I am not sure where you want to include the backdrop, if the space is as restricted as in the example image. And some of the plants may not be movable.
Regarding lighting, you actually could use the diffused light as an excellent fill light. Usually you want to have soft light anyways, and that is exactly what the polytunnel provides.
If you want to take the shot in the tunnel, you have some options:
Try to suppress the ambient light by using a closed aperture and a fast shutterspeed that will result in a black frame unless additional light (by flash) is added. Then use a big softbox as the main light. And maybe rim lights if needed.
Try to incorporate the ambient light by setting the exposure so that it is almost ok, maybe a tad underexposed. Then add a kiss of light via softbox/flash to give it a bit more contrast and direction.
Try to use the ambient light only by setting the exposure so that it is almost ok, maybe a tad underexposed. Then add a kiss of light via a silver reflector (white might not be enough due to the already diffused nature of the light).
In regard to the white background, I see the space restrictions as you main concern. Usually you get a good even white background by using 2 strip light softboxes which illuminate the background in a cross pattern. So the right softbox illuminates the left portion of the background, and vice versa).
For the best results, you would just move the plants to a location where you have full control over the lights and can use a white paper background, which you can extend onto the floor. Product photography is a lot about exact control of the light and setting. You will have neither the space nor the control in the tunnel itself.