Fertilizer 101: When and How to Fertilize Your Lawn and Plants

As the days grow longer and warmer, I can’t help but get excited about the upcoming growing season. It’s a time when lawns come alive, but sadly, many homeowners miss the mark when it comes to proper fertilization. Over the years, I’ve had the unique opportunity to observe countless yards and gardens, testing various fertilizers along the way.

Diving into the world of fertilizers can feel overwhelming at first, which is why I’ve compiled this guide — to help you navigate these choices with confidence. Of course, there’s no shame in calling in professionals like TruGreen if you’re not up for the task. Whether you choose to roll up your sleeves or leave it to the pros, understanding the basics of fertilizer numbers and timing is crucial.

After all, a healthy lawn isn’t just about aesthetics – it’s about creating an environment where beneficial organisms thrive and unwanted visitors struggle to gain a foothold. So, let’s dig in and uncover the secrets to a lush, vibrant yard that’ll be the envy of the neighborhood.

What Are the Numbers and Letters on Fertilizer?

The numbers and letters on the fertilizer will help you identify if you are applying the right mix of macronutrients to your turf. The numbers and letters that you see are called the N-P-K ratio. To better understand this, you may have to take a trip back to your high school chemistry days.

The first letter, N, stands for Nitrogen. The P stands for Phosphorus, and the K stands for potassium. These are three organic ingredients that a beautiful lawn will have to keep in balance to look great.

When you see a fertilizer that is 20-5-10, it means that the fertilizer contains 20% Nitrogen, 5% Phosphorus, and 10% Potassium. After conducting a soil test, you can choose the fertilizer bag that is right for your lawn.

Meaning and Importance of N-P-K

If you don’t have the proper mix of N-P-K in a slow-release fertilizer, you can do both long and short-term damage to the plants and turfgrass you are caring for. Here are a few things that you should know about the Nitrogen, Phosphorus, and Potassium in a fertilizer.

Nitrogen

Nitrogen helps to improve leaf growth and longer shoots. Therefore, green leafy vegetables and turf will like a good mix of nitrogen in the formula to be healthy and strong.

Phosphorus

Phosphorous will promote good root growth. This can be important for plants and vegetables but also for lawn needs as you try and get your healthy grass to spread and leave things like crabgrass from having any room to expand. This component is found in bone meal, as well, and makes for a balanced fertilizer while promoting leafy growth.

Potassium

Potassium is used to help plants and turf become more hearty. Think of potassium as almost a vitamin you can give to your lawn. Just as with humans, if you overdo it on the vitamins, you may not feel all that well. The same can be said when looking to green up a lawn. If you aren’t careful with the fertilizer label, you can certainly overdo it.

Where are the NPK Numbers on Fertilizer

The NPK numbers for your spring, summer, and fall fertilizer are on the front of the bag. If you purchase granular or liquid fertilizer, seeing the N-P-K ratio should be very clear. Homeowners that want a healthy lawn need to also keep in mind the type of turf they have when considering these numbers.

Best Fertilizer Ratios to Use on Your Lawn

Now that you have a basic idea of what the fertilizer numbers mean and why your turf needs them, let’s look at what fertilizer numbers to use specifically in spring. The application rates will vary depending on the type of grass you have and the results of any soil tests that you may take.

Best Fertilizer Numbers for Spring

The best fertilizer to use on your lawn in the spring is something with a higher percentage of nitrogen. A nitrogen fertilizer will help your turf grow stronger when spread in the early spring. That said, getting the ratio right is important because using too many pounds of nitrogen can cause long-term damage.

A few of the different fertilizers that would be best for spring turf will be:

• 15-0-6
• 20-0-5
• 30-0-3
• 35-0-5

I typically recommend starting with the 20-0-5 because 20 percent nitrogen generally is a safe amount for most turf. For those who want a lawn that is not going to damage on the first warm day, or can make it through a fungus issue, keeping that first number (Nitrogen) a bit higher is an excellent idea.

The reason that you will see the zero as the middle number for the phosphate is that lawn fertilizers typically have a 0 in the middle. If your lawn needed phosphorus, the best way to tell would be a soil test.

Choosing the Best Fertilizer for Your Garden in Spring

The best thing you can do for your spring garden is to plant it at the right time. Unlike cool-season grasses or warm-season grasses that can survive all year, if you plant the garden at the wrong time, no fertilizer will help bring the plants back. For the best option for gardens in the spring, you will look for a high phosphorous rate. Something like 12-55-6, 16-20-0, or even a very standard 20-20-20. Some organic fertilizers with much lower numbers may be best for a brand-new garden.

