How to Use Worm Castings: An Expert’s Guide with Tips and What You Need to Know (2024)
Let's start with the #1 reason we love worm castings - worms and their castings are the culmination of 500 million years of evolution and are much safer, less toxic, and more effective than harsh chemical fertilizers.
Frequently referred to as “black gold” and "the superfood of plants", worm castings contain minerals, nutrients and active biology that are proven to improve soil structure and aid in plant and root growth.
Castings are so effective in plant growth they are a popular product among organic growers and are coveted by vineyard and cannabis growers, home gardeners, and many other grow industries.
And unlike chemical fertilizers, castings are an all-natural, long-lasting, and versatile superfood that helps plants increase yield, grow larger root mass, and resist diseases, without the damaging side effects of chemical fertilizers!
Benefits of Earthworm Castings
Worm castings are alive with microbes and bacteria and are full of organic matter, and this biology has been proven through numerous scientific and academic studies to help plants grow larger root mass which leads to greater yield and the prevention of disease from insects and the environment.
So how do castings have this effect on plants? They do it with:
- A rich population of beneficial fungal and bacterial feeders - This active biology helps create optimal, fertile soil structure by releasing nutrients that are easy for plants to uptake.
- Improved soil porosity - The size and composition of castings promote air flow and improve water retention, both of which aid in root growth.
- A wide variety of water soluble minerals and nutrients - These nutrients are easy for plants to absorb and better than traditional fertilizers, which are toxic and often lost before plants can absorb them.
Where to Use Worm Castings
Castings are an excellent soil amendment for virtually any plant type or grow system, including:
- Vegetables, fruits, and herbs
- Gardens and raised gardens
- Lawns, trees, shrubs, and vines
- Perennials and Annuals
- Potted plants and hanging baskets
Worm castings can also be used with any type of planting - from new seedling plantings or established plants and flowers to lawns, trees, and shrubs.
Basically, if it has a root system, lives in the soil, and grows, it will love worm castings!
How to Use Worm Castings
Worm castings can be used with existing potted plants, gardens, and lawns, or with new plantings and seedlings.
The application and use will differ, though, with castings being mixed into the potting medium for new plantings and placed on the surface and gently kneaded into the soil for existing plants.
Here’s a breakdown for using castings for virtually any home horticulture or garden project:
Application & Amount to Use
Existing Vegetables, Perennials, & Annual Flowers
Spread 1-2 inches of castings on the soil above the plants’ root systems. Gently work the castings into the soil and water lightly.
2-3 times per year in early spring, early summer, & early fall
Existing Potted Plants, House Plants, & Hanging baskets
Sprinkle ½ to 1 inch of castings on the top of the soil. Gently knead the castings into the soil and water lightly.
Every 2-3 months
Existing Trees, Shrubs, Vines, & Roses
Apply a 2-3 inch layer of castings around the base of the plant and root system. Gently knead or rake into soil and water lightly.
2-3 times per year in early spring, summer, and fall
Existing Gardens & Flower Beds
Apply a 1-2 inch layer of castings (~5 pounds per 10 square feet) to the garden or flower area. Knead or rake into the soil where possible and water gently.
2-3 times per year in early spring, summer, and fall
New Vegetables, Perennials, & Annual Flowers
Line each seedling hole with ½ to 1 inch of castings, or each plant hole with 1 to 2 inches of castings, insert seed or plant, and cover with soil or your grow mix.
If you are planting a new garden or raised bed, you can instead mix 15% to 20% (1:5 ratio) castings with your soil or favorite planting medium before seeding or planting.
Once at planting, then top dress with ½ inch of castings every 2-3 months from spring to fall.
New Potted Plants, House Plants, and Hanging Baskets
Use 15% to 20% (1:5 ratio) castings with your soil or favorite planting medium.
At planting, then top dress with ½ inch of castings every 2-3 months
New Trees, Shrubs, Vines, & Roses
Mix 15% to 20% (1:5 ratio) castings with your soil or favorite planting medium before planting. Line plant hole with mixture, insert plant roots into the mixture and cover with additional 1:5 castings to soil mixture.
New Gardens & Flower Beds
Mix 15% to 20% (1:5 ratio) castings with your soil or favorite planting medium before planting.
At planting, then top dress with 1-2 inches of castings every 2-3 months
New & Established Lawns
Apply 20-25 pounds of castings for 100 square feet of lawn space. Water lightly.
2-3 times per year (spring and fall)
How to Store Worm Castings
Here’s one last tip on how to store worm castings - the nutrients and biology in castings are so dense that a little bit goes a long way. If you have castings left over, just store the worm castings covered and away from direct sunlight or moisture. Covering the worm castings will keep them from drying out and protect them from UV radiation which can harm the beneficial biology in castings. Worm castings will usually last 12-18 months if stored properly.
Frequently Asked Questions: How to Use Worm Castings
We’ll end this post on how to use worm castings by covering a few frequently asked questions and tips we’ve learned.
Are worm castings good for all types of plants?
Yes, worm castings can help any plant, tree, shrub, or grass grow. The biology, minerals and nutrients, and composition in castings will improve soil structure and provide the building blocks plants and their root systems need to thrive.
Can I use too many worm castings?
No, castings will not burn or harm your plants at any application level. Although castings are chock full of nutrients, minerals, and active biology, plants will only absorb what they need to thrive. Studies have shown, however, that worm castings do not help plant growth above a certain amount, so the generally accepted amount of worm castings to use is 15-20 % castings to 80-85% potting or growing medium.
Are there different types of worm castings?
Yes! Worm castings will either be fungal (meaning they have lots of fungi) or bacterial (meaning they have lots of bacteria) in nature. Although most plant life will benefit from fungal or bacterial castings, annuals and vegetables like bacterial soils while perennials, trees, lawns, vines, and shrubs prefer fungal soils and castings.
Are all worm castings the same?
No - all worm castings are not the same. Castings are impacted by several factors during their life cycle. Bedding and feedstocks (like quality compost and manure) that are high in beneficial microbes will generally result in higher quality castings with higher quality. Quality in = quality out! In addition, how the castings are processed and stored will impact their biological levels over time.
Finally, pH and the density of nutrients and minerals varies across castings based on the bedding and feedstock used by the worm farm.
How many worm castings should I use per gallon of soil?
We recommend using a ratio of 15-20% castings to 80-85% soil or potting mix.
Where can I buy castings?
Castings are available through many worm farms, either in their online store or on Amazon. You can also buy worm castings at many retail locations, form big box locations like Home Depot to smaller lawn and garden shops. We sell them in small batch and bulk sizes at Brothers Worm Farm.
Good luck and happy growing!