Fertilizing 101. part 3: fertilizers for the fall

Fertilizing 101. part 3: fertilizers for the fall

Here we are in the third day of fall in southern Ontario and it’s a steamy 31 deg celsius (88 F).  I have watered more in the last week than all of the summer!  Annuals are blooming madly and the butterflies are still hanging about.


A swallowtail on my lantana

However, it is fall and time to plant bulbs, trim back dead summer leaves and apply fall fertilizer to your lawn and garden.  Fertilizer whether synthetic or organic is not all the same and different types should be applied at different times of the year.  Now is the time for a high potassium (K) fertilizer to be applied, the third number on the bag of synthetic fertilizer.  Organic sources of potassium include greensand, kelp meal and hardwood ashes.  Potassium slows new growth and prepares grass, plants and shrubs for winter dormancy.  A little phosphorus for plants can be included to encourage healthy root growth while the top growth slows in the cooler weather.  The last thing you want to add is high nitrogen as it encourages new growth that will be killed off by a sudden frost and can cause deeper winter kill in your roses and shrubs.  So look for a low or preferably zero N number, a low P number (5 or lower) and a higher K number (above 10).  Fall lawn fertilizers have some nitrogen as the grass stores it for new spring growth.  Bonemeal is a good organic source of phosphorus.


A typical fall lawn fertilizer with a little nitrogen, no phosphorus and high potassium

Apply when growth slows down and the leaves have turned their fall colour, usually October and even November depending on where you live.  Too early and the plants will try to grow, especially roses!  The silly things just don’t know when to stop sometimes 🙂


My Dainty Bess, an old but beautiful tea rose from the 1920’s showing rose hips, blooming roses and new growth on Sep 25.  She grows until she freezes!

Don’t forget to water!  Even though plants are starting to die back, water is very important in the fall.  Make sure your plants, shrubs and evergreens are well watered up to the first frost and they will overwinter much better.  Apply your fertilizer first and then water it well into the soil if there is no rain in the immediate forecast.

Plants take up nutrients in the exact same way whether it comes from a synthetic or organic source.  The major differences in synthetic fertilizer vs organic is the nutrient content and how they are made.  A bag of synthetic fertilizer must contain a guaranteed analysis of the N-P-K ratio, list any other ingredients such as a combined pesticide, directions for use, cautions etc.  Synthetic fertilizers generally have a much higher concentration of nutrients which are quickly released (unless stated as a slow release fertilizer) , increasing the risk of burning your plants if you over apply.  It is very important to follow the application instructions.  If I use a synthetic fertilizer, I always use less than the label states. Synthetic fertilizers contain an inert material as filler to make it easier to spread but doesn’t add any thing to the soil.  Synthetic fertilizers are more readily available in dissolvable form than organic fertilizers as well.  I once used an organic liquid fish fertilizer.  The smell made every cat within miles descend on my garden with the usual feline reminders left behind!!


Organic compost to be added to soil

Whichever you use is personal preference depending on time and budget and ease of application. Organic is better for the environment but if misused, can be almost as damaging as overuse of synthetic fertilizers.  For example, manure runoff into streams causes pollution, algal blooms and can spread disease pathogens just as readily as a chemical fertilizer.  We are stewards of the land and must be conscious of all our actions.


Don’t forget to save and mince your leaves for a winter garden blanket!

Happy gardening 🙂

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