Choosing the Best Fertilizer for Trees and Shrubs in Spring

Many homeowners forget to apply fertilizer to trees and shrubs, but it can help with micronutrients and overall plant growth if you fertilize correctly. One thing to remember with trees and shrubs is that you don’t need them to grow fast. In fact, slow and steady growth is a much better choice for shrubs and trees.

I recommend using a 10-10-10 fertilizer in the spring for all new trees and shrubs. Sometimes if you are growing fruit trees, you will want something like 8-3-9 — sometimes, fruit trees need some more specialized formulas to promote the right type of growth.

Best Fertilizer Number For Your Lawn in The Summer

The best fertilizer ratios for summer are 6-0-0, 15-0-8, and 10-0-4. These ratios will keep your grass strong and healthy while avoiding fertilizer burn in the heat.

Before you get any further in your search for the best fertilizer ratio for your summer turf, you must make sure that you know the type of grass you have. A cool-season grass like Kentucky Bluegrass cannot be fertilized in the summer. You can fertilize Bermudagrass or St. Augustine in the summer.

Even when fertilizing in the summer months, you will have to be careful to use a more mild fertilizer with slow-release nitrogen. Fertilizing correctly and with the right formula is key to preventing fertilizer burn on your summer turf.

Typical summer grass fertilizer numbers include:

• 6-0-0
• 15-0-8
• 10-0-4

Best Fertilizer for Your Garden in the Summer

The 20-5-20 fertilizer tends to be the best bet for the garden in the summer. Make sure that you are fertilizing before it rains so the fertilizer can absorb properly and make its way into the soil. It’s typically best to save fertilization for the early part of the summer when possible.

Best Fertilizer for Trees and Shrubs in the Summer

Fertilize the trees and shrubs in your yard in the early summer. However, if it gets towards the late summer and you have still not fertilized, it makes sense to skip it and wait for the fall. The 16-8-8 is good for early summer fertilization for trees and shrubs.

Best Fertilizer Number to Use on Your Lawn in the Fall

Again, I recommend that you do not fertilize your cool-season grasses in the summer as they won’t handle it well. The fall is when you should leave your warm-season grasses alone.

The key here is to use a lower Nitrogen ratio — the last thing you want is to have Nitrogen trying to work when temperatures get too cold for the grass to grow. This will stress the lawn and can create bare spots on your turf.

The best fertilizer number to use on your lawn (cool-season grasses) in the early Fall include:

• 6-0-0
• 20-0-5

The 20-0-5 is very commonly sold in stores for fall fertilizer, but you should test your soil before applying this much nitrogen in the late fall. Fall fertilization aims to ensure your grass is healthy enough to make it through the winter.

Best Fertilizer for Your Garden in the Fall

The best fertilizer for your garden in the Fall is 24-4-12. This is a much higher Nitrogen percentage than earlier in the year, and it makes sure that your plants will stay alive throughout the winter months. At this time of year, make sure that you are not giving garden plants too much fertilizer. Forcing them to grow faster when temperatures are not conducive to growth can strain the plant.

Best Fertilizer for Trees and Shrubs in the Fall

Once all of the leaves have fallen from your trees and shrubs in the late fall, you can apply a 12-6-4 fertilizer to get you through the winter months. This fertilizer will give a nice mix of nutrients to help the trees and shrubs have better health throughout the winter, but it won’t encourage growth during the wrong season.

When to Fertilize Your Lawn

When deciding when to fertilize your lawn, there are three things you need to consider, the temperature, the weather, and the growth of your lawn.

• Temperature: Cool-season grasses should be fertilized when the temperatures are between 60-70; warm-season grasses should be done in the 80-90 range.
• Growth of Lawn: The best time to fertilize is when your grass is actively growing; if it is not growing, wait a few more weeks to fertilize.
• Weather: Wait until your lawn is dry before you apply fertilizer. Try to time this so that you put the fertilizer down the day before a rain shower. The fertilizer will eventually need to be watered in, and a nice rainstorm can help your efforts.

How to Fertilize Your Lawn

Fertilizing grass is all about paying attention to the minor details. I’ve encountered many homeowners in the industry who simply want you to listen and do as they tell you, regardless of the time of year and weather.

It doesn’t work that way. There’s a right and a wrong way to fertilize your lawn, and if you choose the wrong way, you’re not going to get the results you want.

If you choose the right fertilizer numbers and follow these tips, you’ll have a lawn that’s treated for months.

1) Test The Soil

Before treating your lawn with fertilizer, it is good to have your soil tested. Sometimes if it is a new lawn or the grass has not been aerated in years, there will be micronutrients that are missing and will need replacement. I think at-home soil tests are good enough for most homeowners.

2) Purchase High-Quality Fertilizer

The better brands have a more nutrient-rich formula worth the extra money. Make sure to get a bag of fertilizer without a lot of filler or amendments like mulch. The best fertilizers contain magnesium, calcium, and a good amount of nitrogen. Lawn care providers like TruGreen have access to more potent fertilizers that are harder for homeowners to get without an application license.

