The plant is probably just an herb that someone was growing and left behind.
The marks in the yard just look like sticks perhaps that someone may have arranged.
Though these items have some magical use or significance they don't seem to be connected and are probably just coincidences. I wouldn't worry about these things too much.
What you can do since this is a recently purchased property is cleanse the space and even put up a protection spell for the house and grounds.
First smudge the house and yard with sage or any other cleansing incense. You can also use holy water to cleanse if you don't want to use smoke. As you walk clockwise through the house and around the yard you can make whatever symbols with the smoke , water, or just your finger while you carry the water or burning incense that you feel necessary. Some examples would be a cross, pentagram, star of David, etc. Personally I first draw a pentagram and make the circle counter clockwise for banishing and then I follow with another pentagram with the circle going clockwise for protection. I make as many of these as I feel necessary while I am cleansing. I specifically make them in each of the four corners of the house and yard as well as one for every room of the house, every faucet, and over each window and door. I will also cleanse all out buildings as well.
Once you have cleansed the space you should put up a protection spell or ward. A spell bottle for the house would be good place to start. This is a glass bottle filled with oatmeal, sea salt, and other protective herbs and items placed in a the middle part of the house. For the yard I suggest a scattering. This is an mixture much like the spell bottle but only contains items that can decompose. Also instead of keeping it sealed in a bottle you scatter it along the property line as well as around the outside of the house and through the yard.
There are many different ways to cleanse and many more different ways to protect your house and property. Be sure to research the topics if you are not familiar with them and pick the best way you feel comfortable with to proceed.