Knowing when to fertilize boxwoods and how to do it right can spell the difference between robust growth and spindly, ho-hum shrubs. Fortunately, with the right soil pH and mix of soil nutrients, the foliage of the common or American boxwood (Buxus sempervirens) can look glossy and vibrant nearly year-round. You can easily meet your boxwood’s fertilizer needs with periodic applications of either slow-release or water-soluble nutrients. Want to spur new leaf growth or encourage more root development? The decision you make about when to fertilize boxwoods partly depends on your goals for your plants.

Signs that your boxwoods need to be fertilized

How can you tell when to fertilize boxwoods? Whether you’re growing common varieties like Sprinter, Green Beauty, Green Mountain, or any of the dozens of other boxwood varieties, nutrient deficiencies will commonly show up as very slow plant growth, yellowing foliage, smaller-than-normal leaves, and branches which are sparser and less full than their healthier counterparts. Let’s discuss each one of these symptoms in turn.

Sign 1: Yellow leaves on boxwoods

Yellowing leaves definitely can indicate a lack of nitrogen in the soil. Nitrogen is responsible for spurring on green growth. That said, although yellowing leaves can signal when to fertilize boxwoods, discolored foliage also can be the result of:

• Feeding damage from pests like spider mites and boxwood leaf miners—Look for tiny holes and other damage from feeding insects on the undersides of leaves.
• Soil moisture levels that are out of whack—Adequate drainage is essential for these plants, so make sure the soil is not too heavy or too easily compacted.
• Wintertime bronzing—When subjected to very harsh winds or too much bright, direct sun in winter, some boxwoods can turn a bronzy yellow color. (Wintergreen is just one cultivar that sometimes goes from green to bronze.)

Sign 2: Slow growth on your boxwood

What about when to fertilize boxwoods in order to correct seemingly slow growth? Provided you’ve ruled out a pH problem (see section below) and your soil is truly low in nutrients, slow growth can be another indicator that you need to give your boxwood a boost. Choosing a fertilizer with equal amounts of nitrogen, phosphorus, and potassium is a safe bet if you are applying nutrients very early in the growing season.

Sign 3: Thinning boxwood foliage

Changes in leaf color and slowed growth aren’t the only possible signs for when to fertilize boxwoods. Spindly, thinning foliage can also indicate trouble. Again, as long as you’re certain that your pH is OK, you may be able to correct the problem with equal parts nitrogen, phosphorus, and potassium which you’ll learn more about below.

Also worth noting, if you see branch and foliage dieback only within the interior of the plant, this can indicate poor airflow and a need to prune.

The best pH for boxwoods

Of course, the best way to prevent these potential issues is to use a soil test kit and pH tester. That way, you can amend the soil appropriately with nutrient-rich organic matter like well-rotted compost and worm castings and adjust soil acidity levels (pH) if needed.

Unlike most other broad-leaved evergreens that prefer acidic soil conditions—such as rhododendrons, holly, and azaleas—boxwoods thrive in more neutral pH levels, between 6.5 and 7. In low-pH soils (acidic; below 6.5), your plant’s root zone may be surrounded with plenty of nutrients, but the roots will be unable to access them. You can work dolomitic lime into your soil to raise the pH and unlock the nitrogen, phosphorus, potassium, calcium, and other nutrients that may already be present in your soil but are tied up because of improper soil pH.

If your soil’s pH is too high (alkaline; above 7.0), you can add a fertilizer formulated for acid-loving plants or a sulfur fertilizer to drop the pH down closer to the target.

The only way to know how much fertilizer to add, and what product to use, is to conduct a soil test. That way you won’t overdo it and send the pH too far into one direction or the other. Soil tests aren’t very expensive, and you really only need to take one every 4 or 5 years. They’ll also tell you what nutrients your soil is lacking and which it has in excess. Here’s an article with more details on soil pH and why it matters.

When to fertilize boxwoods

Generally speaking, the best time to fertilize your boxwoods is in early spring, but fertilizing in early fall also has its place.

You should avoid fertilizing in late summer, because, during drought conditions or just the heat of summer itself, your plants’ naturally shallow root systems are more susceptible to stress and potential fertilizer burn.

Fertilizing too late in the fall is also a no-go, because it can encourage new leaf and branch growth that will simply die off with the onset of very cool fall temperatures and the first frosts.

