Terrariums are incredibly easy to make and maintain. However, there are some pitfalls that you'll want to avoid.
Too Much Light
It's easy to roast plants living in terrariums. The glass can act as a magnifier and burn the leaves and stems. Temperatures can get hot inside your jar, and before you know it, your terrarium can get as steamy as a sauna. Most plants can't take this kind of heat, so it's best to keep terrariums out of the direct sun.
Too Little Light
While too much light can cause problems, most plants need at least some light to survive. You can use grow lights or fluorescent lights, or place it near a window that receives indirect light.
Too Close to Radiators
The heat of a radiator can kill your plants quickly. Think of how fast a car sitting in the sun can get hot. If you put your terrarium on or near a radiator, most plants will not tolerate the additional heat.
Keep an eye on your terrarium plants, and when they get leggy, prune them back. To keep plants small, you can also prune their roots. This helps to ensure that your terrarium will not become overcrowded.
Leaving Dying Plants
If a plant is looking bad in your terrarium, get rid of it fast or remove dead or dying leaves. If the plant is diseased, take it out immediately as it can infect other plants. Dig the plant out with a small shovel, terrarium tool, or long spoon, being careful not to disturb the roots of other plants. Replace the plant with one of a similar size and light requirements, making sure to surround the roots with soil, leaving no air pockets.
Every once in a while, you'll want to clean the glass of your terrarium both inside and out. The glass needs to be cleaned because if it's too dirty or foggy, it will be harder for light to reach your plants. You can use a damp piece of newsprint or a lint-free cloth. Do not use any harsh cleaning products on the inside of the terrarium because it may present a danger to your plants.
It is easy to over-water terrariums. One way to prevent it is to use a spray bottle instead of pouring water. If you do over water, try to absorb any extra with a paper towel. Leave the top off of your terrarium until it has dried out.
Most terrariums do not need any fertilizer at all. Because you want to keep your plants small, you shouldn't feed them, which will cause new growth, and the plants will quickly outgrow their confined space.
Choosing the Wrong Plants
While it's possible to grow almost anything in a terrarium, it is important to choose plants that will thrive in the type of terrarium you are creating. If you're making a closed terrarium, select plants that like to be moist. Also, make sure to include plants for the amount of light they will be exposed to. Low-light plants generally work best.
Growing Succulents in Closed Terrariums
Succulents generally thrive in high light and low moisture environments. If you put them in a closed terrarium, it is usually far too humid for most to thrive. You can solve this dilemma by simply creating a glass dish garden without a top. Note that even a large jar will be too humid—you want air to be able to circulate around your succulents.