All morning the wren had watched me digging. Now the hole was as deep as a nice bath. Wrists aching, arms throbbing, I dug out the final, sodden plug of earth. I was building a bog garden, the latest horticultural fashion. The idea is to create a place of poor drainage, in which many of the jewels in the botanical crown thrive. Once the hole was a foot and a half deep, and four foot by four foot, it was time to put in the pond liner. The liner in place, I took up my garden fork and pierced it a few times before beginning the sinew-stretching operation of backfilling two tons of excavated claggy soil. The submerged liner will keep the ground sodden, but the holes will allow just the right kind of slow drainage. Heavy work finished, I could begin planting. First of all, ragged robin whose p