Westerosi crops.

[Ser Lepus]

A long time ago, I argued that at least the Reach, Arbor, Dorne and the southern Free Cities could realistically survive through winter by switching to winter grains, beets, sugar beets, radishes, carrots, lettuces, spinaches, legumes, onions...etc., and breeding goats, sheep and cattle, since these regions should experience little in the way of frost and snow...

However, I have realized something: Most of the fruits that are mentioned couldn't grow at all... peaches, plums, apricots, apples and pears need at least a few cold weeks before some warm months in order to produce fruit...

Grapes, pomegranates, lemons and oranges, on the other hand, could grow fine during summer (but oranges would be green-colored... it is the cold of autumn what makes oranges go orange...).

Do you know if GRRM has ever offered an explanation to the growth of fruits despite the lack of seasons?

[The Hoare]

I don't think there's a explanation at all, it's just fantasy. With seasons like that humans should be stuck at hunter-gatherer levels forever.

[Lord Vance II]

On 5/27/2019 at 12:00 PM, Ser Lepus said:

Do you know if GRRM has ever offered an explanation to the growth of fruits despite the lack of seasons?

Short answer, no, and I doubt he put much thought into it. He isn't a farmer, though he sure can write about food after it's cooked. 

If you want an in-world explaination - their plants aren't our plants. Ours grow the way they do because they evolved with our conditions and seasons. Their plants have, presumably, evolved over millions of years of their wacky seasons and have adjusted. I'm not a botanist, so don't ask me how. Besides, it seems like a mute point if they've adjusted by storing food.