Nitrogen is the key to producing good quantities of tender chard with luxuriant, dark green leaves. But don't overdo it. Too much nitrogen, and stems are less colorful. Where summers are hotter and, more significantly, drier, plants may bolt prematurely. If you live where hot and dry summers are predictable, be sure to mulch plants by mid-June and irrigate often enough to keep soil evenly moist because it's not the heat that causes bolting, but water stress.
Swiss chard is relatively free of pests.
Leaf miners are occasionally a problem. Prevent damage by covering young plants with a floating row cover in spring when leaf miner flies are most active. If these insects do damage plants, it affects only appearance, not yield. Two fungal diseases sometimes occur. Fusarium wilt causes seedlings to wilt and shrivel and older plants to wilt and turn yellow.