Because all these vegetables grow so quickly, they do best when given very fertile soil and a steady supply of water. In dry weather, water twice a week, or use drip irrigation. If you want to extend the harvest even longer into fall, protect Asian brassicas with row covers or cold frames. Light frosts don't harm them, but night temperatures in the 28o to 25oF range will reduce quality and slow growth, even when plants aren't damaged. You'll get several weeks more production if you cover the beds with polyethylene tunnels.
Flea beetles are devastating to seedlings, and beetle shot holes can disfigure the leaves badly.
Cover the plants with a row cover, or use rotenone or pyrethrum insecticides. A row cover will also prevent cabbage moths from laying eggs. Once plants grow large and the covers need to come off, control the moth larvae with Bt. Slugs are also inordinately fond of these greens. Control them by handpicking or with traps.