The 5 Best Fertilizers for Watermelon

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Not all fertilizers are created equal. The difference between the best ones out there and an average one is drastic. It can mean the difference between juicy, sweet watermelons and bland, pale watermelons.

Set yourself up for success when you are growing by buying the highest quality product possible. Many fertilizers do not have the proper nutrient ratios or use poor ingredients. They are not difficult to grow with the proper nutrients and watering. Keep reading below for the ones that we recommend and also a care guide.

Comparing Our Top 3 Recommendations

Fertilizers for Watermelon

At A Glance

These products use quality ingredients and much better than the other products on the market. Please keep reading below to learn more about each one. 

Best Fertilizers for Watermelon

Osmocote Smart-Release Plant Food Flower & Vegetable

Osmocote Smart-Release Plant Food Flower & Vegetable is an excellent choice for all of the stages of its life cycle. Osmocote has developed a special granule coating. This smart-release granule permits water to enter and dissolve the encapsulated nutrients. The nutrients are then slowly released over an extended period of time as temperatures change and as more water is applied. More nutrient is released during warmer growth periods. Osmocote’s plant food will help you successfully grow juicy, flavorful watermelon. Furthermore, because of its balanced N-P-K ratio, it is provides the proper amount of nutrients for many other plants and vegetables as well.

As stated above, mix the granules into the soil around the stem of your plant. Make sure to water well after application and don’t let the granules touch the plant itself.

Dr Earth Premium Gold All Purpose Fertilizer

Growers can feel great about using Dr Earth Premium Gold All Purpose Fertilizer because it’s made from organic materials that are safe for both people and pets. The formula is specifically crafted to grow healthier vegetable and fruit. It is perfect for establishing your plant and also as it continues to mature. It contains a blend of humic acids, carbohydrates, and trace elements and minerals that help to create a better soil quality and nourish plants from the roots up. It contains no GMOs and no chicken waste. Plants treated with Dr. Earth will develop healthy vines and ultimately full, rich fruits.

Mix the granules into the soil. Scatter it around the stem and make sure to water it in well. This will allow it to be absorbed into the soil and prevent it from being blown away by the wind. Make sure that the granules do not come into contact with the plant. This can cause plant and leaf burn.

J R Peters Jacks Classic All Purpose Fertilizer

J R Peters Jacks Classic All Purpose Fertilizer is an excellent choice for a water soluble nutrients. It contains a balance of phosphorus, potassium, and nitrogen that works well for both the immature and mature stages of growth. The water soluble formula sinks into the soil, nourishing the roots to create healthier, stronger fruit.

Tip for application: When you are applying water soluble nutrients, mix it and then pour it right at the base of the main stem.

Triple 10 All Purpose Liquid Fertilizer

Triple 10 is an excellent liquid based fertilizer

Triple 10 All Purpose Liquid Fertilizer contains a balance of nutrients, amino acids, and seaweed extract that help to elevate soil quality and nourish a range of plants. The Triple-10 formula is specifically designed to help plants grow larger and become healthier, even during later growth stages. The product is simple to measure out and pour.

Similar to above, when you are applying it, pour it right at the base of the main stem.

Shultz All Purpose Plant Food Plus

Shultz plant food 10-15-10

Shultz All Purpose Plant Food Plus is the perfect choice, particularly once the fruits begin to set. It has a higher phosphorous content and lower nitrogen so as to not encourage excessive foliage growth. This liquid fertilizer is easy to measure and apply. Adding a few drops of the fertilizer to the soil at the base of the main stem is enough to nourish your plants for an extended period of time. Nutrients will be released into the soil, and thus to the roots, every time it rains or it is watered. Gardeners will find that watermelon treated with this formula experience more juicy and sweet fruit production.

What Type Should You Grow?

There are many different types. The varieties that we recommend are:

How to Grow

Watermelons need warm soil for the seeds to have the highest chance at germinating. It is ideal if the soil can be warmer than 70 degrees Fahrenheit.

Sow the seeds at roughly a depth of 1 inch and keep the soil well watered until germination occurs. To help the germination process in cooler areas, you can start the germination process indoor. Sow the seeds roughly ½ inch deep and place it in a south-facing window or under grow lights. Keep the area warm or use a seedling heat mat. Try to keep the soil at roughly 80 degrees fahrenheit. Once it has germinated, transplant it out doors.

Watermelon 3

Ideal N-P-K Ratio

As you may know, all plants need macronutrients and micronutrients. N-P-K are the primary macronutrients, so you should always be checking labels to find the N-P-K ratio.

Watermelons grow most robustly with higher amounts of nitrogen when they are young and higher amounts of potassium and phosphorus during active melon production.  Using high nitrogen fertilizers after the melons are setting can result in excessive foliage growth rather than melon development. This will nourish the vine rather than the fruit! An N-P-K ratio of roughly 1-1-1 (such as 10-10-10 or 14-14-14) can be used throughout the lifecycle of your plant. Just make sure that if you are using a fertilizer with higher nitrogen components that you switch to one that is more balanced when your melon set.

Fertilize it three times throughout its life cycle. Once before are first planting the seed or transplanting your seedling. Once when the vines begin to ramble. And a third time when the fruits are set.

If you are using a granular fertilizer, make sure it does not touch the leaves. The leaves are sensitive and can be damaged by contacting the fertilizer. After application, make sure to water well so the roots can take in the nutrients.

Where to Plant

Watermelon vines can grow quite long. Sometimes the vines can reach up to 15-20 feet long. Plant in an area where there is a lot of open space. Ideally, you should add some organic matter such as compost to the area. Sow about 8 seeds or germinated seeds in a hill at about a depth of 1 inch. Once the seeds start taking off, leave only the healthiest 3 seedlings.

Make sure that you watch the soil and remove any weeds. You can also prevent weeds by adding a layer of mulch.

Water Only When Soil is Dry

Because watermelons have deep roots, frequent watering is not necessary. Oftentimes, rain itself is enough. Watering is only necessary if the weather turns dry for a long period.

Another tip is to try to withhold even more water when your plants start to mature. This will help your harvest become even sweeter.

Use Boron for Sweeter Fruits

When your plant’s vines begin to ramble, give your watermelons some boron. You can dissolve 1 tablespoon of Borax in 1 gallon of water and spray the base of the plant.

Methods to Determine Ripeness

It can be difficult to know when a watermelon is ripe. Use the seed’s harvest time as a general guideline. Watch the tendril. The tendril is the leave closes to the melon stem. When it turns brown and dries, you know that it is almost ripe. However, this isn’t always accurate as sometimes you need to wait a couple days after it falls off.

You can also tap your watermelon. You want a deep and hollow sounding thud. This suggests that the inners are softening up and ready for eating.

Another clue of ripeness is to look at the bottom of the melon. As it ripens, it should turn from a white to a yellow color.

Bottom Line

Again, I recommend buying the Osmocote’s fertilizer. Don’t make the mistake of buying any average fertilizer at your local gardening store. Many of them will simply not produce the same results. You cannot go wrong with any of the products above. 

If you want to read our more comprehensive guide for growing watermelon, you can do so here. 

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About the author: Carley Miller is a horticultural expert at TheGreenPinky. She previously owned a landscaping business for 25 years and worked at a local garden center for 10 years.

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