The answer to whether 12-12-12 fertilizer kills weeds is no. This product is formulated as a balanced nutrient source intended to promote healthy growth across a wide range of plants, including lawn grasses, vegetables, and ornamentals. It functions as a plant food, not as a selective or non-selective weed killer. The fundamental difference is that the product’s purpose is to nourish, which often benefits weeds just as much as the plants you intend to grow.
The Purpose of 12-12-12 Fertilizer
The designation “12-12-12” refers to the percentage by weight of the three primary macronutrients: Nitrogen (N), Phosphorus (P), and Potassium (K) in the blend. This equal ratio makes it a general-purpose, balanced fertilizer suitable for supporting overall plant health in various soil types and for many different species.
Nitrogen, the first number, is responsible for promoting vigorous vegetative growth. It contributes to the deep green color of leaves and stems because it is a component of chlorophyll, the molecule central to photosynthesis.
Phosphorus, the middle number, aids in energy transfer within the plant and is vital for strong root development, flowering, and fruiting. Potassium, the final number, supports overall plant resilience by regulating water uptake, enhancing disease resistance, and activating necessary enzymes. Applying a 12-12-12 blend is designed to ensure a steady, comprehensive supply of these nutrients to support growth throughout the season.
Why Balanced Fertilizer Does Not Kill Weeds
A standard balanced fertilizer lacks the specific chemical components required to act as a herbicide. Herbicides function by containing active ingredients that disrupt a plant’s cellular processes, such as photosynthesis, cell division, or hormone regulation, leading to death. The fertilizer, in contrast, only supplies basic elements that all plants utilize for survival and growth.
Weeds are often highly efficient at nutrient uptake and can sometimes out-compete desired plants for resources. When you apply a nitrogen-rich fertilizer like 12-12-12, you are essentially feeding the weeds and encouraging their growth alongside your lawn or garden plants. This input of general nutrition can actually make a weed problem worse, as the unwanted plants become more vigorous and robust.
Understanding Fertilizer Burn
The belief that fertilizer can kill weeds often stems from the phenomenon known as “fertilizer burn.” This is not a targeted form of weed control but a chemical injury that occurs when too much fertilizer is applied. Most synthetic fertilizers are composed of highly soluble mineral salts.
An excessive concentration of these salts in the soil or on the plant tissue causes the water flow to reverse due to osmotic stress. Instead of the plant roots absorbing water from the soil, the high salt concentration draws water out of the plant cells, leading to dehydration. The resulting damage appears as yellowing, browning, or wilting of the foliage, giving the appearance of a burn. While an uncontrolled, heavy application may damage or even kill any plant, including weeds, this is an undesirable, non-selective outcome that harms the entire landscape.
Targeted Strategies for Weed Control
To effectively manage unwanted plants, employ targeted weed control methods rather than relying on general nutrition. Chemical controls are specifically formulated to disrupt plant growth.
Pre-emergent herbicides are applied before weed seeds germinate, creating a chemical barrier in the soil that inhibits the development of the shoot and root. Post-emergent herbicides are used after weeds have already sprouted and are actively growing, working by being absorbed through the leaves and moving systemically through the plant to kill it entirely.
Some products, referred to as “weed-and-feed,” combine a fertilizer with a post-emergent herbicide, offering a dual action of feeding the lawn while simultaneously eliminating broadleaf weeds. Non-chemical controls, like manual removal or proper cultural practices, also play a significant role. Maintaining a dense, healthy lawn through correct watering and mowing at a higher height can naturally suppress weed growth by limiting light and space.