According to Ayurvedic literature, fruits and vegetables grown in other than their natural season should be completely avoided. Every fruit or vegetable has a natural growth duration and environment from which it derives its natural properties and constitution. A fruit grown in unnatural duration and unnatural environment will have unnatural properties, which might not be beneficial for the body.
Let us take the example of construction of a house. If appropriate time is not given to the construction ingredients to settle, the building will be completed fast but it will be frail and unsafe to stay in.
Besides seasonal fruits meet the natural demands of the body in their respective season, for example consider the fruits of summer – watermelon, litchi, mango etc. All summer fruits are juicy and help in maintaining the water and electrolyte balance in the body, which is much required due to continuous loss of water through perspiration in summers.
Nowadays fruits and vegetables are semi grown and unnaturally ripened, by exposing them to chemical compounds. Such products might have a very attractive external appearance, but they bring comparatively low amount of nutrition to the body.
In Ayurveda, the ease of digestion and absorption of a food item is more important than the amount of nutrients chemically present in it. For example, we need iron but we cannot eat iron bar or plain iron powder, because its digestion and absorption is impossible in our body.
Similarly fruits, vegetables or grains grown unnaturally, (these include products grown by using chemical growth promotors or genetically modified products.) are Guru or heavy to digest and absorb. On the other hand, due to incomplete digestion, they might putrefy and produce toxins in the body.
Therefore use only seasonal fruits and vegetables, organically grown if possible. Here is a link to the indicative list of seasonal fruits and vegetables.