Very few Witches talk about this and I think that's because most go with one then forget the other. I do both. Both methods have their merits and as long as you keep a few things in mind, you can practice either one.
The first herbal magick is the one we are all most familiar with and it's the one that comes to mind when discussing Witchcraft. That is using herbs as medicine. It's also cooking with herbs, or using certain leaves as a bandage. Some part of the herb physically affects the body.
The other herbal magick is invoking the spirit of the plant. Here is where things get confusing. When a Wicca 101 book lists a poisonous herb as a "cure" because it has been used "by Witches throughout history" then whoever wrote the book has no actual experience with plants or magick.
Mistletoe is an example of this. Mistletoe is poisonous. It was used by Druids and some of that as carried over into our tradition of kissing under mistletoe. But I assure you, the Druids didn't consume mistletoe. They used the totem energy of the plant.
Tree magick is more in the totem realm than the medicine. We talk to the tree. We recognize it as a sentient being. Even when we cut the wood we make choices based on the tree's corospondences such as using oak to make furniture because it is beautiful and strong. Willow gets turned into wicker furniture giving things a much more airy, relaxed feel. Sometimes the plants associated with each system overlap because aspirin is made from willow bark. But while you can invoke the spirit of any plant, you cannot eat all greenery. Some of it you can't even touch.
The main benefit to totem herbal magick is making use of toxins without causing physical harm. An example of this would be stuffing a poppet with poisonous plants thus causing a person to feel discontent or to feel sickened about something. Maybe a poppet of a stalker who begins to feel unwell whenever he enters your neighborhood.
In my practice I cook with spices, keeping their magickal properties in mind. I grow food. I grow other things, too. I grow plants for beauty. I grow plants to attract bees and hummingbirds. (Thought I'd throw that in so you wouldn't have an entirely sinister view.) I grow aloes for medicine but they cannot be consumed. Aloe has an unpleasant laxative effect so just ignore those pills which promise stomach relief. See, pharmaceutical companies use information without researching it just like wannabe witches.
To use totem plant energy sit before the plant and establish mind contact. Ask if it will help you. Ask if you may take part of the plant with you. You have to be content with leaves on the ground or dirt underneath. Leave an offering as a thank you. Fertilizer or water works well. You can also clear around the plant to help it grow. If the plant is poisonous wear gloves. Do not use kitchen tools to harvest cuttings. Do not use plants you cannot identify.
And as long as we're on the subject of identifying, when you buy spices do you know which part you're purchasing? If you aren't sure about root or seed ground into spice, do research. I have met people who didn't know pumpkin pie could be made from pumpkins. They think the filling came from a can and they never gave any consideration to what was in the can.