Answer: Your Japanese plum trees should begin producing fruit now that they are 5 years old. It can take a year for the roots to become established and then 3-4 additional years for them to mature enough to produce fruit. The normal bloom and fruiting cycle is spring through summer. It takes about 3 months for the fruit to ripen so anything that is produced in late summer will obviously not have time to mature before cold weather arrives.
I wonder if late frosts are killing the flower buds on your trees. Plum flower buds tolerate -15 to -30?F when dormant, and the wood is killed just below this level. Open flowers and young fruitlets of both species are killed by brief exposure to 28?F or below in spring. Japanese plums bloom early, generally with or before the earliest peaches (late February to early March in the south, a month later in the north), and are therefore prone to frost damage.
Another concern is that plums have chilling requirements ranging from 550-800 hrs for Japanese, and >1000 for European types. Therefore, "low chill" cultivars must be grown in areas like the gulf coast, Florida, and southern Texas where less than 600 hours of chilling occurs each winter. The Japanese cultivars ?Robusto' and ?Segundo' are low chill, but there are no low chill European cultivars. Although Santa Rosa and Methley are commonly grown in Texas, your garden may simply have some cold pockets where your trees are planted and they are therefore subject to late frosts.
Hope this answers your questions!
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