Growing watermelons can be a challenge, but with the right fertilizer, proper conditions, and care, gardeners can enjoy abundant harvests. A tender annual fruiting vine, watermelon is a member of the cucurbit family, which also includes cucumbers, pumpkins, squash, and muskmelons. It’s the quintessential summer fruit, conjuring images of picnics and backyard barbecues.
Watermelons grow best in sunny locations at temperatures between 77 and 85 degrees Fahrenheit in warm loamy soil with a pH between 6.5 to 7.5. The plant’s large, sprawling vines require ample room to roam so they don’t overtake other plants. They have a long growing season, with most plants taking 80 to 100 days from seed to fruit. Choosing the best fertilizer for watermelon is the key to a quality harvest.
- BEST OVERALL: Dr. Earth Natural Wonder Fruit Fertilizer
- BEST BANG FOR THE BUCK: Espoma Garden Tone 3-4-4 Plant Food
- PREMIUM PICK: Organic Plant Magic Super Premium Plant Food
- BEST ORGANIC: Jobe’s Organics Granular Fertilizer
- BEST LIQUID: Neptune’s Harvest Hydrolyzed Fish Fertilizer
- BEST BALANCED: Burpee Enhanced All Purpose Plant Food
- BEST CONTROLLED-RELEASE: Osmocote Smart-Release Plant Food Plus
What to Consider When Choosing the Best Fertilizer for Watermelon Plants
Watermelons are heavy feeders that need very fertile, nutrient-rich soil. Selecting the best fertilizer for the plants depends on several factors, including the garden’s native soil conditions and texture, its organic matter content and pH, and individual application preferences. When choosing the best fertilizer for melons, consider the type of product as well as its value and ease of application.
Types of Watermelon
More than 100 cultivated varieties of watermelons exist, which grow in assorted sizes, shapes, and colors, including hybridized seedless varieties. That said, these are the four basic types:
- Picnic melons are jumbo size, typically oval shaped or round with bright green rind and deep red flesh (15 to 45 pounds).
- Icebox melons are perfectly round and small enough to fit in a fridge. Most have dark green rind and deep red flesh, but some have yellow- or cream-colored flesh (5 to 15 pounds).
- Seedless melons are hybrids that, despite their name, have soft pale seeds, albeit far fewer than seeded varieties. These hybrids are sterile, so they require interplanting with seeded varieties to enable pollination for fruit production. Interplant one seeded vine for every two seedless vines (10 to 20 pounds).
- Yellow/orange-flesh melons may be round or oval, seedless or seeded, and they’re often sweeter than their red and pink siblings (10 to 30 pounds).
The secret to choosing the right watermelon variety lies in matching the number of days to harvest with the length of the growing season. Most larger watermelons are ready to harvest 80 to 90 days after sowing, while smaller icebox varieties, such as the sugar baby or yellow baby, mature in 70 to 75 days. Seedless varieties, such as crimson trio and queen of hearts, require 80 to 85 days to ripen.
Types of Fertilizer
Gardeners can choose synthetic (inorganic) or organic fertilizers in either a granular or liquid formula. Both organic and synthetic fertilizers provide the necessary nutrients for growth, but the key difference is in their delivery. Choosing between the two depends on preferences and consideration of cost and environmental impact.
- Synthetic fertilizers are manufactured from chemicals. They contain higher concentrations of nutrients, which are supplied directly to the plants but do not improve soil health. While they generally act more quickly and may be less expensive, they are less eco-friendly as their runoff can pollute the water table.
- Organic fertilizers are derived from plant or animal sources. Although they contain lower levels of nutrients, they break down slowly to improve soil health and nourish plants longer. Organic fertilizers are also less likely to cause “fertilizer burn,” which occurs from overfertilizing.
Both organic and synthetic fertilizers are available in liquid or granular form. Liquid fertilizers are controlled and fast acting, whereas granular fertilizers generally break down slowly over a period of months.
The NPK ratio on a fertilizer label represents the percentage of nitrogen (N), phosphorus (P), and potassium (K) the package contains by volume. A balanced fertilizer might have an NPK ratio of 10-10-10, and a nitrogen-rich fertilizer could have a ratio of 24-4-12 or 20-2-6.
