Working at a garden center, a big part of my job is providing gardening advice by answering questions about plant biology and science in general. Here are some frequently asked questions:
"How often should I water?"
When your plant is dry. I don't intend that to be flippant, that's the most accurate short answer. Different plants have different requirements for watering based on anatomy, rate of photosynthesis, and environmental factors. Succulents and cactus store water in their leaves and stems and therefore require less frequent watering. Plants that receive more light require more water for two reasons: (1) more light causes more photosynthesis to take place, which uses more water and (2) more light means faster evaporation of water from the soil's surface and from the plant's leaves. Plants receive more light in the summer than the winter, so more water is needed in the summer than the winter. Temperature, humidity, and soil composition all affect a plant's need for water too and House Plant Journal offers a more detailed explanation of these factors.
As you get to know your plants, you will learn how to "speak their language." Obviously, a wilted plant is desperate for water. Thirsty foliage plants will begin to droop and succulents will wrinkle when they need a drink. For many plants (but not all), a good way to tell if it needs water is to stick your finger into the soil and feel if it is wet or dry. Above all, it is best to do your research about the specific watering needs of specific plants.
"Which houseplants improve indoor air quality?"
All of them. No wait, none of them. Yes, I mean, none of them.
All plants engage in photosynthesis, which captures carbon dioxide and produces oxygen. Since we humans breathe in oxygen and expel carbon dioxide, it's the Earth's biomass of plants that make our lives even possible! So if you interpret "improving indoor air quality" in this way, the answer to the "which houseplants..." question is all of them. But if you are seeking specific plants to remove specific volatile organic compounds (VOCs) from indoor air, I have more questions than answers.
A big piece of the NASA Clean Air Study copypasta that is often overlooked is the recommendation for at least 1 plant for every 100 square feet. How big should those plants be? Are we measuring plant size by weight or by volume? Do you include the basement and closets in your square footage calculations? I have no idea. But I don't think that a windowsill with a pot of echeveria is going to cut it. I mean, I definitely don't have the space (or budget) for an indoor forest of 10-15 fiddle leaf figs in my rowhouse. And whenever my house gets a little stuffy, I open a window.
I firmly believe that the better solution to indoor air quality is to address the root cause of the problem: the building materials and furnishings used in homes, workplaces, and schools that emit VOCs. If you have the available finances, you may be able to renovate your home to remove harmful materials. Not everyone can do that, which is why the EPA considers indoor air quality to be an issue of environmental justice.
NOTE: this response has been revised to clarify my point of view in response to reader feedback (thank you!).
"Which plants repel mosquitoes?"
None of them. Think about it, if an effective mosquito-repelling plant actually existed, then no one in DC would grow anything else! Some culinary herbs contain essential oils that can repel mosquitoes, but you have to extract those oils and slather them on your skin in order for them to be effective. You know, like bug spray. And since essential oils are volatile, you have to reapply them after they wear off. You know, like bug spray. Here, have some bug spray.
"Which plants are good for sleep?"
There is no scientific evidence to suggest that plants have any effect on sleep. If you are having trouble sleeping, please talk to your doctor about ways to improve your sleep hygiene or see a sleep specialist. I underwent a sleep study several years ago, was diagnosed and treated for a sleep disorder, and I now enjoy better overall health.
"Which plants can I not kill?"
This question is typically asked in a joking manner by someone who is new to gardening or who has not developed their green thumb yet. So then I have a split second to decide if my auto-reply of "faux plants" would be met with further laughter.
Truth is, there's a learning curve for plant care. Even experienced gardeners kill plants. All the time. Myself included.
You will be most successful with gardening by selecting a plant that prefers the environmental conditions of your chosen site. Consider the following for the site where you would like to grow plants (indoors or out): light, water, and climate as well as your commitment level for care. Having these conditions in mind when you go shopping will make it easier for your friendly garden center clerk to match you with the right plants.
"Which plants can I grow in my bathroom?"
If you have a sunny window or sky light, then try an epiphyte such as an orchid, air plant, or staghorn fern, as these plants will appreciate the higher humidity. If you don't have a source of sunlight in the bathroom, you don't wish to have a grow-light on all day, and the room is usually pitch black because you keep the bathroom door closed, then I would suggest that you reconsider keeping a living plant there.
"Which plants are cat/dog safe?"
Being a responsible pet parent means not allowing your fur babies to eat your plants. The ASPCA's list of toxic plants is enormous...of course it is, their mission is to protect animals, not plants. Most plants are not acutely toxic, but anything that is not part of a normal healthy diet may cause vomiting or diarrhea if consumed. Talk to your veterinarian about specific concerns.