What project does Christopher work on with his father while his mother has the flu?

Christopher and his father work on the garden.

And Mother got flu and I had to spend three days with Father and stay in his house. But it was OK because Sandy slept on my bed so he would bark if anyone came into the room during the night. And Father made a vegetable patch in the garden and I helped him. And we planted carrots and peas and spinach and I’m going to pick them and eat them when they’re ready.