- Nitrogen is important during the early phases of planting for vigorous growth and leaf development.
- Phosphorus helps the plant develop strong root systems and promotes flowering and fruiting.
- Potassium keeps roots healthy and helps them resist drought and disease.
Have an early-season soil test done to determine if the soil is lacking in any specific nutrients and how much N, P, or K it may need to bring it up to optimal conditions.
Watermelon plants require different nutrients depending on the plant’s stage of growth. At the initial planting stage, the plants benefit from a nitrogen boost to help develop foliage, but when the vines begin to flower, the plants need less nitrogen and more phosphorus and potassium to encourage fruiting.
Give melon plants a healthy start by preparing the soil with up to 4 inches of high-quality, well-composted organic matter to help maintain moisture, temperature, and aeration and improve soil structure. When first planting watermelons, use a nitrogen-rich organic option, such as blood meal (dried blood), cottonseed meal, fish emulsion, and seaweed. After vining, bone meal and rock phosphate provide a healthy dose of the phosphorus needed for fruiting.
Adding probiotics, the beneficial bacteria that support plant and soil health, helps increase nutrient uptake and promote the plants’ resilience to environmental stress. To add probiotics to the soil, gardeners can mix in worm castings, unsulfured blackstrap molasses, and organic liquid or powdered kelp concentrate.
Most synthetic fertilizers contain natural elements— nitrogen, potash, and phosphorus—that are not harmful to pets. Manufacturers generally claim that most fertilizers are harmless after 72 hours. The danger arises from ingesting large amounts of fertilizers, either organic or inorganic. Blood meal and bone meal fertilizers can be mildly toxic to dogs and cats, causing gastrointestinal symptoms. To be safe, choose EPA-certified fertilizers that include ingredients like cottonseed meal, cornmeal, fish emulsion, or seaweed. Make sure to follow the package instructions carefully and keep unused fertilizers out of reach of curious pets. Watermelon flesh is safe for pets to eat, but make sure they don’t consume the seeds or rind, which can cause gastrointestinal issues.
Our Top Picks
With the myriad options available, choosing the best fertilizer for watermelon can be challenging. We’ve researched and curated this list of the best watermelon fertilizer choices for enhancing growth, flowering, and fruiting based on brand reputation, quality of ingredients, performance, and value.
Dr. Earth’s Natural Wonder fertilizer is formulated for fruiting vines like watermelon and other melons as well fruit trees and berries. Gardeners can use it during the initial transplanting phase and at any point of the growing cycle because it releases nutrients quickly and feeds plants for months. Its 5-5-2 ratio gives plants a good jump-start with nitrogen, while the potassium helps promote fruiting.
The fully organic formula is certified by the OMRI (Organic Material Review Institute), OIM (Organic Input Materials), CCOF (California Certified Organic Farmers), and the NOP (National Organic Program). It’s designed specifically to nourish the soil by providing probiotics to help create the optimal soil structure and better facilitate water and air infiltration. The organic ingredients work together to produce a nutrient-rich soil that’s easy to work with.
- Type: Granular
- Size: 12-pound bag
- NPK ratio: 5-5-2
- Certified organic
- Can be mixed with water for foliar feeding
- Can also be used with fruit trees and berries
Get Dr. Earth Natural Wonder Fruit Fertilizer on Amazon, at Lowe’s (4-pound bag), or at Walmart (25-pound bag).
Espoma has sustained a solid reputation since 1929 for producing organic fertilizers that break down slowly from the action of soil microorganisms. Garden-tone is no exception, and it comes in a 36-pound bag.
In addition to six types of beneficial probiotics for growth and yield, this organic fertilizer watermelons can benefit from contains Bio-tone, the brand’s proprietary ingredient that feeds microbes to enhance soil fertility, which is essential for plant health. The low salt content ensures adequate water and essential nutrient absorption in the soil, while the 3-4-4 ratio offers an extra potassium and phosphorus boost for robust fruiting.
- Type: Granular
- Size: 4-pound bag
- NPK ratio: 3-4-4
- Includes beneficial micronutrients and probiotics
- Versatile and can be used for other plants
- Long shelf life
- Low salt content
Get Espoma Garden-tone at Walmart, True Value, or on Amazon (4-pound bag).
