How To Calculate Weed Population?


Knowing the types of weeds and their population on a given piece of land will help you determine many things before planting crops. The weeds compete with food plants, and the more weeds growing in the soil, the more competition the food plants will have.

Rainwater calculation, amount of fertilizer, when to apply fertilizer, what type of weed killer will be most effective, and the best time to use it are all questions that can be answered by calculating the weed population.

Potential harvest amounts and possible pests and disease issues for crops can also be determined by calculating the weed population. Before turning the soil of a new agriculture location, assess the weed population density for the best results from the new farmland.

Count the Number

Weed density counts the number of weed species in a specified area, sometimes expressed as a percentage. By measuring the number of existing weeds, you will know what to expect in the future and plan your agricultural practices around your findings.

The higher the percentage of weeds on a piece of land, the more competition the food plants will have. It also translates into more work from you to eradicate the weeds from the property to reduce the competition and increase the available resources for crops.

Weed Seeds


Weed density counts only consider the number of germinated and visible weeds in the soil. Therefore, many more viable weed seeds remain in the ground awaiting the right germination conditions.

The dormant weed seed requires after-ripening for it to become capable of germination. After-ripening of weed seed usually occurs in the soil when deposited into the earth during the growing season until it germinates. This could take one winter, and the weed seeds germinate as soon as the soil warms in the spring, or it could take a few years for the weed seeds to germinate.

Depending on the type of weed seeds, the soil, and weather conditions, these unwanted seeds could lie dormant in the ground for up to five years. When the soil is turned before planting crops, it brings the weeds seeds to the surface and exposes them to sunlight, warmth, air, and rainwater. The dormancy will be broken, and the roots will soon germinate and create a field of weeds that will grow fast and overtake the planted crops.

Crop Loss

The more weeds a piece of property has, the greater the crop loss. Crop losses caused by diseases, animals, pests, and weeds are calculated to be up to 40% of the potential yield harvested from a piece of land. Crop loss due to weeds alone is one-half of the calculated amount. If the crop loss is estimated at 40%, 20% of the loss will be rampant weed growth.

Weeds are the known enemy of crops, and learning how to calculate the weed population is the first step in reducing crop loss and increasing the harvest.

Mark Off A 1 Foot Section

Mark off a 1-foot square section of land and use it to sample how well the land produces and what it produces. Then, use it strictly for testing and observation.

You can conduct rainwater calculations and soil absorption rates to determine how much water will be needed to bring a crop to harvest. The rainwater amount will vary significantly among crops, and this testing section will help you decide what the land will grow best.

For example, sugarcane and asparagus are water hogs and require a tremendous amount of water to bring them to harvest. However, if your soil has plenty of organic matter and retains moisture well, and your climate gets plenty of rainfall during the summer, these are good crops to grow.

Okra, sweet potatoes, and cowpeas are drought-tolerant crops that require very little water during the growing season. However, if the soil within the sample section reveals heavy soil that retains moisture poorly and your climate receives little rainfall during the summer, these will make good crops choices.

Summer HeadCount


It’s not always feasible to wait until spring or fall to calculate the weed population. It can be done by taking a headcount of all the weed seed pods during the summer. When the weeds are setting seed during the mid-to-late summer, count the number of seed heads or pods within a small area. Mark the site to create a 1-foot by 1-foot square area and do a summer seed headcount. Break open one of the seed heads and count the seeds inside. Multiply that number by the number of seed heads within the marked-off sample area. This will estimate the total number of weed seeds produced within the land or agriculture field plot.

For example, if you consider a location to plant a new garden that is 50 feet by 50 feet, calculate length feet x width feet = square feet. Therefore, 50 x 50 = 2,500, so you will have 2,500 square feet of agriculture growing space. After marking off a 1-foot sample area and counting the seed heads, then multiplying the number of seeds inside the seed head, you will get a reasonable estimate of how many weeds you and your crops will be contending with.

If the marked-off sample area of 1 foot contains two weeds, and each develops a seed head with five seeds, you will multiply 2,500 x 5. The 50 x 50 piece of land can grow 125,000 weed plants in one season.

Now that you are aware of the vast potential of weed seed germination that would threaten to destroy your planted crops, you can take precautions to prevent and control the weeds.

Soil Sample

A more complex but very accurate method to calculate weed seeds in a given area is through a soil sample. First, take soil cores from 4-5 locations within the marked-off 1-foot square sample. The soil cores should extend down 6 inches into the soil and be 1 inch in diameter.

Sieve the soil through a fine-mesh sieve to reveal the weed seeds within the soil core and count them. A light spray of water may be needed to wash the dirt off of the sources. Spread the seeds out on white paper for a better view and assessment. Weed seeds can be very tiny and easily overlooked.

This soil sample technique is time-consuming and is only accurate if you can identify weed seeds.

rainwater on lupine leaf

Rainwater Calculation

Water a small area (use rainwater if possible) of the property so you will be able to count and identify the germinating weeds. This is best done in the spring, when the seeds usually germinate. This will create a controlled environment to get an accurate count and identify weeds that grow when rainwater is available.

This can be done during late summer or early fall but does not always provide an accurate weed count due to the varying natures of weed seeds their dormant seasons.