Fish seaweed is mild and well-balanced for fall fertilizing.
Reduced nitrogen
Horticulturists, bonsai teachers and others have long been advising people to reduce nitrogen fertilizing in late summer and fall. The keyword is “reduce.” Some nitrogen is necessary when you fertilize otherwise the phosphorous, potassium and other nutrients can’t do their job properly.
Fall growing season in mild climates
Here’s what Michael Hagedorn of Crataegus Bonsai has to say about nitrogen and the fall growing season: “We might also begin thinking about fall fertilizing. I think fall fertilizing is more important for bonsai, particularly developed ones, than spring fertilizing. In Japan more fertilizer is sold for bonsai in the fall than in the spring. If you live in the Pacific Northwest or other areas with a long, mild fall, re-fertilizing the trees is very wise as it is as long and nearly as productive a growing season as spring.
Don’t discount at least some nitrogen in the fall…there is a lot of talk out there about cutting out nitrogen in the fall, but if you’ve been fertilizing with nitrogen fertilizers throughout the year the trees will shut down naturally due to shorter day length and cooler temperatures by themselves, regardless of nitrogen. Nitrogen is used in all cell processes; don’t cut it out of your fall fertilizer.”
Cold climates
If you live where winters come on fast and last for a long time, don’t wait a day longer. It’s not too late as roots will continue to grow while the soil still holds some of the summer’s warmth (time is running out for trees in pots, but for field grow trees, roots will continue to grow for a while). I use fish-seaweed fertilizer in early September (I live in northern Vermont). It’s mild and because it’s in liquid form, it goes to work immediately.