Okay, so this is a fairly lengthy article that delves into some details about fertilizer and then provides instructions for how to fertilize shrubs and trees. If you want to get to the basic instructions scroll down a little.
The two primary reasons for fertilizing ornamental shrubs and trees are to encourage growth, and to create healthy, vigorous, attractive plants. That being said, there is often a temptation to over-fertilize in the hopes of producing an even healthier, larger or more beautiful plant. But be careful! If you force a plant beyond its natural growth rate by over-fertilizing, you can cause it to grow too quickly. This can result in structural problems, predispose the plant to insect or disease infestation, and reduce tolerance to drought or temperature extremes.
So, to grow healthy shrubs and trees, where to start?
Evaluate Soil Conditions
Before fertilizing your shrubs or trees, the best starting point is a soil test. Soil tests are done to determine soil pH and essential nutrient levels. Your local Extension Service might provide soil testing services or you can test soil yourself with a soil testing kit or soil pH testing probe.
Keep in mind that construction and many other factors often result in soils that differ from one landscape to the next, and even from one area in a landscape to another. In other words, the soil in your front yard may be much different than the soil in your backyard.
Once you know the soil pH you can either look for shrubs and trees that will grow well in it, or you can adjust the pH to meet the needs of specific plants.
Soil pH is Important!
Soil pH is a measurement of the alkalinity or acidity of soil and is measured on a scale of 1-14, with 7 as the neutral mark. Any measurement below 7 indicates acid soil conditions, and anything above 7 indicates alkaline.
If you're unsure about the pH of your soil, and whether or not it's suitable for growing the various shrubs or trees you intend to plant, it's a good idea to test the soil pH in the planting area. You can quickly test soil pH with a inexpensive soil test kit or pH tester probe.
To raise the pH (make more alkaline) you can add pelletized limestone to the soil. To lower the pH (make more acid) you can apply Soil Sulfur, Aluminum Sulfate, or Chelated Iron. Adding organic compost to the soil or using compost as mulch can also help to increase acidity and maintain acid soil conditions.
Learn More: What is Soil pH and How To Adjust It?
Soil Nutrients
When selecting a fertilizer, the first question to answer is, "What type fertilizer do I need?" Determining what fertilizer to use can be an overwhelming task.
Which formulation do you need for your particular situation or specific plant or tree? Should you get the bag of 10-10-10 or 16-4-8, go with a specially formulated shrub & tree fertilizer, or maybe a natural or organic plant food? If you did a soil test, the fertilizer you choose could be based on the test results and/or the needs of your plants.
What do those three numbers on a package or bag of fertilizer mean anyway? It may seem a little confusing, but you can figure it out.
The analysis is the three large numbers you see on every fertilizer label, such as 12-6-6 or 10-10-10 or 6-2-0. These numbers represent the percentage (by weight) of the three major nutrients required for healthy plant growth. These three major elements are always in the same order: Nitrogen-Phosphorus-Potassium (N-P-K). Each of these nutrients affects plant growth differently.
The first number represents nitrogen (N). This element provides plants with the ability to produce more chlorophyll, which in turn sponsors more rapid growth to the part of a plant that is above the ground: the foliage. With each additional nitrogen application, plants should grow taller and greener. Typically, fertilizers that are high in nitrogen content are not recommended for use on most ornamental plants, such as shrubs and trees. A well-balanced shrub and tree fertilizer might contain 10 to 15 percent nitrogen, such as 12-6-6, which contains 12 percent nitrogen.
The second number represents phosphorus (P). Phosphorus aids in root development and therefore can help to increase bloom size and bloom production. A fertilizer high in phosphorus content can be used when planting new trees and shrubs, helping them to "root in" more quickly. If you have blooming plants that aren't blooming as they should or not at all, and your soil indicates a phosphorus deficiency, you might consider using Triple Super Phosphate (0-45-0) to boost root growth and increase bloom production.
The third number represents potassium (K). Potassium has many functions. It guards the plant against diseases and aids in drought protection and cold tolerance. It also serves a role in improving root development, and helps in the process of photosynthesis.