3) Check The Temperature

There are ideal temperature ranges for fertilizer for both cold-season and warm-season grasses. For warm-season grasses, the temperatures are usually in the very high 70s to mid-80s. For the cool season grasses, the temperature range will likely be in the 60–70 range. The exact fertilizer you decide on will tell you the perfect temperature.

4) Check The Forecast

It is never a good idea to put your fertilizer down on wet grass. The key is to put the fertilizer down when the grass is dry and then have it rain within the next day or so. Make sure there is no rain in the forecast for the day you plan to fertilize.

5) Use A Spreader With Settings

When applying fertilizer, you must not do it haphazardly. Use a spreader that has settings on it. The settings will tell you how many pounds of fertilizer are dispersed per square foot. You can determine the type of turf you have and the amount that your lawn needs with a soil test. The best fertilizer spreaders also allow homeowners to guarantee that the fertilizer does not pile up in certain areas.

6) Protect Yourself

Fertilizer may be good for your grass, but it is not great for humans. Make sure to wear the proper personal protective equipment when applying fertilizer to your turf. Additionally, give the fertilizer a few days to sink into the soil before you mow the lawn. If you are worried about this, use a lawn care provider like TruGreen that can help ensure your safety.

Types of Fertilizers

1. Granular Fertilizers: These are my go-to recommendation for most homeowners. They’re easy to apply with a spreader and provide a slow, steady release of nutrients. I’ve found they’re less likely to cause fertilizer burn when applied correctly.
2. Liquid Fertilizers: These work faster than granular types, which can be great for a quick green-up. However, I always caution homeowners about the risk of overapplication, which can lead to nutrient runoff and potential harm to beneficial insects.
3. Organic Fertilizers: Made from natural materials like compost, bone meal, or fish emulsion, these are excellent for improving soil health over time. They’re slower-acting but safer for pets and wildlife. In my experience, they also tend to attract fewer pests than synthetic options.
4. Synthetic Fertilizers: These are chemically manufactured and provide a quick nutrient boost. While effective, I’ve observed that overuse can lead to soil imbalances and potentially attract certain pests. I always advise careful application.
5. Slow-Release Fertilizers: Available in both organic and synthetic forms, these provide nutrients over an extended period. They’re great for busy homeowners who can’t fertilize frequently, and I’ve noticed they tend to promote more consistent growth.
6. Water-Soluble Fertilizers: These dissolve in water and are often used for container plants or hydroponics. While efficient, I’ve seen cases where improper mixing led to plant damage, so I always stress the importance of following instructions carefully.

When choosing a fertilizer, consider your soil type, the plants you’re feeding, and your local climate. In my line of work, I’ve seen how the right fertilizer can not only promote healthy growth but also help plants naturally resist pests and diseases. Remember, a well-fed lawn or garden is more resilient and requires fewer interventions in the long run.

Final Thoughts About Fertilizer Numbers

At this point, you should feel considerably better about what fertilizer numbers to use in the spring. Always make sure that you check the current chemical makeup of your turf and do soil testing before fertilization. If all of these numbers are too much, hiring a company like TruGreen can give you peace of mind that you’ll have a beautiful, green lawn soon.

Methodology: How We Rank Lawn Care Companies

At Today’s Homeowner, we strive to bring you the most accurate information for your home improvement projects. Our lawn company ratings emphasize what matters most to you — a wide range of useful products with reliable service and impressive results.

Our ratings are based on publicly available information about each company, secret shopping via phone and internet, customer-review analysis, and discussions with lawn and landscaping specialists.

Each company can earn up to 100 points divided across five categories. We assess those point scores, then convert them to a five-star scale to determine rankings. We researched popular lawn care companies across the country to develop the following category breakdown:

• Plan Options (36 points): Lawn care companies earn the most points in this category if they offer a wide range of plans to fit various homeowner needs. Companies providing services like fertilization, aeration, seeding, and tree/shrub care will earn more points than providers with base-level plans.
• Additional Offerings (11 points): We score this category by evaluating additional offerings that enhance a company’s overall service selection. Companies that provide pest control services, organic lawn care products, and a la carte options score the highest in this category.
• Customer Service (25 points): We considered how far each company goes to ensure customers have convenient, intuitive access to products and services with contact forms, online chat features, and additional learning resources.
• State Availability (8 points): We factored each company’s state availability into its overall score. Companies with more locations and wider service areas score higher than those with limited availability.
• Trustworthiness (20 points): We score each company’s trustworthiness based on its Better Business Bureau accreditation and score, number of years in business, and Google Star ratings.

We monitor company data on an ongoing basis to keep rankings and information up to date.