The stress of change

Just like humans, some plants don't like change! When you keep your plant outside, it gets used to its surroundings and acclimatizes to its conditions. This means that when you bring it inside and everything changes very suddenly it has to adjust to new lighting, temperatures, airflow, humidity etc. As a result of this, your plant may show a stress response, e.g. by dropping leaves, wilting or becoming discolored (such as yellowing of the leaves).

If your plant is accustomed to colder, draftier conditions then the warmth and lack of sufficient airflow in most homes may additionally cause it to develop mold and / or mildew. While some molds (depending on the type) in themselves aren't harmful to plants, they can lead to the development of additional problems such as by interfering with effective photosynthesis or increasing the risk of pest infestations.

Pesky pests

Speaking of pests, it's unfortunately quite common for outdoor plants to accidentally bring in pests with them when you bring the plant inside. Pests often thrive in indoor environments, as many of them enjoy the heat and dry air, so any hitchhikers on your plants may quickly start to multiply. This can have detrimental effects on your plants' health and could even kill them in the worst-case scenario.

They can also spread to other plants in your home - pest infestations tend to spread quickly - so always be sure to thoroughly check over your plants when you bring them inside. It's also a good idea to keep them somewhat isolated from your indoor plants at first if it's possible to do so, as this will help prevent any potential hidden pests from spreading.

The right plant for the job

No matter how well you take care of them, some plants will never be able to truly thrive indoors as they're just better suited to being outdoors and need the increased airflow, humidity and temperature drops that an outdoor environment offers. This doesn't necessarily mean that you can't ever grow them indoors, just that they'd technically be happier outdoors. Examples of plants that can sometimes struggle indoors include Rosemary, Lavender, Eucalyptus and Conifers.