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Weed Biology Part 2 : Stages of Growth

Last week we covered the differences between monocots and dicots. (If you missed it, click here.) The next step to effective weed management is to be able to understand and identify the four different stages of growth. The effectiveness of pesticide applications will vary depending on the phase of the growth cycle. Since the purpose of this series is to talk about weed biology, I’ve focused on the life cycle of weeds which can differ from turf and ornamental plants. Most weeds are annuals which we’ll cover in more detail next week.

Phase 1: Seedling

As you might guess, the seedling phase is when all plants (weeds and desirable plants alike) are the most tender. Seedling are the infants of the plant world. Seedlings have immature root and shoot systems. The water and nutrient content of the soil needs to be almost perfect. Any disturbance in the root system will be highly detrimental if not fatal. A pesticide application can be severely detrimental to seedlings even if the pesticide is not an herbicide. Mechanical controls are also highly effective in managing seedlings. When using mechanical controls take care not to damage any seedlings of the desirable plants, for example turf.

Phase 2: Vegetative

During the vegetative stage weeds go through an enormous amount of growth. This is the pre-teen/teenagers of the plant world. Weeds in the vegetative stage are taking up as much water and nutrients as their roots can find. This is why dandelions appear to spring into existence almost overnight. The number of stems and roots increase rapidly during this phase making weeds a little harder to pull by hand. However, since the weed is actively growing, any systemic pesticide applied will be moved through the plant quite rapidly. Non-selective systemic herbicides will be highly effective during this phase of growth.

Phase 3: Flowering/Seed Production

By the time the weed reaches the flowering/seed production phase it will have reached “full size” and has started directing energy toward flower and seed production. Weeds in this phase are the young adults of the plant world. Water and nutrients taken up are directed toward flower and seed production instead of shoot and root growth. The effectiveness of herbicide applications will be slower if not overall less effective than at the seedling or vegetative phase in the plant’s life. Systemic non-selective herbicides will still kill the whole plant, but it may take a week and a half or more instead of a few days.

Phase 4: Maturity

The final phase of weed growth is maturity. They’ve germinated, grown, flowered, and set seed. They’re done. There is very little, if any, weed growth during this phase. Demand for water and nutrients is very small, and there is very little energy production. Annual weeds have reached the end of their lifecycle and will die, biennial and perennial weeds will enter a dormant period. Herbicides applied at this phase will be largely ineffective. With no demand for water or nutrients the herbicide will remain localized in the leaf.

Understanding the lifecycle of weeds and being able to identify each stage of lifecycle is an important skill for turf and ornamental managers. Pesticides are an effective tool in the never-ending battle with weeds like dandelions, but knowing when to apply pesticides will save time, money, product, and help keep pesticides out of the environment.

For more information on weed management in turf or ornamental situations, check out the following courses:

IPM for Turf Management

IPM for Ornamental Plant Pest Management

Common Turfgrass Weeds

Common Ornamental Plant Pests

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