You might have noticed that all the numbers in the fertilizer analysis don't add up to 100 percent. That's because there are other nutrients and fillers often added to the mix. For example, specially formulated shrub and tree fertilizers will contain what is called a "micro-nutrient package." This package may contain other nutrients that shrubs and trees appreciate, such as iron, sulfur, manganese, zinc and others. Other fillers might be included to help apply the nutrients more evenly over an area.
Experienced gardeners, nurseryman and professional landscapers may recognize a plants need for fertilizer, or the need for a particular nutrient, by simply looking at the foliage.
For example, if the foliage of a plant is turning light green or yellow, this might indicate a nitrogen or iron deficiency, or perhaps there's too much moisture around the roots. Similarly, purple foliage (on an otherwise green plant) might be a sign of phosphorus deficiency.
The only true way to determine what fertilizer or nutrients are lacking in the soil is by way of a soil test. As mentioned, most states, and many counties, offer soil tests through their Cooperative Extension Service at no charge (or for a small fee). Or you can buy your own soil test kit. A soil test will indicate nutrient levels in the soil and pH. Based on a specific plants needs, this will help you choose the right fertilizer and other nutrients or minerals needed.
Fertilizing Instructions
Once you've determined the type of fertilizer you need based on existing nutrient levels in the soil and plant needs, feeding your shrubs and trees is very easy.
What if you didn't do a soil test?
In the absence of a soil test, you can assume that most plants in a landscape setting require more nitrogen and other elements than they get from natural sources. One reason is the leaves that have fallen from the plant, which could have put nutrients back into the soil, are often raked and removed. So, if your plants are growing or looking as green or healthy as they should, I recommend feeding with a good, slow-release shrub and tree fertilizer, preferably one that contains a "minor nutrient" package, and follow application rates and instructions on the product label.
Alternatively, you can feed your shrubs and trees with a natural or organic plant food, such as Nitroganic. Organic fertilizers are usually "goof proof," which means they are very mild and will not burn your plants, even when over-applied. If you do a summer feeding it's always a good idea to go with an organic fertilizer.
When to fertilize shrubs and trees?
Non-Spring Flowering: I usually feed most of my shrubs and trees, excluding spring flowering shrubs such as azaleas, in late winter or very early spring, before or right when leaf buds begin to swell, or when new leaves just begin to emerge. I feed again in late summer or early fall. Cease fertilization two months prior to the average first frost date in your area.
Spring Flowering: Wait to feed spring flowering shrubs until halfway through their bloom period or when flowers begin to fade.
How much fertilizer?
This will depend of course on the size of the shrub or tree you are fertilizing. You'll find instructions for amount of fertilizer to apply on all specially formulated shrub and tree fertilizers. For shrubs, the amount you apply is usually based upon the height of the shrub. For trees, the amount of fertilizer you apply is usually based upon the trunk diameter. Simply follow the instructions on the bag or container.
How to apply fertilizer?
Shrubs and trees feed themselves from their root system. For most established shrubs and trees, their feeder roots are found at and beyond the outside perimeter of the branch system, what many professionals call the "drip line," and this is where most of the fertilizer should be spread.
How far outside the drip line you spread fertilizer will depend on the age and size of the shrub or tree you intend to fertilize...
For shrubs, you can spread the fertilizer a 3-4 inches beyond the drip line for each foot of shrub height. For example, if a shrub is two feet tall, spread the fertilizer about 6 to 8 inches beyond the drip line.
For trees, you can spread the fertilizer about 2 feet beyond the drip line for about every 10 feet of tree height.
Other Helpful Fertilizing Tips
- Be careful not to apply fertilizer too heavily. Doing so may cause the plant tissue to burn, or even result in plant death. Read product labels carefully and follow directions to avoid toxicity problems.
- If over-fertlilizing your plants is too much of a concern for you, consider easing your mind by using a "goof proof" natural or organic fertilizer, which are made with natural ingredients, such as composted manures and other organic matter so are much less-likely to burn your plants.
- Some plants, like conifers and junipers, don't respond well to excessive nitrogen. Feed them with a fertilizer than contains slow-release nitrogen. Most shrub and tree fertilizers are slow-release.
- As a general rule, the slower the plants habit of growth, the less fertilizer it needs.
If a shrub or tree in your garden appears unhealthy, or is not actively growing, consult with a local arborist, your local Extension Service agent, or a reputable and knowledgeable landscape contractor.