Organic Plant Magic’s soluble powder mixes with water to create a rich tea with more than 55 trace minerals as well as a proprietary mixture of millions of beneficial microorganisms. The 6-5-5 mixture also contains calcium to protect watermelons from heat stress. The formulation replicates the fungi and bacteria normally found in compost to strengthen plants and make them more resistant to pests and disease.
Don’t be fooled by the small ½-pound bag: The concentrated formula makes 32 gallons to cover up to 1,000 square feet of soil. Gardeners can apply it directly to the soil and plant roots or mix it with water during normal irrigation.
- Type: Powder
- Size: ½-pound bag
- NPK ratio: 6-5-5
- Sturdy, lightweight bucket packaging
- Child and pet safe
Get Organic Plant Magic on Amazon.
A high-quality, reliable, balanced, all-purpose fertilizer, this organic product helps get watermelon plants off to a good start and supports them through their growth stages. Jobe’s Organics Granular Fertilizer improves the soil condition and helps plants resist disease, insects, and drought. The formula contains biozome, a type of archaebacteria that breaks down the fertilizer ingredients into basic nutrients for faster absorption.
The certified organic fertilizer uses feather meal, which is a rich, natural source of nitrogen that’s critical at the first stages of plant life for producing verdant foliage and stimulating overall plant growth. The product also contains manure for a high concentration of the main three macronutrients: nitrogen, phosphorus, and potassium, as well as calcium and sulfur.
- Type: Granular
- Size: 16-pound bag
- NPK ratio: 4-4-4
- OMRI listed for organic gardening by USDA
- Environmentally safe
- Improves long-term soil health
- Unpleasant smell
- Odor may attract dogs and entice them to eat it
Get Jobe’s Organics Granular Fertilizer on Amazon and at The Home Depot.
Neptune’s Harvest organic marine-based fertilizer offers a healthy blend of nutrients made from North Atlantic fin fish, seaweed, and kelp. Fish emulsions offer the long-term benefits of improving soil health and increasing soil fertility.
The product provides a source of burn-free nitrogen, along with the other primary nutrients, phosphorus and potassium. It also contains the trace elements calcium and magnesium and a variety of other minerals necessary for developing strong foliage and plentiful fruit yield. Easy to dilute and apply, the water-soluble liquid acts quickly to provide a quick nutrient boost.
- Type: Liquid
- Size: 8.4-pound container
- NPK ratio: 2-4-1
- Cold processing protects heat-sensitive nutrients
- No unpleasant odor
- OMRI certified
- Easy to apply
- Somewhat pricey
- May be more diluted than some fish emulsions
Get Neptune’s Harvest Hydrolyzed Fish Fertilizer on Amazon or at Neptune’s Harvest.
Produced by a well-known American seed company, this organic balanced 5-5-5 fertilizer contains equal amounts of nitrogen, phosphate and potash, along with other micronutrients. It’s further enriched with BIOMIX, Burpee’s natural blend of beneficial microbes and fungi designed to promote water uptake and nutrient absorption. This versatile formula can also feed herbs, trees, shrubs, container garden plants, and flower beds.
- Type: Granular
- Size: 4-pound bag
- NPK ratio: 5-5-5
- Secure, resealable bag
- Feeds up to 3 months
- Organic formulation contains smaller amounts of nutrients than inorganic products
Get Burpee Enhanced All Purpose Plant Food at Burpee.
Osmocote’s fertilizer granules are coated to provide a controlled release of nutrients over time at the same rate as the plant’s root system needs them. The manufacturer recommends applying it only every 6 months.
With a 15-9-12 ratio, it provides the nitrogen boost that new watermelon plants need, and it also contains a balance of other major nutrients as well as a variety of beneficial micronutrients. One scoopful can feed 4 square feet.
- Type: Granular
- Size: 8-pound bag
- NPK ratio: 15-9-12
- Controlled release prevents overfeeding
- No-burn formula
- Long shelf life
- Can be used on indoor plants
- Not organic
- Not pet friendly
Get Osmocote Smart-Release Plant Food Plus on Amazon, at The Home Depot, or at Lowe’s.
