One reason gardeners often have too much zucchini, not enough, carrots, and bitter lettuce is that they forget to plan for how long each crop will last.
Some plants are only harvested once; carrots just give the one root for example. Lettuce can be stretched to several harvests if you cut leaf-by-leaf rather than the whole head, but the plants will start to get tough and bitter as they bolt to seed. To have good quality over a long period, you need to have new ones coming along at the same time your first ones are maturing. This is called succession-sowing. If you wait to replant until after you've harvested, it's too late.
For crops like this, you need to leave some empty space when you first plant, so there will be room for the second wave. You'll need more seeds for this type of plant than for longer-lasting ones.
Root crops, and many leaf crops fall into this category.
Other plants give many pickings. Some go all summer. One indicator of long harvest is a vining habit. For example, pole beans produce all season, while bush beans give a big harvest and quit. Determinate, or bush, tomatoes cover themselves with fruit and then stop. Indeterminate, vining tomatoes go on and on, bunch by bunch.
Below is a guide to which plants need replanting and which ones don't.
Plants that need succession-sowing: These will need to be planted every 2 weeks for a continuous supply.
Sow summer crops in succession through June.
Sow hardy crops* again starting in mid-to-late July for fall and winter harvests.
*arugula, bush beans, *beets, *heading broccoli, *cabbage, *carrots, burdock, *chicory and endive, corn, fennel, *lettuce, *Asian greens, *radishes, *spinach, sunflowers, determinate tomato, *turnips.
These can produce most of the summer from one planting: If you season is very long and hot, you may want to plant a second wave or a later variety to keep up quality and yield, but each plant will be picked over and over and produce many harvests.
Starred * plants are often planted in July for a fall crop.
Pole beans, *sprouting broccoli, chard, *collards, cucumber, eggplant, *kale, melon, okra, peas (replant once) summer squash (zucchini), indeterminate tomato.
These usually make one large crop, so they are planted once, (or twice for a longer harvest period.) Crops that are normally succession-sown can be handled in this way if they are meant primarily for storage--freezing, canning, drying, or fermenting. In garden planning, these will usually be succeeded by a different crop or a cover crop. For example, when corn is harvested, there will be no time for more corn to mature; the space will be planted to a winter cover crop or a fall vegetable.
Starred crops take a long time to mature but are winter-hardy and can be used slowly out of the garden over the winter, as they hold a long time in cold weather.
Dry beans, corn, winter squash, onions, *leeks, *parsnips, *storage beets, *winter cabbage, *rutabagas, grains