From Lost Fables Wiki
Tree cucumbers do not actually have their own plant, rather, they are a curious outcome of a fruit tree growing atop a ley line, though not atop one of the wellsprings. Given a couple decades of growing above a ley line, the tree will stop producing the normal fruit, and begin to produce these oddly shaped fruits. Tree cucumbers don't have a definite form, though they're almost always yellow or green and gourdlike in shape, and both their flavor and the meat inside is largely dependent on the original tree.
Color, Flowers, and Activity
The cucumbers are almost always yellow or green, while the trees themselves take the shape, flowerings, and activity patterns of the original plant.
Eating these cucumbers is said to be good for one's health and supposedly, if one eats their body weight in tree cucumbers over the course of a week, eating an equal amount every day for seven days, they can be cured of any disease. Alchemists prize them for their odd effects, wildly altering the effects of any potion they're used in.
In order to plant one of these, you'll have to find a ley line, and a fruit tree which will grow in the region the ley line is located, plant it, and wait thirty or so years for it to begin producing tree cucumbers rather than its own fruit.
Though they basically follow the same conventions as whatever fruit tree they would normally be, if one wants to produce larger tree cucumbers, and faster, planting them with a good eight to ten meters is the best course.
Because of their location, being on a ley line, they grow about 10% faster on average than their base fruit tree.