Dry winter air causes houseplants to dry out quickly. At the very least, check plants weekly to assess soil moisture. Sticking a finger onto—or even into—soil is an easy way to determine if plants need a drink. With small plants, lifting the pot is another good way to figure out how moist soil is. Dry soil is light; wet soil is heavier. Soil color also changes as moisture evaporates. Wet soil is dark; dry soil is lighter in color.