Fertilizing evergreens (conifers)
Like all landscape plants, evergreens remove nutrients from the soil. In the forest, needles and twigs accumulate on the ground and return nutrients to the soil. Under cultivation, evergreens usually receive fewer nutrients from this source because some needle and twig litter is removed beyond the drip line or tips of their branches.
At some point it might be necessary to fertilize your evergreens, though evergreens generally require less fertility than deciduous trees. In many landscapes, evergreens also benefit from fertilizer applications to the lawn.
The plant itself will often indicate when it needs fertilizer. If growth rate and needle color are normal for a particular variety, fertilization is not necessary. If new growth is sparse or slow, or the needles are not a healthy color, or are shorter than normal, you should probably fertilize. Keep in mind, however, it is not unusual or abnormal for newly transplanted evergreens to exhibit slow growth until they're re-established.
Regular fertilization may be recommended if you are trying to grow evergreens in a less than ideal site, such as very sandy or heavy clay soil, or if the plant has suffered damage from insects or disease. You might also wish to fertilize to encourage more rapid growth in relatively young evergreens.
What to use
A complete fertilizer that supplies nitrogen, phosphorus, and potassium, such as 10-8- 6, is often suggested. This formula can vary somewhat, but usually the nitrogen content (the first number) will be higher than the phosphorus (second number) or potassium (final number).
It is always best to have a reliable soil test run before fertilizing, as much of our soil already has sufficient amounts of phosphorus and needs no extra. Testing will also show whether the soil is acidic or alkaline. Generally, evergreens grow better when soil conditions are acidic; many nutrients may be unavailable to the plant when soil is too alkaline.
Timing fertilizer application
The best time to fertilize is early April, before new growth expands, but you can apply fertilizer anytime until midsummer (roughly July 15). Applications beyond this period will stimulate growth late enough in the season that it may not have time to harden off before cold temperatures arrive. Such growth is much more likely to suffer winter injury and dieback.
An exception would be the use of slow or timed-release fertilizer such as Osmocote or Sta-Green. A light application in late summer or early fall may help nutrient-stressed trees come through winter in better shape. Mid to late autumn applications of slow-release fertilizer are also usually safe.
Never fertilize drought-stressed plants. If conditions become quite dry after you've fertilized, it's doubly important to water your evergreens regularly.
How much fertilizer to use
Fertilizer recommendations are usually given in pounds per thousand square feet. To find out how much to use, figure out roughly how many square feet your tree or shrub covers. For instance, a spruce that measures five feet across would cover twenty-five square feet. If it were large, with branches extending five feet in each direction, it would be ten feet across, and cover about one hundred square feet, in total.
A common "maintenance rate" of fertilizer is two to four pounds actual nitrogen per thousand square feet of soil surface, applied every two to four years. For mature, slower growing trees, one pound of actual nitrogen is probably enough.
Figuring actual nitrogen
It's easy to figure out how much actual nitrogen is in a bag of fertilizer, because the numbers in the fertilizer analysis are actual percents, by weight.
A forty pound bag of 10-8-6 is ten percent nitrogen. Ten percent of forty is four, which means the entire bag contains four actual pounds of nitrogen (plus phosphorus, potassium, and an inert carrier). A thirty pound bag of 21-0-0 is 21 percent nitrogen. Twenty-one percent of thirty is 6.3, which means the bag contains 6.3 pounds of actual nitrogen. (The rest is an inert carrier.)
As long as the rate used is two pounds actual nitrogen per thousand square feet or less, you can spread the fertilizer with a drop spreader under the evergreen branches and slightly beyond, then water it in well. Any higher rate would burn the grass it was spread onto, even if it were watered immediately after application. A more effective method would be to make holes about eight to twelve inches deep, then place fertilizer right in the holes. Space the holes about two feet apart in large concentric circles, starting one and one-half feet from the trunk of young trees or three feet from the trunk of larger, more mature trees.
Thoroughly soak the area where you will fertilize, to soften the soil so it's easier to make holes. Use a crowbar, soil auger, metal rod, or other tool to create holes roughly two inches in diameter. Divide the amount of fertilizer needed, and place it equally into each hole. Then water the area. There's no need to refill the holes with soil.
Using these guidelines for making the holes, the following table gives you a short-cut way to determine how much of several common fertilizers to place in each.
Table: Amount of fertilizer per hole, at approximately two pounds actual nitrogen per thousand square feet.
|Fertilizer composition||Amount per hole|
|10-8-6||2 1/2 tablespoons|
|21-0-0||1 1/4 tablespoons|
Using spikes or root feeders
Fertilizer spikes are a convenient and simple way to fertilize evergreens, and are effective when used in sufficient quantities. However, since each spike contains only a small amount of fertilizer, they are not cost-effective compared to granular products.
Applying liquid fertilizer through a root feeder is another option on all but heavy, poorly drained soil. But it can also mean additional work when applied to light, sandy soil. Liquid fertilizer leaches through sandy soil quickly, and may need to be applied several times throughout spring and early summer. Again, this method of fertilizing, though convenient, is more costly than applying standard granular fertilizer.
Keep evergreens healthy by mulching the soil surface under their branches and slightly beyond. Start a couple inches out from the trunk, and apply the mulch three or four inches deep. Not only can you apply fertilizer right over the mulch (watering it in well), but mulch will help retain soil moisture and insulate roots from fluctuating temperatures. In the landscape it has the added benefit of protecting plants from grass and weed competition as well as injury from lawnmowers or weed whips.
Typically, evergreens are mulched with wood chips or shredded bark. However, any mulching material will do; well-rotted manure or seasoned compost will add a small amount of nutrients as they break down further. Do not lay plastic sheeting under the mulch. Usually weeds are not a problem, but if you want to put down a barrier, choose a landscape fabric that "breathes" and allows moisture to both enter and leave the soil.
Thanks to Gary Johnson, Urban Forester, and Beth Jarvis, Horticulturist, for their help with this Brief.