To grow watercress, you can either sow from seed or propagate from shop bought stems.
I have found the quickest way is to do the latter - new leaves can be harvested just weeks after planting out.
Watercress can be grown even in hot summer conditions so long as the soil it is growing in is always moist as it thrives in wet conditions. Remember that watercress usually grows in water like chalk streams.
Place your cuttings into pots filled with soil and then place the pots on to a plate or a water tight container. The idea is that water will fill the plate beneath and not run off ensuring that the soil is kept damp.
Watercress plants are a magnet for caterpillars. Use mesh to protect your plants during the summer. One of my pots had a total of 30 caterpillars one day even when covered with mesh (so secure it tightly). The grubs ate through every single leaf on my plants.
Watercress can bolt during the heat of summer. They tend to grow better in cool conditions so autumn is actually a perfect time to grow watercress from a cutting (the butterflies have also disappeared!).