There are a number of ways to work with plant spirits as a spirit worker; plant spirits also have many ways to work with you. Entheogens specialize in trance states, visionary experiences (like divination), deep healing work, and spirit communication. For some people, deciding the right plant for the job is easy: you are told or use only a single plant for a certain application. Others have their own way of discerning which entheogen is needed. Certain plants will work better in some applications over others, and experience with a variety of plants will give you the knowledge base to know which tool is right for the job. There is no need to be experienced in all the entheogenic plants, trees, and fungi on Earth, and there is no competition to be skilled in using the most potent or scary. Mugwort and wormwood are chemically benign compared to datura or peyote, but datura and peyote won't and can't be used for the same jobs as the Artemisias can; wormwood and especially mugwort have a high level of spiritual potency that negates the need for overwhelming chemical effects. One's nature, personality, and body chemistry will be better suited to some plants over others, just as the personality of some spirits will be compatible with you and others will ignore you completely. Forcing the issue with a plant you desire to work with but who is not extending an invitation or accepting your advances is, needless to say, inadvisable. There are sure to be very valid reasons for a spirit's unwillingness and heeding those wishes are in your best interest.
Aside from the application that comes most readily to mind (altering awareness) which is covered in " Entheogens and Trance", there are other important uses for sacred plants. As was mentioned elsewhere, some of our most important medicine has come from the healing traditions that utilize these plants. I would add one more warning to all the others I have already given: do not self-prescribe and do not prescribe for others. The ability to treat ailments with these plants would require a detailed knowledge of chemistry and human physiology (or animal physiology, if such is the case); that knowledge goes beyond one's ability to communicate with the spirits or to dose correctly for the desired consciousness-altering effects. In some cases the therapeutic dose is very nearly the toxic dose and in many cases a less dangerous alternative is available; only a trained professional should attempt that work.
All that said , there are healing benefits that can be gained by just having the plant in your vicinity. Calm, protection, and the slow treatment of grief and depression are some of the ways these plants assist us through their presence. However, even this apparently straightforward form of healing has disadvantages. Some plants are quite toxic; aconite and mandrake are contact poisons, and handling them can lead to ill-effects, especially in children. Children and animals present another problem; curious hands and paws explore without knowing the potential danger and accidental illness and fatalities are not unknown. Even without animals or children, plants propagate in many ways, and seeds may be dispersed into areas we can't control (neighboring yards and fields, for example). It's also not unheard of for thrill seekers to pilfer from gardens. Close attention is required. For more on this, please refer to the FAQ .
When it comes to magical applications such as spellcasting , many entheogens have a traditional role. Europe prized the Solanaceae botanical family, which includes mandrake, belladonna, and henbane; these are classical witches' herbs and are listed as the ingredients in various "flying potions." In North, Central, and South America tobacco, datura , and brugmansia (also all in the Solanaceae family) had extensive traditional use. In modern recipe-style spell books, substitutes for the more dangerous herbs are often given. Given the sanitary nature of many of these books, I'm surprised that some of these herbs are mentioned at all. While the diligent magician or witch can still locate many of these plants, it seems we've forgotten how to use them.
An herb may have several given associations (love, luck, wealth, etc.) that it shares with many others; this doesn't guarantee that one may be cleanly substituted for another. Skilled herbal magic involves being able to get a sense of the spirit and energy of an herb, what it feels like and how it fits into the other herbs being used in a magical blend. Both mandrake and vanilla have associations with powers of love and lust, though they are not interchangeable. Crafting your blend around the spirit of the ingredients to augment the power you most specifically want to manifest is skilled work and takes a lot of time getting to know a variety of plants, both in their dried and fresh form (if possible). Introducing entheogenic plants into these blends is sure to open a new dimension of possibility in herbal magic endeavors. Sprinkling an herbal mixture while preparing a space for rituals involving high spirit activity could be useful for warding and to begin to "open" the pathways between Worlds. If a good working raport is established with the spirits related to the herbs you are working with, they may be willing to assist in your magical goal.
Plants are connected throughout almost all the Worlds; wherever plants are found, they have some power. This far-reaching perspective is useful in several ways if you are allowed to become familiar with the network energies of plants; the spirits are often very close to the Threads as well, and being familiar with one will help bring about an understanding of the other. This network communicates behind-the-scenes of the Worlds accessible through Journeying. Some plants have recognizable counterparts on other Worlds, some only outwardly resemble familiar Midgard plants. The plant Grandparents are more universal and having their attention through your work with them often gives you an in when you encounter plants and plant spirits in other Worlds. As on Midgard, the small local spirits of plants will be looked after by larger spirits of place---mountain spirits, valley spirits, patrolling land spirits, and others. When you seek to get to know these Otherworld plant spirits, be sure to introduce yourself to the spirit of the place you will be in and ask their permission. Courtesy is a universal language.