First of all, it´s one noun is uncountable if it´s taken as a group concept:http://www.Englishpage.com/minitutorial ... -nouns.htm
Here we could add "vegetables" (and we can see plenty of QA sections of sites around the Web with "fruits and vegetables" along with "count uncount nouns".)
Well, back to crux then. Cucumber is in the grouped as "fruit".
When you don´t think about it as a group, but of their different "groups" or type, categories etc. Then you can think of them as a countable noun.
Several examples in the following site -http://www.grow-it-organically.com/cucu ... eties.html
(another way to think about that, is to think that slicing/pickling/specialty is what "adjectives" the cucumber there, so "slicing cucumbers" is one of the varieties wich have a subset there - Armenian and Lemon cucumbers are grouped within slicing cucumbers.)
And when it´s "distinguishable" you can refer to it in such way (then you are not thinking about it in general terms, but - for example- plants):
Plant a few cuccumbers and tomatoes just because smile emoticon I've heard beans are easy but I didn't have that kind of space
But when you don´t want to tell them apart, when you say it in general ways, then it´s uncountable:
Locust, pawpaw, cucumber, buck-eye, black mulberry and wild cherry trees also abound, and the grape, raspberry and strawberry are native fruits. (source: sentence.yourdictionary.com)
See? We are back to the "grouped concept" square!
This guy gives another (rather good) tip:In general, natural food items are both countable and uncountable. Again, in general terms, they are countable as whole units and uncountable when prepared in some manner.
Food is uncountable when prepared because it is usually preceded by an adjective: roast potatoes, stir-fried vegetables, sliced cucumber, etc. Uncountable words that are qualified often become countable: e.g. "He has a basic knowledge of chemistry."
A: "James had lettuce in his salad." -> it is assumed that the lettuce is [prepared as] "pieces of lettuce", and hence uncountable.
A: "James ate a lettuce." -> James has eaten a whole lettuce.http://forum.wordreference.com/threads/ ... r.2860170/
Another very good rule of thumb (and a way of training) is to look for "quantifier" words, they will give useful clues if the "cucumber" (or other word.) is
Thought as countable or uncountable. I say "thought" as per the above rules.http://www.edufind.com/English-grammar/ ... uantifier/
That´s my twopence´s worth, but other more advanced learners may comment on, and add their thoughts and comments.
I hope this helps.