Working with herbs is all about relationships. What you cultivate is key, and if you don't have a relationship with the spirits behind the fleshy, fibrous mass that is their corporeal form, the roots, stems, leaves, resinous chunks, seeds mean nothing to you and have no power outside of what you create and will. What you imbue into them, if you will.
This is a major point of contention where the divide between low and high magic is concerned. The Magician is concerned with commanding the Powers do [X], [Y], or [Z], whereas the rootworker is concerned with cleverly making things occur by way of persuasion and massaging events into place. The trick is to find a balance between the two in a way that works with you and your spirits.
Forming relationships can be a difficult task, as there are a myriad of ways to do so. One of the most powerful ways to develop a relationship with a plant spirit is to do so by caring for a living, breathing, oxygen-producing version if it. That's not always possible with certain plants due to the area of the world you live in, so I suggest starting out with something that naturally grows in your area. As you do so, if you're open to it, you'll become aware of how the plant "feels" and its patterns. When you get the vibe that its time to open up an official line of communication, do so by talking with the plant. After that, evoke the spirit thereof and engage in conversations geared towards creating a pact with the spirit. It doesn't need to be anything grandiose, but having an official agreement does help, as then you have more leverage. For more information about pacting, you can read my posts on the subject here.
When you can't form a relationship by caring for a living plant, I recommend giving offerings to the spirits behind the plants. Candles, incense smoke, water, etc. all work. Rehydrating something like a Rose of Jericho can prove to be an endeavor that generates blessings for you. Doubly so if you elicit permission from the spirit thereof and then use its parts in oils.
For those of you who prefer a more structured ritual format to use, rather than simply going with the flow, I recommend the following:
Create an oil of Hyssop & Basil, using high quality virgin olive oil as a base. Provide low heat for the elixir in a stainless steel pot for five to ten minutes.
When done, strain the mixture through a cheese cloth, pouring the oil into a jar to be kept closed up, away from light for three days.
During that three day period, if it safe for you to do so, fast, eating and drinking only bread and water. Recite the Lord's Prayer and the Invocation of the Bornless One.
At the end of the three day period, cross yourself and anoint three green candles with the aforementioned oil. Place them in the shape of a triangle, with the plant material you wish to form a relationship with the spirit of in the center of said triangle.
Light each candle, saying "Domine, permitte me spiritus et ad angelos tuos, varias sanctissimo me herba materia. Invocarem [plant name, in Latin] efficiunt foedere alloqui utiles ambobus." Repeat this phrase until the spirit appears. From there, I leave it to you to work out terms that work for the both of you.
When complete, provide the spirit with license to depart, cross yourself, thank the spirits for their audience, and go about your day. Resume your normal eating habits thereafter.
I'll leave it to you to play with this. ;)