Just as warm season annual flowers that bloomed through spring and summer get removed to relinquish their space to cool season annuals, summer vegetable plants need to vacate the garden for cool season vegetables. Fortunately, removing vegetable plants is not as unpleasant as removing the pretty flowers might have been, because most have finished producing whatever it is that they were grown to produce.
Besides, cool season vegetables are even cooler than warm season vegetables, since some have distinctive foliage that is appealing beyond the vegetable garden, in borders, pots and mixed plantings, out in the landscape. Swiss chard is a striking foliar plant, whether it gets eaten or not. The outer leaves can get eaten without compromising the appeal of the inner leaves that continue to grow and fluff outward to replace them. Arugula, kale, mache, loose lettuce (non heading) and collard, mustard and turnip greens do the same.
This ability to function in more places in the landscape is a definite advantage since cool season vegetables are not quite as productive as warm season vegetables are. They would certainly like to be as productive, but grow slower through cool weather.
Most cool season vegetables should be sown into the garden as seed. Root vegetables, like beet, carrot and radish, might be available as seedlings in cell packs, but do not recover very well from transplanting. Besides, seedlings of such plants in cell packs are either too abundant and crowded, or to sparse to produce much. Leek, loose leaf lettuce and most of the greens likewise grow best if sown as seed in rows instead of planted in tight clumps as they are grown in cell packs.
Only greens that are grown as individual big plants, like chard, kale and collard green, can be as productive from cell pack seedlings as from seed. However, each cell pack produces only six plants, maybe with some extras. A package of seed costs about the same, but contains more seed than most gardens can accommodate.
Actually, the only cool season vegetables that might be more practical to grow from cell pack seedlings than from seed are broccoli, cauliflower, cabbage and Brussels sprout. These vegetables grow as even larger plants, so minimal quantities, like one or two or perhaps three cell packs, are sufficient.
Like so many of the warm season vegetables, many cool season vegetables should be planted in phases every two to four weeks to prolong harvest. By the time the first phase finishes, the next phase will be ready. The frequency of phases depends on the growth rate and duration of production of the particular plants. For example, chard has a long duration of production, so does not need many phases.