Yes, it does not rotate -- when you downloaded it to the iPad, it should have shown as an iphone app. These kind of apps appear on your iPad just as they do on your iPhone (if you use Instagram, you will see that it functions the same way). If you would like to see it in landscape view, go to the web app. I prefer the web app when on the iPad as I can see moisture level graphs and details much easier than on the phone app.
Can you post a screen shot of what you are seeing? And is your iPhone app up to date?