March to July is the best season because in subtropical countries light intensites increases upto 12-14 hrs a day and during this period plants synthesize almost all important groups of natural products. from sap, flower, seed, bark, leaves and pods. 

There is  a clear view point about collection of different vegetative plant parts in different seasons, e.g., roots in autumn, leaves and stems along bark before autumn; moreover flowers obviosuly during flowering period and same for the pods and seeds...this is very obvious strategy

Dear Sudheer

Check this interesting publication on traditional knowledge on season and time of collection of medicinal plants.

Thank you so much Dr. Rajeswara Rao.

You can visit the seminar website and check other interesting publications.

Good luck

In the flowering season you will find a larger amount of the flowers' phytochemicals, so you can isolate contrary to nonflowering time

Dear Sudheer Kumar. The season of collection depends on the type of plant part going to be screened.

For example. I you want to collect the biggest amount of metabolites you should answer first this questions: In what part of the plant is the metabolite you want produced? If it is produced in the roots, perhaps the wet season is ideal, since the roots will be growing and unstressed... or maybe it will be produced as an answer to humidity to prevent fungal infection... Also, If the metabolite is produced by a stressful situation... maybe you should collect during the dry season.

What I want to say is that every plant is different and you should be guided by the main events that trigger the production of the metabolite that you want... if you know the metabolite... If you don't know the metabolite... then maybe a search in databases of other jobs with the plant will be helpful. If there is not I recommend the dry season (I mean no rain) since the metabolites will be more concentrated in the tissues of the plant, and sometimes the rain washes out many metabolite from the cortex. If what you are looking for is a medicinal metabolite responsible of the medicinal properties of the plant... You should really consider consulting to the traditional healer and collect the plant in the time he recommend you... However, since plant organic extracts and medicinal potions are very different you must make an important choice... Do you want to do a phytochemical screening (Then season is not too important), find the active metabolite (then you must look where it is).

There is much to say about this matter...

I hope my answer is helpful to you. Kind regards :)

Agree with B R R Rao and Blanca Marina. Roots are collected when the plant shed their leaves and depends upon the type of plants and geographical regions

It depends  upon the part of plant which you wish to collect. Leaves are concern Feb-March duration is suitable as we all know that maximum phytoconstituents are present in tender leaves.

My experience is very difficult to say exactly when can you collect leave or any other parts, it depends on the plant, climatic conditions etc.

It depend of your goal of your investigation: biological, ecological, ...ect. For screening it is better to collect the plant at different stages. Thus, you can follow the variation of composition and establish a profile according the season variations. "Good yield" ? you can have "good yield" but you lost important compounds.

the plant part collected  at its mature condition

I will say the best season is the one you can identify the plant, that is, in its flowering season. Once identified, run a preliminary search to evaluate if the plant was already studied; if not, try to find what sort of compounds are reported for the genus; then you can use a different approach and collect only those parts of interest to you.

It depends on what part of the plant you investigate. For leaves, the best time is when they are still young. Most of plant secondary compunds are already present in young leaves, but the biomass is still not so much developed. When leaves grow, they mainly produce biomass thereby diluting the secondary constituents; this has well been shown for phenolic compounds (flavonoids) and (in our own case) for cyanogenic glycosides; but also for others. For roots or rhizomes, the best period may be after the season when plant store compounds and energy in them. For flowers you should harvest them just after beginning of blossom. Fruits in an early ripe condition.

However, in any case you may miss compounds as generally the physiological condition of a plant part, the condition of temperature and water supply, the ecological conditions at all influence the biosynthetic process. So if you plan to go deeper into the material, you should measure plant natural compounds during their deveopment during a whole season time. 

The stage of growth at which a plant is harvested has a bearing on its chemical constituents.  Therefore, a  plant needs to be harvested  at the right ontogenic stage when the desired active compounds are present in significant amount. For example, roots of Withania somnifera have fairly good content of withanolides when they are harvested from plants 6-7 months old and bark of Holarrhena antidysenterica has high content of alkaloids when the tree is in full bloom. Vasicine content of Adhatoda vasica leaves is highest at full bloom stage and xanthotoxine content of Heracleum candicans is highest when the aerial portions dry up. Ocimum sanctum is to be harvested at full bloom stage. On completion of ripening the fruit of coriander  will have low aldehydes and borneol, but maximum content of linalool, responsible for the characteristic aroma of coriander.  The right stage of harvest is judged from the colour of the umbels. Terminalia chebula fruits are to be collected at near maturity, but before they turn yellow and Aegle marmelos fruits are to be harvested  at full ripe stage.   Similarly, young leaves of peppermint contain the toxic monoterpene pulegone, while older leaves contain more desirable menthol and menthone   Likewise, the morphine content of Opium poppy is highest 2-3 weeks after flowering.  Contrarily, the solasodine content of many solanaceous berries disappears once the berries mature.  Commonly, leaf drugs are collected as the flowers begin to open, flowers are harvested just before they are fully open and rhizomes and roots are collected as soon as aerial parts wither away.

 In addition,the time of collection/harvesting of plant materials (before sunrise, at noon and at sunset) at a particular season is very vital as studies have shown that most volatile compounds are very sensitive to UV-light.