Plants notice it before people do. They respond accordingly. Vegetative growth slows or stops completely. Most remaining bloom does likewise. Fruit and seed finish developing. Less fresh produce is available from the garden. Boston ivy and Japanese maples might already be changing color. They all know that the days are shorter and nights are cooler.
Warm season produce gets less abundant as summer ends because most is fruit, which contain seed. Most plants naturally finish with seed production prior to autumn. Most cool season produce is truly vegetative and lacking seed. It naturally grows during autumn or winter, with the (‘fruitless’) intention of blooming and fruiting during the following season.
It seems as if warm season vegetables were replacing cool season vegetables from last winter only a few months ago. Perhaps they were. That process began about half a year ago, but never completely stops. Various phases of various vegetables start and finish at various times. Now, some warm season vegetables might continue to produce until frost.
Corn is of those warm season vegetables that remain productive. Seed for the last phase would not have been too late if it got into the garden two weeks ago, but will take time to produce. Broccoli seed sown at the about same time could be the first of the cool season vegetables in the garden. However, it is likely more practical to plant seedlings a bit later.
Seed for cabbage and cauliflower can go into the garden now. Alternatively, it should do as well in flats or cell packs, for later planting. This procedure delays their occupation of garden space, which might still be occupied by late warm season produce. Besides, it is easier to defend tiny seedlings in flats or cell packs from slugs, snails, birds and insects.
For big and leafy cool season produce, it may be more practical to purchase seedlings in cell packs rather seed. Since only a few seedlings of each type are needed, they are not too much more expensive than seed. However, confinement to cell packs disfigures root vegetables, such as beets and carrots. Their seed can get into the garden in two weeks to a month.
The difficult part is pulling up the warm season vegetable plants while they still seem to be productive. It gets much easier after that. Putting new cool season vegetable plants back into the garden is much more gratifying. Vegetable gardening never stops here. One season starts before the previous season ends. If cool season vegetable plants get a late start, they take longer to produce.
If there is enough space in the garden, the cool season vegetable plants can go somewhere else, while the warm season vegetable plants finish with what they might still be working on. Tomatoes might last until frost. Even when it get too cold for the foliage, green fruit can still ripen on the vine, and a bit more on the kitchen windowsill. Green tomatoes remaining after that can be pickled.
If the soil does not need too much amendment, and the spacing works out well, new cool season vegetable plants can be planted in with the warm season vegetable plants so that they can start to grow and disperse roots while the warm season vegetables finish. By the time the warm season plants succumb to cooling weather, the new cool season plants will already be maturing nicely.
This technique does not work so well with last season’s tomatoes only because they are so greedy with nutrients. Fertilizer might compensate for plants that seem slow where tomatoes had been previously. A bit of amendment can be added to the soil when new cool season plants get planted, and more can be added between them when old warm season vegetable plants get removed.
Broccoli, cabbage and cauliflower should all be planted as seedlings, so can work nicely with interplanting with aging warm season vegetable plants. Since only a few of each type are needed for a well outfitted garden, one or two cell packs of six or eight plants each should be sufficient. Each cell pack does not cost too much more than a packet of seed. Seedlings get established quickly.
Beets, carrots, lettuce, spinach and peas should be grown from seed sown directly into the garden, so do not work so well with interplanting. Too many individual plants of each type are needed. It just would not be practical to grow them from cell pack seedlings. Besides, they grow so efficiently from seed. Kale can be grown either from seed of seedlings. It might be too late for cucumbers.