Just a narnia question...

You've got your "plantings" mixed up, lol! In Magician's Nephew, the lamppost happens when the Witch tries to throw an iron bar at Aslan. The bar hits Him (though it doesn't hurt Him in the slightest) and it falls to the ground; and a new lamppost grows as a result. Totally unplanned and unexpected.

What you're REALLY thinking of is the Tree of Protection. Aslan sends Digory, Polly and Fledge on a quest to go to a magical garden in the mountains, where they find a tree of silver apples that can grant your heart's desire.

They had to take one and bring it back to Aslan, uneaten, so it could be planted in Narnia. The Tree of Protection would grow and repel the Witch for centuries to come. Only when it died would she be able to break into Narnia again.

Hope that clears up the confusion! =)

The lampost was not planted to keep Jadis out of Narnia. It sprouted when she threw a bar of iron on the ground when the world was trumming to Aslan's song. Polly, Digory, and Fledge are sent to repair the evil that has entered the new world. They fly west, towards the garden on top of a hill, to fetch an uneaten apple. Digory goes in alone. Inside, he meets Jadis, who tells him that Aslan only wants the apple for himself. She eats an apple while she tells him that the apple can make his mother well. The apples aso grant eternal life.
Aslan never ate a bite of that apple. Instead, he had Digory throw it towards a riverbank. While they waited for the tree to grow, the animals crowned Frank and Helen as the first rulers of Narnia.
Afterwards, Aslan tells Digory to take an apple from this new tree back to his mother. Only while the tree flourishes will Narnia be safe. The witch is far to the north, growing stronger in her dark power.
In England, Digory makes his mother well with the apple. He then plants the core in the garden. The tree that grows there responds exactly as the Narnia tree does. Later, the English tree is cut down to make the wardrobe in which the Pevensies will later have their adventures, as the wood of the tree longs to be back in Narnia.