If it's not possible to get good photos of the whole plants, I would go for closeups instead. Then you can get more depth in the photos. Closeups also makes them easier to light in a pleasing way.
If you don't have better light, you can use an iPad (which I know you have :-) or another phone that you open a blank Pages document in, so the whole screen is white. Try putting it at an angle 45° above and beside the camera. You will have to experiment with what works best. The closer to the plant you put the light source, the softer the light will look. Remember that you can adjust the brightness of the iPad screen.
If possible, put the iPhone on a chair or something and use the self-timer to minimize shake blur.
Using the iPad as a light source also works excellent on portraits. Try putting the camera right in front of the face and the iPad at a 45° degree angle to make interesting shadows on the face. The 45° degree angle to the face has several advantages. It doesn't blind the person, it makes a face look less flat than light from straight ahead, and it doesn't make the nose look big. You don't want a long nose shadow on the cheek in most portraits.
For other effects you can change the color of the iPad screen (draw a colored square in Pages). Try blue to get a cold midnight look, orange or red for sunset, etc. Experiment with light positions. If you go for blue moonlight, the light might come from above. Sunset is shoulder level.
Try different portrait angles too. Turn the head 45° away from the camera but still look into it. Most people have a “better” side of their face, but not everyone realize it. Ears and eyes can be different sizes. If one eye is bigger or more open, you may want to angle the face to have that eye further away from the camera so they look more evenly sized. Or if an ear is bigger, the smaller ear side may be best faced to the camera, and vice versa.
