Cool season vegetables will replace warm season vegetables during autumn. Also, cool season annuals will replace warm season annuals. Neither simple task is easy for warm season plants that continue to perform too well to remove. Conversely, some finish early. Autumn annuals may compensate until cool season annuals become more seasonable.
Summers here are long, dry and somewhat warm. Even petunias and other annuals that enjoy warmth and tolerate aridity may not want to perform for so long. Without occasional grooming, they can get shabby by late summer. The recent unusually warm weather only accelerated the process for this year. Summer weather might continue into early autumn.
Consequently, it may still be a bit too early for some favorite cool season annuals that do not appreciate arid warmth. Pansies can get scrawny and lay low in response to warmth. Ornamental cabbage and kale is likely to bolt (general floral stalks) after exposure to too much warmth. Such annuals perform better or for a longer season after summer warmth.
This is why autumn annuals are so popular. They replace tired warm season annuals by the end of summer, and bloom until the weather is cool enough for cool season annuals. Autumn annuals generally bloom for shorter seasons than warm or cool season annuals, but most last until frost or significant rain. After bloom, a few are actually perennial plants.
Chrysanthemums and marigolds may be the most familiar of autumn annuals. Marigolds are actually warm season annuals that can perform just as well during spring or summer. Late installation allows late performance, which can continue until frost or sustained rain. Chrysanthemums are actually perennials, so can grow and bloom again for next autumn.
Celosia and alyssum, like marigold, are warm season annuals that perform late after late installation. Sweet William is a cool season annual that, after early installation, begins to perform earlier. Several types of aster naturally bloom late in summer or early in autumn. Even if nasturtium already succumbed to warmth, fresh seedlings may perform until frost. Old plants, prior to deterioration, could have provided seed, albeit feral, for their own replacement.
Tony Tomeo 
It is getting close to that time here. Unfortunately, we are still in the 90s and won’t be cool for a while.
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Yes, our weather will be in the nineties later in the week also. It is not yet time for cool season annuals, but because warm season annuals get tired after such a long season, transitional color is popular. Although they are some of my favorite annuals, I do not like adding them for such a brief season. In my own garden, I might grow marigolds as a full fledged warm season annual, even if they would not last into autumn. Alternatively, they could bloom through summer, and then be replaced by more of the same for autumn!
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We seem to be getting a second spring here in the East of England after our very hot summer. Spring bulbs are starting to show again, so I am a little worried about them. Autumn flowers are also out and looking great, my favourite at the moment are my Cosmos.
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Oh my, that would be a concern. For us, it is normal for daffodil to begin to extend foliage about now. It just grows slowly through autumn, and delays bloom until winter. For your climate, that would not work out so well. I suspect that it gets cool enough to ruin exposed growth, and even ruin bloom. The warmth there was weird to read about, not because it is unfamiliar, but because it is not what one expects to read about in that region. It happened in Oregon also, and the vegetation did not know what to do.
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It is a concern. Our spring bulbs usually start coming our late winter. I may try to put some mulch down to protect them from frost, it may not help but I’m thinking it’s worth a try. I’m hoping we don’t get the same heat next year. Poor plants and insects could do with a cool and rainy summer.
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Do you find that bulbs that are naturally endemic to mild climates are more easily confused than those of harsher climates? It seems to me that they do not care if they come up early, since they do not expect major frost. They are generally fine if they do so here, even if not there.
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This is the first time I have seen they come up this time of the year. The ones coming up are mainly crocus’ which, if my memory is correct, are native to woodland in the Mediterranean. So that would fit with your point about bulbs from mild climates are more likely to be confused.
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My saffron crocus were strangely confused, and this is a Mediterranean climate! I can not explain what happened. They were labeled as saffron crocus, and looks just like saffron crocus, but bloomed in early spring, shortly after the rest of the Dutch crocus. Of course, I did not mind, since I sort of expect crocus to bloom in early spring.
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