Watering Tips for Shrubs & Trees
Contrary to popular belief, more newly planted shrubs and trees die from too much water than from too little water. When we plant a new shrub or tree in the ground, especially during the hotter summer months, the tendency is to think it has to be watered every day in order to grow roots and become established in its new home.
Fact is, rarely will a newly planted shrub require daily watering to survive and thrive. More often than not, daily soaking will lead to problems with the plants roots, including the dreaded "root rot," which almost always results in death of the plant. Splashing just a little water on plants every day isn't beneficial either.
It‘s difficult to teach proper watering techniques, necessary to establish and maintain woody trees and shrubs. Part of the problem is that there are many variables that must be considered - each landscape situation has unique components that influence how much and how often water needs to be applied. Then factor in the weather conditions that vary from site to site and season to season, and the issue becomes more confounding.
In the final analysis, proper watering requires that the person responsible must become familiar with the water requirements of the plants in their landscape, and the site and soil where the plants are growing. They must regularly check the soil moisture as a guide, and then apply common sense to supplement rainfall as required to maintain uniform moisture in the plant root zone.
Proper watering is especially crucial during the first year or two as the plants are working to establish themselves in the landscape, and to overcome any transplant shock.
Summer Watering
At planting time
Water deeply at planting time, making sure the soil around the root ball is moist all the way down to its base. Then, each day thereafter, check the soil moisture and only provide water if the soil has dried out somewhat or is just lightly damp. After several days of checking you should establish how many days you can wait between waterings. Now you'll have a watering schedule. If there's a good soaking rain you can count this as a watering.
During the first summer
If the weather is hot the soil will dry out quicker and you'll have to pay closer attention to watering, making sure not to over or under water plants. Keep in mind that wilting of leaves can be the effect of either dry soil or when root rot sets in from overly saturated soil. Whenever watering newly planted shrubs or trees during summer, it's best to water deeply less often than to splash plants with a little water every day. Deep soaking promotes deep root growth and can reduce water loss by evaporation.
In following summers
After established, many shrubs and trees won't require nearly as much attention to watering. That being said, in the absence of rainfall, especially during the warm season when plants are actively growing, plants may need supplemental water. During a drought, wilting leaves are usually a sign that your plant could use some water. To be on the safe side though, check soil moisture before watering. Deep soak when necessary to keep the soil damp to moist.
Winter Watering
At planting time
When planting a new shrub or tree during the winter soak the root ball and the surrounding soil deeply just as you would during summer.
During the first winter
Check the soil moisture every few days until you've determined how many days or weeks go by before the soil has dried somewhat and the plant needs watering again. With average rainfall you may not need to water plants during the first winter. Whenever watering newly planted shrubs or trees during the first winter, it's best to water deeply less often than to splash plants with just a little water every day. Deep soaking promotes deep root growth and can reduce water loss by evaporation.
In following winters
After a shrub or tree becomes established, if there is average rainfall, supplemental water will not be required during the winter dormant season. If there is prolonged drought it's a good idea to check soil moisture at least once a week or so and provide a deep soaking if the soil has become dry. If an exceptionally hard freeze is expected, it's a good idea to deep soak around all the shrubs and trees in your yard to make sure there is water around the roots. This water will form an insulating ice sheath around roots that can help protect roots from freeze-drying.
Note: If you have an automated irrigation system it's best to cut it off during the winter, only running it manually when and if necessary. Only provide water during the winter if there's been a prolonged period of dry weather.
Other Helpful Watering Tips
- Never water outdoor plants in the late evening or at night as doing so can cause fungus and diseases to develop. The early to mid-morning hours are the best time to water. This allows moisture to dry from the leaves.
- If the weather has been dry and forecasters are calling for a deep freeze deep soak the ground with water around all of your shrubs. Doing so will help to insulate the roots of plants from the freezing temperatures.
- Hand-watering using a garden hose or drip soaking with a soaker hose are the best methods for watering newly planted plants until they are established, at which time automated irrigation systems will usually provide enough moisture.
- When possible, provide water at the base of the plant, avoiding splashing water on the leaves.
- Mulching around your plants can reduce the frequency of watering. Avoid placing mulch at a thickness greater than 2 inches and keep it away from the trunk of the plant or tree.
- Control weeds as these will rob water from your plants.
Plant Long & Prosper!