Trees, Shrubs and Conifers: cross pollination question, 1 by Decumbent

Decumbent wrote:
Hi Christie,

This is a complex issue, which requires a lot of in depth knowledge that is not readly available or even known sometimes. I'll take a swing at this and hope that others can correct and/or complete what I offer.

Three things are required for pollination between two plants. The first is easy. Flowers must happen at the same time and pollen must be vectored from one flower to another. Given that, the other two issues are:

1) Flowers must be fertile. This is the norm. This is the way nature intended, and almost 100% of the time flowers are indeed normal and perform as expected. In the realm of ornamental plants, however, sometimes they aren't, particularly in regards to what are often called "double" flowers. This is a freakishly rare occurance, which usually happens because normally sexual flower parts, such as stamens, are not produced in the flower. In their stead are an extra set (or sets) of non-sexual petals. Extra petals often mean a uniquely beautiful flower, so trees with this feature are often selected and sold as "ornamentals.," Also, many, if not most, normal, fertile flowers drop their petals and start developing into fruits almost immediately following pollination. Infertile double flowers, since they are never pollinated, often remain fresh and on the plant much longer and consequently give the gardener a longer bloom time. Again, a reason for selection and cultivation in the ornamental trade. Cherries, plums, peaches, almonds (all genus Prunus and closely related) are the fruit trees most given to having double flowers, although I recall hearign about double crabapples too. This is a bit over my head, so I hope I've explained this aspect reasonably well, and I hope there is some truth to what I've said. To complicate things a bit, I don't believe that every double-flowered plant is necessarily infertile. Just many of them.

2) In order for two plants to cross pollinate each other, they must be closely related. This is really complex, and all I can hope to do is throw some ideas out. I don't feel very qualified on this topic. Usually, two cultivars or two seedlings from within the same species, let's say Prunus persica (peach), if they bloom at the same time, will pollinate each other. But sometimes two closely related species will too, such as Prunus persica (peach) and Prunus dulcis (almond). Apples (the genus Malus) seemingly all pollinate each other provided only that they flower at the same time. They are easy to the point that the domestic apple now cannot even be attributed to a naturally occurring species--its genetic heritage has become so complex. And, usually, any old crabapple will pollinate any old domestic apple.

Now, what makes two species or two cultivars closely related enough to pollinate each other? Way out of my league here, but... Sometimes I think there are physical mechanisms that prevent pollen that isn't exactly right from successfully completing pollination. Also, differing chromosome counts is an issue. You'll hear people refer to tetraploids or diploids, which refers to the numbers of chromosomes a plant carries. The numbers must be equal for pollination to happen. This is most often talked about with plants such as Iris or daylilies, things that have been highly bred, but it could apply to fruits, which are highly bred, too. I don't know.

Now, I'll step back and hopefully someone with more background on this will clean up the mess I've created! In the meantime, here is a picture from last year of a double Prunus of some type. I'm guessing an almond, but I really don't know.

Scott