How to Solve Tents and Trees Puzzles
Tents & Trees was invented by Dutch designer Leon Balmaekers in 1989. Sometimes, you’ll see it as Campsites, or even under another theme. For example, one popular app uses cars and charging stations, calling the game Charge Up. Your goal is to place a tent next to each tree and fill out the campground.
Rules
- Each tent-tree pair must be orthogonally adjacent. Think of them as a domino. Each tent belongs to only one tree, and vice-versa.
- A tent might be adjacent to more than one tree, but it can’t solve both of them.
- Tents may not be next to each other in any direction, even diagonally.
- Number clues indicate how many tents must be in that row or column.
- Rows and columns without numbers may contain any number of tents.
Each tent belongs to one orthogonally adjacent tree, as though they form a domino. 
A tent in the corner could belong to either tree, but not both. One tree needs another tent. 
No tent may be adjacent to another in any direction. So a tent placed here might belong to either tree, and a second tent would have to be outside this 3×3 area.
Basic Techniques to Solve
- Mark known empty cells.
- Examine large clues, and count in cell pairs.
- Look for impossible tent-tree pairs.
Embrace Emptiness
When you’re first learning to solve Tents and Trees puzzles, marking out all empty cells is a powerful technique that helps you see possible tent placements. As you become more proficient, you might not need this step.
Before you place a single tent, you can eliminate:
- All cells in a row or column with a clue of 0
- Any cell surrounded by empty cells
- Cells with no orthogonally adjacent trees (remember, diagonal trees don’t count)
In this simplified puzzle, I’ve eliminated all cells that can’t hold a tent. This leaves only 8 possible places for 5 tents.
Count in Pairs
In many puzzles with outside clues giving you a count, you want to start with the higher numbers. This is simply because your solution will include more of those cells, so you have fewer to eliminate. When counting in a Tents and Trees puzzle, remember that a tent can’t be next to any other tent. This means there will always be a gap of at least one cell between any two tents. So when you’re counting, look at the cells in pairs, rather than individually, and you might have an easier time finding places your tents must reside.
For example, in this row, you need 4 tents. At first glance, it looks like you have 6 possible positions.
But in those central positions, there can only be one tent each. Counting by pairs, we quickly find where two tents must be located.
Impossible Pairings
Remember that each tree must have its own tent associated with it. Look around the grid carefully for cells which, if they contain a tent that belongs to one tree, they prevent another nearby tree from being solved. It might also force a solution that then exceeds the limit for a row or column.
When this happens, you can eliminate the pairing between the cell and THAT TREE. The important thing to understand is that just because a cell can’t contain a tent for one tree, it might still hold one that belongs to a different tree. In that situation, you would simply mark the cell wall as invalid.
When you look for impossible pairings, you can often eliminate a cell for all trees around it.
In this clue-less example, a tent tied to this tree prevents the other one from being solved. This works both ways in this specific puzzle.
Solving the Puzzle
These are the basic techniques that will get you through most puzzles. So let’s see about solving our example.
Getting Started
First, we’ll mark out all cells that can’t hold a tent. This includes the 0 clue, and all cells not orthogonally adjacent to a tree. 
This reveals a tree with only one option for a tent. 
Whenever we place a tent, we can mark out all the spaces around it.
We can solve this row, since there are only 2 spaces left. One of them must belong to the highlighted tree. 
Mark out surrounding cells, revealing the tree the yellow tent is tied to. Now the purple trees only have one cell left for a tent. 
We’ve satisfied the 2 clue, so we can mark out the remaining cells in this column.
Remember Impossible Pairings?
This column needs 3 tents. If we count cell pairs, there are only three blocks, two of which are singles. The single cells must each hold a tent. 
Now we have a better example for the impossible pairing tip. This cell might contain a tent belonging to either the purple or yellow tree. 
If we tie a tent to the purple tree, the resulting eliminations force two tents next to each other, which is not allowed.
After eliminating that wall, the purple tree has one option left. So we’ll place that tent. 
This eliminates the first cell, giving us the last tent in the 3 column. This row is solved. Finally, the tree on the bottom that already has its tent, so we mark out another cell. 
These trees all have only one cell left for a tent, so we can place all of them.
Finishing Up
This row is solved. The 2 column only has two cells left for tents, and this tree still needs its tent. 
This tree now has only one cell left for a tent. In addition, the 1 column has a single cell, so we can place a tent. 
This row is now solved. That leaves only one cell open in the grid, so we can place our final tent.
Here’s the completed puzzle.
