19, 2013 2:26:39 PM
Subject: [Aroid-l] Arum purpureospathum x Arum palaestinum seeds...Possibly?
I have an Arum purpureospathum and an Arum palaestinum. The palaestinum flowered first, then the purpureospathum. The palaestinum inflorescence died with out fruit but the purpureospathum produced a berry custer. I live in northern California and am almost 100% certain there are none of these any where close to me. The purpureospathum must have been pollinated by a fly that visited the palaestinum and the visited it when it opened a couple days later. They are both outside so its a chance encounter I think. My first question is, is this possible? I would think that these plants can cross pollinate. My second question is how do I preserve these seeds or when/ how do I grow them? I know nothing about growing aroids from seed. Im not sure I even have the patience or the talent to wait/take care of them for the years required to get them to maturity so I may just
give these seeds away if I deem it something I dont wish and or cant do. Some of the berries are starting to shrivel, all of them are orange but upon opening one of the shriveled berries, there appears to be a nice healthy seed inside. So I dont even know if I need to leave them in the berry like you do with amorphs the seed looks completly different from an amorph seed though. Im thinking it can be removed from the berry. And how do I preserve the seeds if I decide I cant start growing them this year and how long will they stay viable.