This morning an app update has gone out which allows you to combine watering and feeding.
This is a beta feature at this time, so you have to opt-in to get it.
Go to the settings screen and enable the "see the future" option
For _future_ times when watering and feeding come at the same time, you will get them as a single task rather than as separate tasks.
If a plant does not need feeding, the regular watering task will appear.
Please do let us know how this works for you!