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I have long been taught that fall fertilization is preferred over spring fertilization for many plants, but there are some exceptions.
We had a tough 2008 growing season with late summer drought and a windstorm that only added insult to injury. Add ice and a generally windy winter and some plants are in need of a little energy boost.
Summer drought and early fall leaf drop (or the severe desiccation that many deciduous plants experience from the wind storm) means that stored energy may be low.
First what not to fertilize: hold off on fertilizing most deciduous trees, if there are significant signs of stress mulch with a rich compost (or fertilize very lightly with 10-10-10); after the leaves fall in autumn you can add some nitrogen for maximum benefit.
Do not fertilize lawns right now, either.
Yes, apply a preemergent weed control but hold off on nitrogen until the fall, as well. Nitrogen is put to better use in the fall on the lawn and for deciduous trees because it gets stored and used slowly; in the spring it gets used up right away on rapid growth, which is not always desirable.
Okay, what we should consider feeding.
Houseplants need some extra energy as daylight increases. Container plants should be fertilized regularly with a diluted water-soluble solution; they rely on use for all their needs so don't forget to feed them from time to time.
Assess you spring flowering bulbs now, too.
If you notice lackluster performance, they may benefit from some fertilizer now.
Bulbs store energy during the late spring and early summer for next year's display. Feed them now with a bulb boosting formulation which is typically higher in phosphorus (bonemeal is a source of phosphorus, as well)...and retain that foliage until it naturally dies back.
The bulb needs the foliage to photosynthesize in order to process the fertilizer and convert it all into stored energy...foliage and fertilizer work together in this case.
Spring flowering shrubs like azaleas, rhododendrons, lilacs, viburnums, kerria, etc. will all benefit from a little extra energy as they bloom.
Spring flowering shrubs bloom, push new foliage and start to set buds for the following year all in a matter of a couple of months, thus some extra food is agreeable. 10-10-10 has long been the catchall fertilizer in the spring, which is fine; if you prefer using organic fertilizers mulching with composted manure, using seed meals like cottonseed or alfalfa, and fish meal and fish emulsion are good all alternatives.
Fertilize your perennial border now, as well.
I generally add leaf mold and compost back to the beds each years which has built up a layer of really rich soil, this is adequate for the long term health of the mixed border.
At planting time, however, I generally water in each new perennial with a water soluble product.
Of you have poor soil consider adding compost each year and using organic mulches (not rocks or landscape cloth).
Organic matter feeds the soil all year round while you supplement it all with an extra spring dose. Same fertilizer choices apply as above.
One other consideration when it comes to feeding plants. Soil pH has much to do with what becomes available to a plant in the form of food.
Certain nutrients are bound to the soil when the pH is too high or too low.
So, know your plant's pH needs: azaleas and hydrangeas, for example, will look pale and yellowish if the pH is too high because iron cannot be taken up by the plant. There are about a dozen micronutrients, too, that may present but unavailable to our plants if the pH is not agreeable.