When it comes to synonyms for the word "planthouses," there are a few options to consider. One of the most common alternatives is "greenhouses," which is a term used to describe a structure designed to cultivate plants and maintain specific environmental conditions. Another synonym is "hothouses," which refers to a type of greenhouse that is specially designed to grow plants that require very warm temperatures. "Conservatories" are another synonym for planthouses, and they are typically used to house and display a variety of different plants and flowers. Lastly, "nurseries" can also be considered a synonym for planthouses, as they are used to cultivate young plants and prepare them for transplantation.