Fertilizer, in simple terms, helps plants grow. It provides a bit more of what growing plants crave. In some situations, it compensates for nutrient deficiencies. Fertilizer can be organic or synthetic. Various formulations serve a variety of purposes. Custom formulations appeal to plants with discriminating taste, such as citrus, orchids and roses. Slow release fertilizer lasts for a month or two.
Of course, proper scheduling of the application of fertilizer is very important. Fertilizer can actually become toxic with excessively frequent application. Fertilizer that provides significant nitrogen to promote vegetative growth after bloom can inhibit floral growth prior to bloom. Fertilizer that promotes root growth for new plants is unnecessary for mature plants. The diets of plants are variable.
Summer lingers later here. Regardless of the weather, it is now autumn. This is when deciduous plants start the process of defoliation for their winter dormancy. Evergreen plants are less obvious about their winter dormancy. Some remain active and even bloom through autumn and winter locally. Nonetheless, cooling weather and shortening days inhibit vascular activity of almost all plants.
Except for cool season vegetables and bedding plants, not many plants benefit from fertilizer applied this late. New plants might appreciate a bit just to help them adjust to their new environment. Some deciduous plants, particularly roses, like a last late application of fertilizer as they get ready for their long winter dormancy. Lawns might want fertilizer this late or perhaps later to stay green.
Otherwise, it is getting late to utilize fertilizer.
Late application of fertilizer can be very detrimental to plants that are sensitive to cool weather. Such plants typically finish growing through summer. By autumn, their mature foliage and stems are either resilient to minor frost, or are dieing back (defoliating) for winter dormancy. Fertilizer can stimulate premature development of new growth that will be much more sensitive to even mild frost.
For many of us, this might seem to be irrelevant. We do not apply fertilizer to our landscapes and gardens. Some of us who use fertilizer do so mostly for seasonal vegetable plants and flowering annuals. Such plants will either not be around long enough to get fertilized again this year, or are cool season plants that are on a completely different fertilizer schedule through autumn and winter.
Two other main exceptions that that might continue to get fertilizer after summer are lawns and houseplants. Some types of turf grasses, particularly those in older lawns, can get a bit pale through the the cooler parts of winter, so appreciate a boost. Houseplants are mostly immune to the cold weather outside, so continue to to crave nutrients, even if slowed somewhat by shorter day length.
So, except for lawns, houseplants, and incoming and outgoing annuals and vegetables, most other plants in the garden do not need to be fertilized again until weather begins to warm next spring. Not only do they not need fertilizer, but some could be inconvenienced by it. They know what time it is, and that they should now be slowing down for winter. Fertilizer can interfere with the process.
There are a few reasons why plants slow down or go dormant through winter. Some of the nutrients that they need to maintain active growth are less soluble or otherwise less available to them at cooler temperatures. There is less sunlight too. Plants from climates with cooler winters tend to be more proficient with winter dormancy. Many are deciduous to limit damage from wintry weather.
Some plants might use a late application of fertilizer to get ready for winter dormancy. Others might just ignore it as it leaches through the soil. Those that are from milder climates might try to use it to continue growing later than they should. For plants that are potentially sensitive to frost, growth that develops too late will not likely mature soon enough to be resilient to even mild winter frost.
This is why fertilizer can be applied to many plants one last time about now, but no later.
No more fertilizer! . . . almost. The main exception is turf that might need a bit of fertilizer if it must stay green through the coldest part of winter, but that is another topic for later. Otherwise, most plants start to go dormant through autumn, and should be as dormant as they get by the middle of winter. Fertilizer later than now could be as logical as espresso just before trying to get to sleep.
Keeping marginally frost sensitive plants up past their bedtime can have serious consequences. Lemon trees are not often damaged by frost in the mild winter weather here. That is because they stop growing through late summer, and the last of their new foliage matures before autumn. Fertilizer applied too late can stimulate late new growth, which is more likely to be damaged by frost.
Plants that like a last application of fertilizer as late as the end of September, such as roses, use it for their last minute preparations for dormancy. It actually takes a bit of effort to abscise (shed) foliage. Besides, healthy foliage is easier to abscise than distressed foliage is. Well fed roots, although significantly subdued by dormancy, work later into winter, and get an earlier start in spring.
Roses are, of course, not as likely to be damaged by late application of fertilizer as lemon trees are. They know better than lemon trees do about what to do when the weather gets cool. They just go dormant. Much of the extra nutrients get ignored as they leach through the soil with winter rain. Many of the nutrients become insoluble and unavailable to plants as winter weather gets cooler.
Autumn is the season for allowing most plants to slow down and get ready for winter dormancy. Trying to get them to be as green and productive as they were in spring can do more harm than good. Only plants that are active through winter, such as cool season vegetables, cool season annuals, and some types of turf, might appreciate moderate applications of fertilizer. If newly installed plants want fertilizer, they should get only enough to keep them happy until they get more next spring.
Too much of a good thing can be a problem. That is why bacon is not one of the four basic food groups. It is why sunny weather gets mixed with a bit of rain. It is why we can not give plants too much fertilizer. Since late last summer or autumn, there has not been much need for fertilizer. If fertilized too late, citrus and bougainvillea develop new growth just in time to be damaged by frost.
Now it is time to start applying fertilizer, but only if necessary or advantageous. Fertilizer really is not as important as the creative marketing of fertilizers suggests. It is useful for new plants, fruits, flowers, lawns and especially for vegetables, but is probably overkill for healthy and established plants. There is no need to promote growth of trees and shrubbery that are at their optimum size.
Some of the specialty fertilizers are a bit fancier than they need to be. With few exceptions, complete fertilizers are useful for most applications. As long as plants get the extra nutrients that they crave, they should not complain. They can not read the labels of the fertilizers that they receive. Plants are more likely to have problems if they get too much of something that they do not need.
Rhododendrons and azaleas might like specialty acidifying fertilizer, but should be satisfied with a complete fertilizer. Citrus might likewise appreciate fertilizer that is specially formulate for citrus, but are probably not too discriminating. Palms only want specialty palm fertilizer if they demonstrate symptoms of nutrient deficiency. (Some palms are sensitive to deficiencies of micro nutrients.)
Too much fertilizer, especially fertilizer with a good amount of nitrogen, can inhibit bloom of several plants. Bougainvillea puts more effort into vigorous shoots and foliage than into bloom if it gets strung out on nitrogen. Nasturtium will do the same. In pots and poorly drained locations, excessive fertilizer can become toxic enough to discolor foliage or even scorch the edges of large leaves.
The most justifiable uses for fertilizers now are for flowering annuals and vegetable plants. Tomato and corn plants respond very favorably to fertilizer because they are so greedy with the nutrients they require for their unnaturally abundant production. (In the wild, the ancestors of tomato and corn do not really produce like garden varieties do.) Flowers, of course, take a lot of resources too.