Weather plays an important role in dravya collection.Because according to weather changes avaibility of active principles in medical plants. Charaka has clearly mentioned that medicinal parts of a plant should be collected in the appropriate season because in that particular season,the percentage of active ingredient is more at that time.More active ingredients in plants gives best results in cure of disease.In case all parts of a medicinal pant are required, Caraka has excluded the rainy season (see following table).
Similarly Sushruta and Bhavprakash have indicated appropriate time for collection of medicinal parts of a plant.

COLLECTION OF PARTS OF MEDICINAL PLANTS AS PER AYURVEDIC TEXTS

Different parts of a plant Seasons as per sushruta Seasons as per caraka Seasons as per Bhavaprakash
Leaves -Do- -Do- May-July
Roots May-July January-March and May-July January-March
Bark September-November September-November
Tubers September-November November-January
Latex November-January September-November
Heart wood March-May November-January
Flowers According to the flowering season of individual plant. March- May
Fruits May- July As per fruiting season of each plant.
Drug as a whole September-November

 

In another Ayurvedic text, fallowing descrition is available acoording to medice collection time-

Plant part Time of collection
Roots (annual plants) Before flowering period
Roots(biennial and perennial plants) Late in fall or early in spring
Bark Dormant seasons
leaves Flowering period
Flowers On opening