Early spring boxwood fertilization

When the growing season starts in early spring, your boxwoods will break dormancy and begin to put on new growth. Making sure they have enough nutrients at this time will help set them up for success over the coming months. As such, the best fertilizer to support your plants during this period typically will provide equal parts of the three essential nutrients nitrogen (N), phosphorous (P), potassium (K). You should be able to find what is known as the N-P-K ratio on your fertilizer packaging. These numbers help you to know how much of which nutrient the fertilizer contains.

For fertilizers you use to fertilize boxwoods in the spring, the label should display equal numbers for the N-P-K ratio (for example, 5-5-5 or 10-10-10). One balanced, slow-release product to try is Burpee’s Organic All Purpose fertilizer.

Our article explaining what the N-P-K ratio is might be helpful to you, especially if you’re a newer gardener.

Late fall boxwood fertilization

Although fertilizing in early spring is best, you can also feed your boxwoods in early-to-mid fall. Doing so isn’t absolutely essential, but, it can be helpful if you’re trying to bolster the root growth of newer or stressed-out boxwoods. In this case, you would offer a light application of a fertilizer that’s lower in nitrogen and higher in phosphorous and potassium (say, 5-10-10 or 3-5-5).

Feeding boxwoods in late fall—and especially adding a lot of nitrogen in late fall—can do more harm than good. Remember, you don’t want to encourage lots of new, green growth just before Jack Frost comes calling. The same goes for pruning boxwoods too late in the season (learn how to prune boxwoods here).

Types of fertilizers for boxwoods

Besides when to fertilize boxwoods you need to decide what kind of fertilizer you want to use. The nutrients you make available to your boxwoods can come from either organic or synthetic sources. What’s more, you can opt for slow-release pellets or granules that break down over time, or you can choose quicker-acting formulations that can be watered directly into the soil.

Organic vs synthetic boxwood fertilizers

Your boxwoods can’t really tell whether the nutrients provided to them come from organic or synthetic sources, but there are important differences between these fertilizer types. Organic fertilizers come from natural sources like fish emulsion, potash, and bonemeal. Because they often have lower overall N-P-K ratios than synthetics, you are less likely to overfeed your plants when using organic fertilizers.

As for synthetic fertilizers, chemists make these by manipulating chemical compounds in a lab. Because some synthetic fertilizers have very high N-P-K ratios, it’s easier to inadvertently overfeed boxwoods with these. This can be harmful for your plants and can negatively affect soil pH and the surrounding environment.

Slow release vs water soluble options

Water-soluble fertilizers are designed to be mixed with water and are then applied directly to the soil. These give plants a quick burst of nutrients all at once. Still, when it comes to feeding boxwoods, slow-release nutrients are often ideal.

Most slow-release fertilizers come in the form of dried pellets or granules. They’re intended to be distributed on top of the mulched ground around your plants. Every time your boxwoods enjoy a rain shower from Mother Nature or a watering session from you, the dry pellets get wet and begin to break down. Little by little, they release nutrients which make their way to your plants’ roots.

How to fertilize boxwoods

To fertilize your boxwoods properly, scatter your chosen nutrients on top of the mulch in a wide arc around your plants. Make sure the fertilizer doesn’t make direct contact with your boxwood’s trunk, bark, or branches, as this can cause damage from fertilizer burn. Keep in mind, too, that you can have too much of a good thing. Applying too much fertilizer can move soil pH levels out of the preferred 6.5-to-7 range or cause nutrient excesses. This, in turn, can render the nutrients you are trying to provide to your boxwoods inaccessible.

Last but not least, give the area a slow, deep watering after you’ve applied the fertilizer. (Drip irrigation systems are perfect for this.) Deeply watering like this will enable the nutrients you’ve applied to make their way through the mulch and the soil surface to the boxwood’s actual roots.

Final thoughts on when to fertilize boxwoods

As you consider when to fertilize boxwoods, recall that early spring is usually best. When your boxwoods are coming out of dormancy, you can make sure they have the nutrients they need for new leaves up top and healthy root development down below. You can choose from organic or synthetic nutrient sources and slow-release or more immediate water-soluble delivery methods; however, slow-release products are particularly well-suited for early spring feeding.

If very high summer soil temperatures have stressed out your plants, you might also offer a light snack in early-to-mid fall. Just remember that the average frost dates in different plant hardiness zones also play a part here. After all, you don’t want new, above-ground growth to emerge just in time to be killed off by your first frost. So, watch your timing and use a fertilizer that’s low in nitrogen and higher in phosphorus and potassium for this application.

Find more shrub care advice here: 

• When to fertilize blueberries 
• Pruning lilacs
• Pruning Japanese maples 
• Why rhododendron leaves turn yellow 
• How often to water new shrubs