Try to cover as many different habitats as possible: in forest, stream edge, ridge top, slope, stone outcrops, cultivated areas, roadside, edge of forest, etc. Even though there are often fewer things to be found easily inside the best primary forest, the rarest plants often turn up there. The roadsides and edges of cultivated areas usually have the most common plants, but may turn up occasional waifs that represent significant distributional records. Frequently the primary forest trees, lianas, and shrubs have their lowest branches at the margin of the forest, so do not neglect this area. It is important to check tree falls; they often bring down branches or twigs of canopy species, as well as epiphytes and lianas. In open areas with more light, plants sometimes flower and fruit which wouldn't otherwise.
If two or more collectors are working together, it is more efficient if they go to different habitats or agree ahead of time to concentrate on different life forms, i.e., trees, lianas, epiphytes, ferns, etc. Avoid two collectors collecting the same plants in the same area, on the same day.
Areas that are being logged or where new roads are under construction, are generally good places to collect, always watching out for falling trees, of course! Trees, lianas, and epiphytes that are ordinarily confined to the canopy are often readily accessible. In areas where the natural vegetation is likely to be destroyed, one should not be too concerned about depleting populations of species. On the other hand, collecting in any conservation unit (park, reserve, etc.) should be made with care and conscious consideration of population sizes. Be open and friendly with local people. Always be ready to explain what you are doing in terms that they will be most likely to understand. If you stop collecting or look furtive, it may be interpreted that you are doing something wrong. They will also feel less threatened if you acknowledge their presence and are friendly.
COMMON VS RARE
It is better to make more duplicate specimens of rare species and fewer of common ones. ("Rare species" means those that are rare in the herbarium, they may actually be abundant locally.) This is especially true for perennial plants or abundant herbs, in which collecting will not significantly affect the population. For example, it is better to prepare only three sheets of a very common plant and 11 sheets of something rare; not seven of each. Both ways require the same effort and occupy the same space in the press. Collect a minimum of three sheets whenever possible (one to remain in the country of origin, one for your home institution, and one for a specialist).
It is more important to get collections of species that are poorly represented in the herbarium, than those that are well represented. But geography also must be taken into consideration, a species may be well collected in one area and be of relatively low interest there, but it may be poorly collected in another area and be of very high interest there. More specimens are needed to document the range of variation in variable species than uniform ones. Also more specimens are needed to understand the differences between very similar species than more distinct ones.
RARE AND NEW SPECIES
Rare or little known plants and new species tend to occur together. If species are found that are rare or new, it is usually worthwhile collecting longer in that area. More often than not, other interesting species will be found.
If you have seen most species at least two or three times in an area, it means you have a rather high percentage of those species. If you have seen a significant percentage only once, it means there are probably numerous other species that haven't been collected yet.
Always recollect the same species if you find it in a different developmental stage or in better condition. Also, it is generally worthwhile recollecting species to show extremes in the size of parts, coloration, or other characters. Collect both sexes of dioecious species. One might even tag a plant or population to be certain of getting the same species at different stages. Collect the two or three (depending on group) stylar forms of heterostylous species, when this is known, e.g. Erythroxylum, Oxalis, Eichhornia, Rubiaceae, etc. Each should be collected under a different number.
UNICATES AND DUPLICATES
If the collection is a unicate, look around a few minutes for more specimens. Frequently after a careful search, you will find the plant isn't as rare as it first appeared. Specimens are needed for the country of origin, specialists, and exchange. Sets of 5 to 10 or more are most desirable.
It is very useful to mark labels "unicate." But if one specimen has been left in the country of origin, and/or one has been sent to the specialist, the single sheet at your institution should not be marked "unicate." Unicate means there are no duplicates anywhere else.
When there is only a unicate, one can often make a second fragmentary specimen. For example, one might take one or two flowers from a multiflowered inflorescence of an orchid and combine it with a leaf or two and a pseudobulb. If you add a photocopy of the good specimen it is even better. If it should be the only representative of the species in the herbarium it would be a valuable addition even though fragmentary. Often a fruit or inflorescence can be cut in half to make an adequate representation for two specimens, e.g., Cyclanthaceae. Combined with a piece of stem and leaves it is a useful specimen, but it is important to indicate on the label it has been divided if that is not obvious.
Some plants are found only as unicates, and are found at another locality also as a unicate. If unicates are not all kept in the country of origin, recollect it even if it is not needed at your institution. Mark the newspaper and the corresponding specific information as to where it is to be deposited, for example, "Unicate for CR."