Gardening requires planning. There is always planning. The vegetables that are getting harvested now are developing mostly on plants that were put out in the garden early last spring. Some of those plants were grown from seed sown even earlier, late last winter. Now that it is more than halfway through summer, it is time to plan for cool season vegetable and annuals for next autumn.
There is still no need to rush cool season vegetables and flowering annuals that will be purchased as small plants in six packs or four inch pots. They are only beginning to become available in nurseries, and get planted a bit later. Broccoli, cauliflower, cabbage and kale are popularly purchased as small plants because not very many are needed, and they are reasonably inexpensive.
However, if varieties of these vegetables that are not expected to be available in nurseries are desired, they must be purchased as seed. If space allows, they can be sown directly into the garden early in September. Otherwise, they can be sown now into flats, six packs or small pots to grow into small plants that will be ready when warm season plants relinquish their space later in autumn.
Root vegetables like beets, turnips and carrots should not be grown or purchased in flats or pots. They get disfigured by transplant. Therefore, they should be sown directly into the garden through September. Carrots should perhaps be delayed until halfway through September. Turnip greens and leafy lettuces should be sown directly as well just because they get distressed from transplant.
Almost all cool season vegetable plants can be grown in phases, or several small groups planted every two week or so, in order to prolong harvest. Those planted first develop and are ready for harvest first. By the time they are depleted, the next phase should be ready. However, because most cool season vegetables develop somewhat slowly, and individual plants within each group develop at variable rates, planting only one early phase, and one late phase, perhaps with another phase in between, might prolong harvest more than adequately.