It may seem futile to pull certain weeds this late in the season. Those in unrefined parts of the garden that get little or no irrigation might be so dry that they only deteriorate and scatter their abundant seeds when pulled. The soil may be so dry that roots are difficult to extract, especially since the drying foliage now separates from the roots so easily. The only hope is that removal of dying weeds might eliminate at least some of the seeds for the next generation of weeds.
Foxtail and burrclover are not only annoying, but are also dangerous to dogs and cats as their seeds mature and dry. After all, the seeds rely on animals for dispersion, so intentionally stick to fur. The problem is that seeds can get stuck in more than fur, and sometimes get into ears, eyes, nostrils and elsewhere. Seeds from a few other weeds can do the same.
Cheeseweed is not a dangerous weed, and is relatively easy to eradicate. The roots even stay attached to the stems when they get pulled. The problem with leaving them to mature is that they become infested with rust (a fungal disease) that spreads to other desirable plants. Saint John’s wort, snapdragons and roses are particularly susceptible to rust.
Feral Jupiter’s beard and montbretia that grow where they were not intentionally planted are often allowed to bloom before getting pulled. However, after bloom, stems separate so easily from roots that most of the roots remain to regenerate as soon as they are able. If left long enough after bloom, both Jupiter’s beard and montbretia sow seeds to infest even more.
Fortnight lily (or African iris) are not often a weed, but can get that way if their seed capsules are not removed before they mature and pop open. Besides, development of these capsules diverts resources from continued bloom. It is best to remove the capsules before they get floppy, and to remove as much of the finished flower stem as possible without removing stems that have not yet bloomed.
Both dusty miller and coleus are grown for their distinctive foliage but not their bloom. Flowering stems stretch and exhibit inferior foliar color and texture, so can actually get snipped before they bloom.
Tony Tomeo 
This is one of the hardest things to convince people to do when I lecture about fall chores. In our climate, besides the inevitable leaf drop and cleanup, we are often racing against cold weather. So weed removal doesn’t seem important. I try to remind them that they will be grateful in the spring, even if all they do is remove seed heads.
Karla
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Goodness, I know! When I grew citrus in the early 1990s, there were times when we were too busy to pull weeds from the stock trees. I explained to the crew that even if we did not do a complete job of weeding, there would be less weeds to pull later if we just cut off the seeding tops of the weeds.
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I totally agree and try to get all the weeds before they go to seed, which is hard to do in my climate where the growing season never ends. I also have many plants that reproduce too much and I try to cut off the seedheads before they ripen.
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Heck, we sometimes do that even with desirable plants! We recently got some yellow flag iris here. I am very pleased with them, but will be intent on deadheading them. They can be invasive in riparian situations. Although they are already established here, I do not want them to get established within our landscapes.
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