Which of the following is NOT a method to keep house interiors humid for plants?

Planting flowers in the garden does not contribute to maintaining humidity levels inside house interiors where indoor plants thrive. The other options—using a humidifier, misting leaves with distilled water, and grouping plants together—are all effective methods for increasing humidity in indoor environments.

Using a humidifier is a direct way to add moisture to the air, creating a more favorable atmosphere for houseplants that require higher humidity levels. Misting leaves with distilled water can also help increase humidity around the plant itself, providing moisture that can be beneficial for many species. Grouping plants together minimizes air circulation between them and allows for transpiration, where water vapor is released from the leaves, thus raising the local humidity around the grouped plants.

In contrast, planting flowers in the garden does not impact the indoor humidity and is therefore not a relevant method for maintaining humidity levels for houseplants.