We recommend Dr. Earth Natural Wonder Fruit Fertilizer for its 5-5-2 ratio of nutrients, which provides the nitrogen for a good jump-start and the potassium to promote fruiting. For gardeners on a budget, we suggest Espoma Garden-tone 3-4-4 for its quality organic ingredients at an affordable price point.
How We Chose the Best Fertilizer for Watermelon Plants
In making our selections, we relied on our writer’s considerable experience covering gardening products and also conducted extensive research into the science behind fertilizing watermelons. From a variety of fertilizers, we selected both organic and synthetic, granular and liquid, quick release and slow or controlled release. We considered several factors, including the quality of the ingredients, value for the quantity of product, brand reputation, and positive consumer reviews.
Tips for a Watermelon Fertilizer Schedule
Always follow the instructions on the product label for the amount of fertilizer (rate) to apply and how often to apply it. When buying fertilizer, it’s smart to consider the cost per pound. Large containers and fertilizers with more than 30 percent of total available nutrients (such as a 15-15-15) generally are less expensive. For example, a 50-pound bag of 10-20-10 may not cost any more than a 50-pound bag of 5-10-5 fertilizer, but the 10-20-10 bag contains twice the nutrients.
- Organic fertilizers can be more expensive up front, but because they remain in the soil for long periods (unlike synthetic fertilizers that can wash away through leaching), organic fertilizers can end up being more cost-effective in the long run.
- Mix dry fertilizers into the top 2 to 4 inches of soil; then water it thoroughly. For best results, apply fertilizer in a band along the planting row.
- To give the soil a chance to renew its nutrients and to minimize pest and disease problems, avoid planting watermelons and other cucumber family crops (melons, squash, pumpkins) in the same spot 2 years in a row.
Consider the answers to some of the frequently asked questions below about how to grow watermelons from first planting to harvest.
Q. How often should you fertilize watermelon plants?
A watermelon fertilizer schedule is determined by a garden’s specific soil condition at the beginning of the growing season. Then fertilize it according to the plants’ growing stage because each phase has varying nutritional needs.
Q. When should I fertilize watermelon?
Start fertilizing right after transplanting. Melons do best when treated with small amounts of fertilizer in two or three applications. During the growing season, liquid (soluble) fertilizers may be applied every 2 weeks and granular fertilizers about every 6 to 8 weeks. Since slow-release fertilizers work for months, one application is generally adequate.
Q. How do you grow sweeter watermelons?
Give the melons plenty of water until the final stage of ripening, about 10 to 14 days before harvest. Backing off a bit on watering helps concentrate sugar content, boosting sweetness. Also, as a general rule, the better the nutrient profile, the sweeter the fruit.
Q. Is Epsom salt good for watermelons?
Epsom salt is magnesium sulfate, a source of both magnesium and sulfur. Some anecdotal evidence exists that reports that Epsom salt produces sweeter watermelons. However, always test the soil to find out if it’s lacking these or other substances. Adding Epsom salt to soil with sufficient magnesium can harm your plants by inhibiting calcium uptake.
To give it a try, prepare a solution of 6½ tablespoons Epsom salt and 3½ tablespoons borax in 5 gallons of water. Spray when the plant starts vining and again when small 1-inch melons appear.
Q. Are coffee grounds good for watermelon plants?
Coffee grounds contain the three major nutrients—nitrogen, phosphorus, and potassium—and they also improve soil structure. Mix them in with compost or apply as a soil amendment, adding about ½ inch of grounds directly to the soil approximately 6 inches deep. You can also sprinkle them on top of the soil, but be careful not to let the grounds form a thick crust.
Q. What is the fastest way to add phosphorus to soil?
Apply bone meal (20 percent phosphate) and rock phosphate (27 percent) because both substances contain high concentrations of phosphorus that releases relatively quickly into the soil.
Q. What is the best time to plant watermelon?
In warm zones, sow seeds directly outdoors 1 to 2 weeks after daytime temperatures are regularly between 70 to 85 degrees Fahrenheit and the soil temperature measures 60 to 65 degrees Fahrenheit. In cooler climates, seed the plants indoors; then transplant them roughly 2 weeks after the last frost date, making sure the air and soil are warm enough.