Open terrariums usually consist of cacti and succulents, species that can tolerate drier conditions and require less maintenance.

Light

Succulent terrariums prefer bright, indirect light, so keep them near any sunny window in your home, but avoid prolonged exposure to direct sunlight. 

When succulents get too little light, they may lose a bit of color or grow long and start stretching towards the light (know as becoming 'leggy'). This won't hurt the plants - they're incredibly hardy - but your terrarium would definitely benefit from more light. 

If you need to provide additional lighting, you may not need to go full-on grow bulb. Start with a bright, full-spectrum LED bulb that plugs into a regular lamp - they're cheaper, easier on the eyes, and may provide just enough extra light to do the trick.

Watering

Depending on their size, small succulent terrariums usually need only a few teaspoons of water every one to two weeks, large ones need only a few tablespoons. 

The general rule of thumb is to wet the soil slowly, and then let dry completely between waterings (soil is dry to the bottom of the planter or glass container). Tap water is safe for succulent terrariums, but distilled will prevent mineral deposits and water spots on the glass and plants.

Water slowly - you want to make sure the water gets absorbed by the soil and doesn't just fall right to the bottom of the terrarium leaving the soil too dry and creating a pool at the base. 

Spray bottles are great for slowly and evenly moistening the soil, just avoid misting the entire surface of the plants. A small squirt bottle, spoon, or small watering can with a very narrow neck works too - helping you avoid the plants and keep overspray off the glass. Using ice cubes isn't recommended since extremely cold water can damage the roots of some succulents.

A small amount of water at the bottom of the container after watering is okay. If water has accumulated at the base, avoid watering until its evaporated, unless the soil is bone dry and the plants are starting to shrivel or brown. If this happens, water VERY slowly to moisten the soil without adding to the pool in the drainage layer.

In dry environments, check for dryness every 1 to 2 weeks.

In humid environments, check for dryness every 2 to 4 weeks.

 

Troubleshooting

If the leaves are plump and yellowing, you're over-watering. If they're shriveled and brown, you need more water.

Wilting leaves usually indicate that a plant is in need of water, but if the soil is still moist, it could be an over-watering issue, which could lead to root-rot.