Every terrarium enthusiast has experienced failure. The difference between those who give up and those who succeed is learning from mistakes—both their own and others'. This guide covers the most common errors that doom terrariums, helping you avoid these pitfalls before they claim your miniature garden.
Mistake #1: Overwatering
Overwatering kills more terrariums than any other error. The enclosed nature of terrariums means excess moisture has nowhere to go. Unlike potted plants where water drains through holes and evaporates from the soil surface, terrarium water becomes trapped, creating waterlogged conditions that rot roots and encourage fungal growth.
Signs of Overwatering
- Constant heavy condensation that never clears
- Pooling water at the bottom of the container
- Yellow or translucent leaves
- Musty or rotting smell
- Mould growth on soil surface or plants
- Soft, mushy stems at soil level
How to Avoid It
Start with less water than you think necessary. For closed terrariums, light misting is almost always sufficient. For open terrariums, water only when the soil is dry to the touch. Remember that it's far easier to add water than to remove it.
The Fix
If you've overwatered a closed terrarium, remove the lid completely and allow excess moisture to evaporate. This may take several days. Resume with much less water once the interior has dried appropriately.
Mistake #2: Mixing Incompatible Plants
One of the most tempting mistakes is combining plants that simply cannot coexist. Succulents in the same container as tropical ferns. Cacti alongside moisture-loving moss. These combinations are doomed from the start—there's no way to meet both plants' needs simultaneously.
The problem often begins in stores where terrariums are displayed with incompatible plants because they look attractive. These displays are short-lived; the plants won't survive together long-term.
How to Avoid It
Research every plant before adding it to your terrarium. Verify that light requirements, water needs, and humidity preferences align. When in doubt, stick to plants from the same category: all succulents, or all tropical humidity-lovers.
Mistake #3: Direct Sunlight Exposure
Glass containers act as greenhouses, magnifying and trapping heat from direct sunlight. A closed terrarium in direct sun can reach temperatures that cook plants within hours. Even open terrariums can overheat, causing sunburn and heat stress.
Signs of Sun Damage
- Bleached or white patches on leaves
- Brown, crispy leaf edges
- Wilting despite adequate moisture
- Soil drying out unusually fast
How to Avoid It
Place terrariums in bright, indirect light. A spot near a window but not directly in the sun's path works well. Sheer curtains can filter harsh light. If you notice morning sun reaching your terrarium, monitor temperatures and be prepared to relocate.
Australian Summer Warning
During Australian summers, even indirect light through windows can be intense. Pay extra attention to positioning from November through March, when sun angles change and intensity increases.
Mistake #4: Skipping the Drainage Layer
Without drainage holes, terrariums need an internal system to prevent water from pooling around roots. Many beginners skip the gravel layer at the bottom, reasoning that careful watering will compensate. It rarely does.
The drainage layer creates a reservoir where excess water can collect, keeping it separated from the soil and roots above. Without this buffer, even slightly excessive watering saturates the growing medium.
How to Avoid It
Always include a drainage layer of 2-5cm of pebbles or LECA (expanded clay) at the bottom of your container. Add a thin layer of activated charcoal above this to prevent stagnant water from developing odours or bacterial growth.
Mistake #5: Using the Wrong Soil
Regular garden soil is too dense for terrariums, holding too much water and potentially containing pests, diseases, or weed seeds. Conversely, using cactus mix for tropical plants doesn't retain enough moisture.
The Right Approach
For tropical closed terrariums, use quality potting mix with added perlite for drainage. For succulent open terrariums, use cactus and succulent mix or create your own blend with potting soil, coarse sand, and perlite. Never use soil dug from your garden.
Mistake #6: Overcrowding Plants
The temptation to fill every space creates terrariums that look lush initially but quickly become jungle tangles. Overcrowded plants compete for light, water, and nutrients. They also impede air circulation, encouraging fungal problems.
Additionally, most terrarium plants grow larger than their nursery size. What fits perfectly on day one may be hopelessly crowded within months.
How to Avoid It
Less is more. Leave space between plants for growth. Consider how each plant will mature—that tiny fern may triple in size. You can always add more plants later, but removing established plants disrupts root systems.
Mistake #7: Neglecting Ventilation (Closed Terrariums)
While closed terrariums are designed to be sealed, they occasionally need ventilation. Excess moisture must be released, and stale air should be exchanged. Some enthusiasts seal their terrariums and never open them again—leading to problems.
How to Avoid It
Open closed terrariums briefly every few weeks to allow fresh air exchange. If condensation is excessive, leave the lid off for a few hours or overnight. Think of it as your terrarium "breathing."
Balance Is Key
A healthy closed terrarium should show light condensation in the mornings that clears by afternoon. If condensation never appears, it's too dry. If it never clears, it's too wet.
Mistake #8: Choosing Containers That Are Too Small
Tiny containers limit plant choices, make maintenance difficult, and leave almost no margin for error with watering. They also heat up faster and create more extreme conditions. While small terrariums can work, they're actually harder to maintain than medium-sized ones.
How to Avoid It
For beginners, choose containers at least 15-20cm in diameter. This provides room for proper layering, multiple plants, and easier maintenance. Save miniature terrariums for when you've developed more experience.
Mistake #9: Ignoring Early Warning Signs
Small problems become big problems when ignored. A slight yellowing leaf, minor mould spot, or single insect might seem insignificant, but these are early warnings of larger issues developing. Many failed terrariums could have been saved with prompt intervention.
How to Avoid It
Check your terrarium at least weekly. Look for colour changes, new growth patterns, condensation levels, and any signs of pests or disease. Address issues immediately—remove affected leaves, adjust moisture, improve lighting. Early action prevents small problems from becoming fatal.
Mistake #10: Unrealistic Expectations
Many people expect terrariums to remain static, looking exactly like the day they were created. In reality, terrariums are dynamic ecosystems that change constantly. Plants grow, some thrive while others struggle, and the appearance evolves over time.
How to Embrace It
Accept that your terrarium will change. Some changes are beautiful—watching moss spread or ferns unfurl new fronds. Others require intervention—pruning overgrown plants or replacing ones that fail. This evolution is part of what makes terrarium keeping engaging.
Mistake #11: Poor Placement Decisions
Terrariums placed near heating vents, air conditioners, or in drafty locations experience rapid temperature and humidity fluctuations that stress plants. These micro-climate issues are often overlooked.
How to Avoid It
Choose a location with stable conditions. Avoid proximity to HVAC systems, exterior doors, and appliances that generate heat. Consider how the location changes throughout the day—morning sun, afternoon shadows, evening artificial light.
Learning from Failure
Even experienced terrarium enthusiasts experience failures. The key is treating each failure as a learning opportunity. What went wrong? What early signs did you miss? How would you approach it differently?
Don't be discouraged if your first terrarium fails. Analyse the cause, adjust your approach, and try again. The knowledge gained from failure often exceeds what you'd learn from easy success.
For more guidance on terrarium success, explore our maintenance guide or learn about choosing the right plants.