Plums don't get sweeter after they are picked, but since they ship better than other stone fruits, they have a higher chance of having ripened longer on the tree.
Black plums have red flesh (whereas other types have yellow flesh). Because of this, they can be used as a dye to keep other fruit red when baking (like in a cherry pie). These tend to be one of the sweeter types of plums and are great in desserts.
These can be hard to distinguish from black plums as the skin colors can be similar depending on the specific variety and stage of ripeness. However, the flesh is yellow (not red). These tend to be more sour-sweet than black plums, making them more common in soups and stews than desserts.
These are much smaller and have yellow flesh with a sweet-tart flavor.
Yellow skin and yellow flesh with a sweet flavor, and slight tart finish. Great for recipes when you don't want anything to turn red.
These are used for drying into prunes as well as making preserves. However, they can also be eaten fresh. The skin tends to be blueish-purple to red (more red is pictured), often with white 'bloom' on the skin.
Sloe plums are a wild variety about the size of a dime. They are very astringent and taste generally unpleasant raw, but makes for an interesting liquor: sloe gin. It is very unlikely you'll find these in stores or even farmers markets (I was able to eat them at a farm-stay in Italy)
Pluots, plumcots, and apriums
All of these are plum-apricot hybrids, with varying ratios of parentage. The cross with apricots makes these hybrids sweeter and more aromatic then regular plums.
The pluot & plum guide lists the varieties and differences.
When are plums in season?
Plums are in season from May through October. Check your farmers markets in July and August for local plums, picked nice and ripe!
If you can't find them at a local farmers market, don't fret. Plums stand up to shipping and handling better than peaches, nectarines, and apricots at similar firmness levels. That gives you a decent chance of finding good ones at the grocery store.
What else is in season?
How to buy & store
To pick the best plums...
Look for plump fruit, paying attention to the stem end - the fuller it fills up and around, the longer it stayed on the tree. Make sure there are no wrinkles in the skin.
Ripe ones yield to gentle pressure. While they can get softer after you take them home, they do not get sweeter or more flavorful.
Leave on the counter for a few days if they aren't yet soft (remember, this won't make them sweeter). If they are ripe, put them in the fridge and they should last for a few days.
Common questions about plums
That is called the 'wax bloom.' It's dusty coating that contains alcohol (among other things) and helps protect the fruit from fungus and water loss. It also contributes to the flavor.
There are two major differences. First, a prune is a dried plum. Second, they are usually made from a specific variety that is rarely sold for fresh eating (because they are better suited for drying).
Yes, they actually do. They have sorbitol (a natural laxative) as well as a lot of fiber.
Recipes with plums
Plums aren't the summer darling of fruits and are under utilized. But that means you can easily use them to add an unexpected twist to traditional recipes.