From Plants vs. Zombies Guide
Wall-nuts are the fourth plants you get in the game, and the first defensive plants. Unlike other plants which expire after the zombies chewed on them for a few seconds, Wall-nuts last a very long time (precisely 45 seconds) Wall-nuts are used to protect other plants from attacking zombies. Wall-nuts are important since they block off zombies and cluster them into easier to kill groups, i.e. for your Potato Mines. They can't block jumping zombies(Pole vaulters and Dolphin riders), however.
Because of their endurance, place Wall-nuts in front of other plants to protect them. While the zombies are busy chewing the Wall-nuts, your shooters should have enough time to finish them off. The best position to place the Wall-nuts in your build would be the 6th column. Further to the left is fine, but your Wall-nuts will expire faster.
You can use Wall-nuts to buy some time while you wait for your Sunflowers to produce sun. Wall-nuts are very effective time-buyers.
Wall-nuts and other instant killers like Potato Mines make a good combination since Wall-nuts block the zombies off, clusters them into small groups, and if any jumping zombies get past the Wall-nuts, the instant killers are there waiting for them. Wall-nuts with Chompers are also a brilliant combination, since the Chompers will take care off the zombies chewing on the Wall-nuts while the Wall-nuts, in return, protect the Chompers while they are chewing.
On Day levels, you may Wall-nuts (and Tall-nuts) on any column you like (from 6 to 9), however on Pool levels Wall-nuts on the pool should be placed on column 6 only in order to fully protect other plants from zombies emerging from under the pool, while still be able to keep a reasonable space for shooters behind.
Note: Wall-nuts and other defensive plants have visual indicators about the damages they have taken.
Wall-nuts have hard shells which you can use to protect your other plants.
"People wonder how I feel about getting constantly chewed on by zombies," says Wall-nut. "What they don't realize is that with my limited senses all I can feel is a kind of tingling, like a relaxing back rub."