(Prunus domestica)

Background

European plum is an ancient domesticated species, known only in cultivation, now cultivated in temperate areas worldwide. Recorded cultivation started with the Syrians and then the Romans and spread to Western Europe during the Crusades. This species is thought to be a hybrid of ancient origin with parentage derived from species native to the Caucasus Mountains and Caspian Sea region. It is the best known of cultivated plums, having been grown the longest and widely distributed.

There are a number of subspecies that cross easily, so numerous intermediate forms can be found. Their sweetness and tartness may vary as well as their colors: from bluish purple, to red, orange, yellow or light green. Most are self-fertile and do not require cross-pollination but may benefit from it. P. domestica is distinctly different from Japanese plum, which is native to the Far East and genetically incompatible for hybridization.

European plum is longer lived than Japanese plum (and its hybrids), but it starts fruiting later. ‘Mount Royal’ and ‘Stanley’ are still fruiting in the original commercial orchard at Carandale after 40 years, though they are in decline. Japanese and American plum crosses are long gone. European plum trees like a moisture-retentive soil and grow well in heavy clay soils if well drained. Winter hardiness varies by subspecies with some hardy to Zone 3. Fruit of this species is generally higher in solids, making it good for drying. Most of the prunes (dried plums) available for purchase are from a subspecies of European plum (often referred to as prune plums). Plums in general are a good source of potassium, vitamins C and K, and dietary fiber.

Observations at Carandale Farm

There are six cultivars of European plum, representing three subspecies, being observed in cultivar trials at Carandale.

‘Mount Royal’ (prune plum) was included as a standard variety with a known history of performance at the farm. It was selected for a side-by-side comparison with the other dark-blue prune-type plums. As the photo shows, ‘Mt. Royal’ is a well-formed (natural) semi-dwarf tree. The fruit is blocky-round, smaller than ‘Stanley’, but better quality. The firm, yellow-green flesh is very sweet. This freestone fruit has thinner skin than ‘Stanley’ and is good for fresh eating. It is also good for drying and great as a processing fruit. It ripens earlier than ‘Stanley’, usually late August or early September at the test site.

‘Todd’ (prune plum) is a typical European prune type plum offered exclusively by St. Lawrence Nurseries. Trees appear to be hardy, but do not have the good structural features of ‘Mt Royal’ (crotch angles and openness). The fruit is deep blue (almost purple) and elongated elliptical in shape (see photo). Fruit is firm with good handling and storage qualities. Fruit quality is good, but not outstanding. Compared to ‘Mount Royal’, it is not as flavorful and yields have been lower, but it has demonstrated better storage qualities. It is acceptable as a fresh fruit and would probably be very good for processing. It ripens after ‘Mount Royal’ at the test site.

‘Ewing Blue’ (prune plum) is another prune-type European plum introduced by St. Lawrence Nurseries. This bonus cultivar was accidentally mixed in the ‘Todd’ order, but has been positively identified by the process of elimination and its fruit characteristics. This single tree is upright and vigorous (as shown in the dormancy photo). The lack of good tree form is probably a training issue.

Fruit is large, similar in size and shape to ‘Stanley’ (see photo). The dark blue fruit is sweeter than ‘Stanley’ (about the same as ‘Mount Royal’) but has softer texture, which makes it harder to handle and decreases storage life. Its size and sweetness gives it good marketability as a fresh fruit, but it needs careful handling and a shortened supply chain between producer and consumer. Compared to ‘Mount Royal’, it has not been as productive and has a shorter shelf life, but fruit quality is similar. It ripens slightly after ‘Mount Royal’ at the test site.

‘Green Gage’ (‘Reine Claude’) subspecies italica var. claudiana is a diminutive round, slightly oval plum, a little larger in diameter than a quarter. Its green/yellow skin is often covered with a brown russeting that does not add to its visual appeal. Despite its appearance, it has a rich confectionary flavor making it one of the finest dessert plums. This is not a plum that sells by its appearance, but once customers taste it at farmers’ market, they always come back for more.

