Growing Concerns 586: Proper fertilization techniques, mulch all year

Early Fall!

To benefit gardens, fertilizer must be moist and covered or dissolved in water


Dear Janet,

Our perennial garden currently has 3 inches of mulch covering it. Do we have to move the mulch away from each plant to fertilize and then re-cover the area or can we simply put slow-release on top of the mulch and ensure that it gets watered into the area? Our garden is quite large and seems like it would be an impossible task if we had to move the mulch away from each individual plant to fertilize.

Also, can 'whole' fall leaves be left on the garden in the winter or do they have to be removed, mulched or shredded and then placed back in to the garden?


Dear C.B.,

If it's woody mulch and granular fertilizer then, yes, you would have to move the mulch aside or at the very lest scratch the fertilizer into the mulch so it won't just lay on top. The granules have to be in contact with moist soil or mulch for some time in order to melt and become useful to plants. Sitting high and dry on top, they do little if anything.

If it's a woody mulch that persists more than one season, use a water soluble fertilizer during the growing season -- fish emulsion, compost tea, manure tea or one of the blue powders you dissolve in water.

The mulches I like best for perennials are leaves, grass clips, cocoa hulls or finely-ground bark. These break down within the year after they are applied. I add slow release fertilizer to those beds right before I renew the mulch each fall, when the old mulch is thin-to-gone. I use these kind of mulches not just because they make fertilizing easier but because they renew the soil better, something a perennial bed needs.

You can leave the leaves as a mulch or an addition to a mulch. I use leaves pretty much exclusively on my own gardens in fall, whole or shredded. A layer that is four or more inches deep in fall will be just about broken down by late spring. However, sitting on top of dry woody mulch they may not break down well over winter or at the beginning of the next season. For this, too, moisture is essential.



Dear Janet,

I have read that mulching in the Fall is a good thing but that it should be done in the late fall or early winter after the ground freezes.

Sometimes the ground doesn't really freeze until very late (after New Year's) if at all, right?


Dear A.,

We (me, and 26 managers of public gardens I've interviewed on the topic) mulch all year, whenever we need to top up a mulch. I throw leaves all over my gardens any time beginning now, when Nature begins to do just that, and have had no problems.

What you have read confuses advice about normal mulching with adding a heavy mulch to protect marginally hardy plants over winter. Such extra-deep mulch should not be applied over a plant's crown until we're pretty sure both the plant and burrowing animals are dormant, so the plant won't rot and the critters won't find an inviting pile to use for a winter den.

Short report

Some fast-growing trees to replace ash.

L.M. writes to ask which fast-growing trees she might plant to replace a back-yard ash tree that was removed after becoming infested with emerald ash borer.

These trees would fit the bill: Redbud (Cercis canadensis), serviceberry (Amelanchier species), golden rain tree (Koelreuteria paniculata), katsura (Cercidiphyllum japonicum), hackberry (Celtis occidentalis), bald cypress (Taxodium distichum), dawn redwood (Metasequoia glyptostroboides), and lacebark elm (Ulmus parvifolia). Each can be expected to grow one to two feet per year or more.

Those and other trees, with descriptions and sources, are on a list you can have by sending me a long, self-addressed, stamped envelope and one loose first-class stamp to cover my copy costs. The list is also posted on

River birch (Betula nigra) is another tree that might be just right for you. (It is not on the list, but only because a list has to stop somewhere.) It's a 40 to 70-foot shade tree that is mistakenly planted as an "small" ornamental by people who do not know its potential. It grows 1 to 2 feet per year and although the flower is not showy and the yellow fall color is not outstanding, the cream- and cinnamon-colored peeling bark on younger trees is attractive. The bark becomes thick and gray-brown on older limbs and the trunk. Grow it in full sun in moist, well drained soil .


Green thumbs up

to cutting it down as it becomes brown. Rather than doing all of your perennial garden clean up at once, cut back each plant to the ground as it begins to detract from the overall look. This way a big job becomes just a few short forays into the garden, and fall-bloomers like aster and toad lily look better since any eyesore companions are gone.


Green thumbs down

to buying dried-out bulbs, no matter how inexpensive. Bulbs stored too long, too dry feel soft rather than firm. They may grow weakly then not bloom or may die and in dying load your bed with pathogens to infect other bulbs.


Originally published 10/2/04