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Weed Biology Part 3: Annual vs Biennial vs Perennial

Last week we talked about each stage of the weed life cycle and alluded to the difference between annuals, biennials, and perennials. (Click here to catch up if you missed last week’s post.) But what does that mean and how does it relate to weed management? In this week’s post we’re going to take a look at the differences between these three life-spans and touch on effective management strategies for each.

Annuals

Annuals have the shortest life span: one growing season. They’ll germinate, grow, flower, set seed and die all within one growing season. (Stay tuned to next week when we talk about the different types of annuals.) Annuals are usually prolific seeders. Think about how many seeds one dandelion head can produce. The prolific seed production is the reason annual weeds appear to be back year after year after year. The best way to manage annual weeds is to get ahead of them and prevent them from emerging at all. When applied properly at the correct time of year, pre-emergent herbicides will provide adequate control over weed seeds essentially breaking the cycle. Now the only thing you need to do is convince the neighbors to do the same thing…


 

Biennials

Biennials are a little harder to manage. Pre-emergent herbicides can be somewhat effective at preventing biennials from germinating, but what do you do with the weeds already present? As the name suggests, biennials take two years to complete their life cycle. Let’s look at an example: Queen Anne’s Lace, or wild carrot.

Year 1: Queen Anne’s Lace will germinate from seed the first year, but unlike annuals, it will not produce flowers or seeds during the first year. Biennials will grow to the vegetative stage and form a compact cluster of leaves with a thick fleshy root. These fleshy roots are difficult to pull by hand. The weed will then enter dormancy at the end of the growing season but will not die.

Year 2: Queen Anne’s Lace will come out of dormancy, finish growing through the vegetative stage and move into the flower and seed production phase. Queen Anne’s Lace is fairly recognizable during its second year but can be mistaken for carrots during the first year. Biennials in year two will flower prolifically, set seed, and then die at the end of that growing season.

Pre-emergent herbicides will help biennial seeds from germinating the first year but will do nothing to prevent established biennials from growing through year two if already established. Selective herbicides can be used if managing biennial weeds in an established turf. Use care in applying herbicides to manage biennial weeds in ornamental situations.

 


 

Perennials

The last length of life cycle we’re going to touch on are perennials. What makes perennial weeds so difficult to manage is the underground structures the weed uses to reproduce, namely stolons, rhizomes, tubers or bulbs. These underground structures serve as energy storage for the plant from year to year. While annuals will put all their energy into flower and seed production the first year, perennials will save a portion of that energy for following years. This energy storage also makes mechanical control difficult. When a perennial weed is mowed the weed has energy reserves with which to grow new flowering shoots or at the very least continue to increase energy stores with the remaining foliage. If a perennial weed is pulled it will most often re-grow unless ALL of the underground root structures are removed, which is really difficult to do.

With the efficacy of cultural and mechanical controls being limited the best option for perennial weed control will be a systemic herbicide. Contact herbicides will kill the top of the plant and leave the root system untouched. Systemic herbicides will be translocated through the root system, including the underground root structures, to kill the whole plant.

 


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