‘Reine Claude’ was named after the wife of Francis I of France and was bred there over 300 years ago. It became known as ‘Green Gage’ when it was imported to England by Sir William Gage in 1724. They reproduce more or less true from seed, but over the years slight changes occur. As a result, selections marketed as ‘Green Gage’ are variable, some better than others. According to Wikipedia, the original ‘Green Gage’ cultivar survives in almost unchanged form as ‘Reine Claude Verte’.

The selection was purchased from St. Lawrence Nurseries in 2004 was called ‘Golden Gage’ and was said to be identical to ‘Green Gage’ (that they also sell), except for skin color. Everything else being equal, it was thought that yellow would be more appealing than green. In fact, the difference is subtle. The russeting probably shows up more on the lighter background. Fruit quality is excellent for fresh consumption, but how it compares with the other ‘Green Gages’ is unknown. As shown in the photos, the trees have an upright growth habit and are quite vigorous. They have a biennial fruiting tendency. The fruit ripens unevenly, requiring several harvests.

There is huge difference in flavor and sweetness between fully ripe and slightly under-ripe fruit. Fruit texture and ease of removal are the guide for hand harvesting, but this can become a time consuming and tedious job, especially considering that it has to be done every few days. For efficiency and expediency, using a tarp to collect the fruit and gently shaking the tree every few days to dislodge the fully ripe fruit works well as a harvesting technique. Bruising does not seem to be an issue, but shaking trees will obviously get more difficult as the trees get larger.

‘Green Gage’ plums could be a profitable addition to an integrated cropping system for local/regional marketing with a short supply chain where the grower and consumer have somewhat of a connection. Growers will need to watch for and anticipate all of the disease and insect pests common to all plum species.

‘Yellow Egg’ (subspecies intermedia) was added to the variety trial in 2008 to observe another branch of the European plum family. Two trees were purchased from Miller Nurseries. They have adapted and grown well. As the photo shows, it has a well-formed, open growth habit with good crotch angles that will require only moderate pruning. As of 2012, it had not yet come into bloom, so there are no fruit observations to share. There are some fruit buds, so bloom is anticipated in spring of 2013.

The catalog description says: “It is a large European plum, oval with yellow skin and flesh. Ripens mid-September. Selected for outstanding quality, freestone, firm when ripe. One of the sweetest, juiciest plums of all.” Another source (See Orange Pippin reference, below), suggests ‘Yellow Egg’ is not sweet and juicy right off the tree for fresh consumption, and might be best used as a puree for preserves and pie fillings. Both agree the fruit is firm, suggesting it has good storage and shelf life characteristics. Fruit characteristics are influenced by soils and climate.

‘Opal’ (a hybrid of two subspecies) was added to the trial in 2008 to further diversify experience with European plums. Two trees were purchased from St. Lawrence Nurseries. They adapted and have shown moderate to good growth. As the photo shows, it has more of an upward growth habit with weaker (more acute) crotch angles than ‘Yellow Egg’. The trees tend to lean and would have benefited from more aggressive tree support at a younger age. It has not fruited yet, so information about the fruit is not from observation. There are some fruit buds, and bloom is expected in spring 2013.

‘Opal’ is a cross between two subspecies of European plums, one of which is a gage plum (subspecies italica). It is said to have retained gage plums’ excellent dessert quality. It is an early mid-season plum that needs full sun and warm summer temperatures for good sugar conversion required to bring out its full flavor. Harvesting it too early and/or allowing it to over produce can reduce sugar content. The fruit is said to be medium-sized, with a reddish skin and yellow flesh. Fruit ripens over an extended period of time (one or two weeks) and, like its gage plum parent, will require multiple harvests. ‘Opal’ has freestone fruit that is aromatic and fairly juicy for a European plum.

References

USDA Nutrient Data Laboratory: Plums, raw
Plums of North America by Richard Ashton
Plants for a Future information on Plum
Wikipedia entry on European Plum
Orange Pippin information on Plum

This article was posted in Prunus.