You mean the parts poking out, not the gizmos right?
Your best bet is to select your plane, and go to weight paint mode (ctrl + tab in 3d view window, then mouse over weight paint)
and paint only on the faces/vertices you want the grass to appear. You may have to subdivide your plane a few times first before you can get this as accurate as you’d like. (select all in edit mode, right click in 3d view and click subdivide) or add some edge loops for maximal accuracy (in edit mode, ctrl+R, click then drag till the edge is where you want, then click to confirm.)
Once you’ve done this, you’ll have a new vertex group in your object data properties panel.
It’s worth noting, that you can skip the weight paint mode entirely if you so choose, and add in a vertex group here, and assign the weights manually using edit mode and the weight slider and assign/remove buttons in the object data properties panel.
After creating your vertex group, go to the particle settings, scroll down till you find the Vertex Groups section, open it up, and then select the group you’ve just made for Density.
Your particles will now only appear in the area determined by your vertex group.
If needed, adjust your vertex group weights till you no longer have glass intersecting with the cube.
You can also use this to rid yourself of any grass that wont be in the camera view, which will save resources, useful if you don’t have a high end computer.
I hope that helped. Let us know if you need more in depth explanation.