I was wondering if it is okay to fertilize conifers in fall. I was told to fertilize shrubs/trees after leaf drop but what about conifers/rhododendrons/hollies? Thanks in advance for your help.
The attached link offers one of the better presentations on fertilizing I have encountered. But pay specific attention to the first line: Fertilizing trees and shrubs in your landscape is not necessarily an annual ritual. Test your soil first to determine if any fertilization is necessary, then apply according to the test results. Often, mulching with a good organic material, like a quality compost, is all the nutrient supplementation necessary.
The remarks regarding fall applications of fertilizer are towards the bottom of the article.
Here is a link that might be useful: Fertilizing fact sheet
Interesting that the article recommends fall fertilization well after the first frost, as not to initiate new growth. I find it hard to believe that nitrogen is taken up and stored by deciduous plants at that time...can someone please explain?
Conifers are not all alike. Members of the Pinaceae (pine family) form buds containing leaf initials in the late summer. Fertilizing at that time or just prior to it, should enhance bud size and content (more leaves). This will lead to enhanced shoot growth in the following spring, and that will stimulate more stem diameter growth. Fertilizing these trees after they shut down in fall may not do any good, as by the time growth begins next spring the fertilizer will have dissolved down through the ground.
Cypress family trees (Cupressaceae) on the other hand, do not form dormant buds, so they can grow anytime conditions allow. You should be able to fertilize them anytime when soil moisture and temperature are permissive.
At least that's how my mental growth model sees it.
Years back, a member of my family had been cleaning off one of the farm vehicles and a good bit of fertilizer had gotten spread around a spruce tree, I was a bit concerned about possible harm being done ... though, come spring that particular tree had absolutely no browning of any needles, this compared to those nearby with some substantial damage.
Since having noted the above, I've been fall fertilizing most of my evergreens and I do think they've been fairing better with my sometimes bitterly cold and sunny winters, winters that can often desiccate them.
Methods researcher and innovator Carl Whitcomb disputed fall fertilization causing late growth of hardy stock in the 1987 (1991) edition of Establishment and Maintenance of Landscape Plants, saying that timing of growth of northern adapted plants was regulated by temperature and day length. When I looked at the 2006 edition in a nearby library some months ago the main discussion of this topic appeared to be essentially the same including talking about why fall is better but in his Summary of Procedures section he now appeared to be favoring spring fertilization over fall (although both were still recommended) - without explanation so far as I was able to see.
In the 1987 (1991) Summary we were advised to
Fertilize all landscape plants as soon as installation and one thorough watering are complete. If soil tests have been conducted and phosphorus and other non-leaching fertilizers have been incorporated, only nitrogen should be applied. Fertilize the plant on the soil surface immediately after planting (spring or fall) or use slow-release fertilizer in the planting hole. Fertilize at the rate of about one pound of nitrogen per 1000 square feet of soil surface area. Spread the fertilizer over the area which has been kept clean of weeds and grass and has been mulched. Repeat fertilization procedure two or three times during the growing season but especially in fall when soils are warm and roots are especially active
To get the exact wording of his most recent recommendations look for the latest editions at nearby college libraries, ones out here with horticulture departments have them and we're a long way from Oklahoma.
Here is a link that might be useful: Whitcomb's Books