The nursery sites are not always accurate, or don't necessarily give the wanted information. The source of the information is not usually given.
Raintree.com - my source for Shiro and Hollywood. These 2 seem to pollinate each other. Shiro is described as partially self pollinating, and Hollywood is described as self pollinating. They bear well in my Vancouver yard, without other visible plum trees in the neighborhood.
Hybrids can indicate any species with another species, but usually refers to Asian species hybridized with native American species. Asian species give larger size and more meaty flesh, American species add to the flavors and give much better cold compatibility. Many of the hybrids are about 100 years old, so are nonpantented, and do not have industry or university sponsors or advocates.
University of Minnesota Ag Experiment Station workers did a detailed project regarding hybrid plum pollination in 1950. Among their tables, abstracted below. I summarized only the varieties that I have seen for sale as trees or scion, various sources.
From Table 3. Hybrid and native plums rated as good pollinizers.
Kaga. P. americana. 12 recipient varieties tested. Early bloom.
Toka. P. american X P. simonii. 22 recipients tested. Early bloom.
South Dakota. P. americana or P. americana hybrid. 27 recipients tested. Medium late bloom
From Table 4. Hybrid and native plums rated as fair pollinizers.
Ember. P. salicina hybrid X P. americana. 24 recipient varieties tested. Bloom season, mid.
Hanska. P. americana X P. simonii. 17 recipients tested. Early.
Superior. P. salicina X (P. americana X P. simonii). 18 recipients tested. Early / mid.
Shiro. P. salicina hybrid. 1 recipient tested.
From Table 5. Pollinizers tested and rated as poor.
|Prunus salicina "Shiro"|
Other comments from the 1950 U Minn paper -
The study began in 1932, and extended for a number of years.
It was noted that native plums had good pollen viability, while hybrids had generally poor pollen viability. Some had 50% with aborted pollen grains. Many of the hybrids produced pollen with empty or aborted pollen grains. Toka was shown to have good viability but a poor pollinizer. However, in the tables Toka is listed as a fair pollinizer.
Lack of fruit production may be defective pollen, low viability of pollen, or pollen incompatibility.
Among hybrids, more of those with female American plum parent, were good pollinizers. P. simonii may also contribute something to hybrids, in terms of being good pollinizers.
The authors also note that many of the hybrids resemble mainly their female parent. They go on to say that this may be due to apomixis, reproduction without sexual fertilization.
So you might think you have a hybrid, based on pollination, when reality the variety is selfed.
Ember. Hanska (G), Kaga (F), Superior (G), Toka (G), S. Dakota (G), not self.
Hanska. Ember (G), Superior (F), Toka (G), not self
La Crescent. Ember (P), Hanska (P), S. Dakota (G), Toka (G), self not mentioned
S. Dakota. Ember (F), Hanska (G), Superior (F), Toka (G), not self
Superior. Ember (P), Hanska (G), Kaga (G), Superior (G), Toka (G), does pollinize self.
Toka. Kaga (G), Superior (G), not self.
Underwood. Ember (P), Hanska (F), Kaga (G), Superior (F), Toka (G), self not mentioned.
Waneta. Ember (G), Hanska (G), Kaga (G), Superior (F), Toka (G), self not mentioned.
This seems to disagree with other reports of Toka pollinizing self.
Finally, there is also an issue of bloom season.
From Table 8. Varieties of plums suggested...
Early bloom season - Superior, Toka.
Mid bloom season - Ember, Underwood.
Mid to late - South Dakota.
Info from TheFarNorthGarden.com website:
"P. salicina hybrids were developed by crossing native wild plums with plum varieties from California that were not hardy, producing hardy trees with good quality fruit. These include 'Pembina' (sometimes called 'Prairie', 'Acme' or 'Elite'), 'Patterson Pride', 'Brookred', 'Geddes' and 'Perfection' (sometimes known as 'Superb'). These hybrids will only produce fruit if they are pollinated by a wild plum, and they do not provide pollination for any type of plum, including each other. This may explain why some growers have poor fruit production with these trees."
Another lesson - plums of the same variety can have different names.
From "Plums on the Prairies" - by Rick Sawatzky, University of Saskatchewan
On terminology - " Pollinators , usually insects, are vectors of pollen movement. Pollinizers are plants which provide the appropriate pollen for other plants"
On Pollinizing - " George F. Chipman who edited the Prairie Gardener for many years and who wrote about plum pollination in 1934. He summarized a study done by Prof. W. H. Alderman at the University of Minnesota by saying, “...very few hybrid plums would accept pollen freely from other hybrids, but they would all accept pollen from native plums”.
A precaution comes to mind - not from a specific source, but from my observation - most of these studies and comments regard plums grown in the Midwest, and usually the North Midwest at that. There, the climate is not friendly to most Asian plums, which is why they are interbred with American species. Therefore, the pollen from Asian plum varieties might not be tested on the hybrids. Both Toka and La Crescent have Shiro as a parent, so Shiro might pollinize those varieties.
From LMtreefarm -
Brookgold - Asian plum
Brookred- Asian plum
Greenville - Asian (Burbank) by P. nigra
Patterson's Pride - P. nigra X Asian plum, 1960
La Crescent - Howard Yellow apricot X Shiro Asian plum 1923
Pembina - Native Canadian plum X Asian plum "Red June"
South Dakota - pollinator for hybrid plums, a selection of Prunus americana. 1949
Tecumseh - Shiro X "Surprise" -
Toka - Native plum X Chinese plumcot P. simonii. 1911.
HardyFruitTrees states La Crescent is "Also known as 'Crescent', 'Golden La Crescent' and 'Golden Minnesota'. La Crescent is a cross between the 'Shiro' plum (Prunus simonii x Prunus salicina x Prunus cerasifera x Prunus munsoniana) and 'Howard Yellow' plum ( Prunus americana). It was introduced in 1923 by the University of Minnesota Fruit Breeding Farm"
The source has some nice things to say about La Crescent - "sweet and juicy...golden-orange color...flesh is yellow like an apricot... melting and not fibrous... freestone... taste has an hint of apricot. and they note that it is a poor bearer and must be pollinized by and American or Canadian plum.
Of Pemina, they state - "hybrid between a Japanese plum (Prunus salicina) and a Canadian plum (Prunus nigra)... self-sterile... must be pollinated by a wild plum tree, or a pure American plum tree (Prunus americana), or a pure Canadian plum (Prunus nigra)....introduced in 1923 by Niels Ebbesen Hansen from the South Dakota Experimental Station in Brookings"
Based on all of this information -
-Of readily available varieties to pollinize Hybrid plums, Toka seems to be the best candidate.
-South Dakota seems equally good as a pollinizer, but is harder to find.
-It's not clear to me that Asian plums have been tested as pollinizers for hybrid plums. It's worth adding them into the mix in this area.
-If a pure American or Canadian plum can be found, those are considered among the best pollinizers for hybrid plums.
-Probably, the more types in the mix, the better the chances for a good crop. If there is not room for multiple plum trees, or ability to are for multiple plum trees, multigraft is an option.
Images are public domain via U.S. Department of Agriculture Pomological Watercolor Collection. Rare and Special Collections, National Agricultural Library, Beltsville, MD 20705. " The majority of the paintings were created between 1894 and 1916."