Collecting wild herbs

Unless you are a botanist that can clearly identify plants you are just as likely to kill instead of heal.

Just a few examples:
Most amateur fungi collectors believe that mushrooms that can be peeled are safe. Not true, many of the toxic mushrooms (toadstools) do have a skin that can be peeped.
Some believe that the colored mushrooms are toxic. Not true some of them are safe.
There is also a theory that mushrooms that grow in deeply shaded areas are safe. Not true many of them are very poisonous.
Some species of hemlock look like parsley. One bite and you are dead.
Aconite has been mistaken for horseradish, with fatal results!

There are many other considerations:
Some healing herbs are both beneficial and very toxic. Rhubarb is consumed in pies and deserts all over the English speaking world but I wonder how many people realize that the leaves of the rhubarb plant are very dangerous.
Angelica root, used for thousands of years as a herbal medicine, contains a volatile oil that is very toxic. When used dried this volatile oil has evaporated so it should never be used fresh.

This does not mean that collecting wild herbs will inevitably lead to illness or even death. Herbalists have collected from the wild for centuries but they knew exactly what they were looking for and were unlikely to come home with a poisonous substitute.

Some plants like the common dandelion are easy to recognize and just as easy to pick (but do make sure that you can accurately identify the plant) others, like many of the berries grow on thorn covered vines so it may be prudent to dress appropriately and wear protective gloves.

If you are going on a field trip to collect the very poisonous herbs like aconite, it is essential that you are knowledgeable in the methods of harvesting and handling. These herbs are so dangerous that just handling them have caused fatalities. Making herbal remedies from toxic plants must be left to the experts.

Some herbs have to be collected at very specific times. These conventions are steeped in age old superstitions and beliefs that the gods bless the plants with healing powers on certain days of the year but the practical issue is one of maximizing the desired constituents or minimizing undesirable constituents.

A general rule of thumb is that herbs should be gathered just before flowering and in the morning but there are many other factors, particularly when the roots or berries are collected. Roots are best dug up before the flowering season or in autumn and berries are normally picked when ripe but this is not always the case (cloves for instance are the buds of the flowers of the Syzygium Aromaticum tree).

Before collecting make sure that the season and timing is right.