With the arrival of fall, many of our landscapes start to wind down. Flowering plants begin to fade, and trees put on their annual display of fall color. There are plenty of seasonal tasks to tackle, from planting perennials and bulbs to cleaning up the vegetable garden and putting away tools and hoses for the winter. One task that often gets overlooked is managing weeds. However, taking some time to control weeds in the fall can save a lot of time and effort come spring.

Why is fall a good time to manage perennial weeds?

As the days become shorter and temperatures cool, perennial weeds move sugars and nutrients into their roots and other underground storage structures to prepare for winter and resume growth the following spring.

Because perennial weeds are moving resources into their roots, systemic herbicides such as glyphosate and 2,4-D can be more effective in the fall. Once absorbed by the leaves, these herbicides move throughout the plant and can reach underground plant parts, improving control.

Fall is a good time to manage biennial weeds, too

Fall is also an excellent time to manage biennial weeds such as thistle, wild parsnip, and garlic mustard. These plants spend their first year growing as low rosettes before flowering and producing seed the following year. Controlling these weeds in the fall, while they are still in the rosette stage, is often easier and can prevent thousands of seeds from being added to the soil seedbank.

Some weeds are just getting started

While we usually think of weeds germinating in the spring and summer (crabgrass, purslane, etc.), some weeds begin germinating in the fall. These weeds, often referred to as cool-season or winter annual weeds, include plants like common chickweed, deadnettle, and henbit.

Winter annual weeds germinate in late summer and fall, continue growing until a hard freeze, then resume growth in spring before flowering, producing seed, and dying. Because these weeds are already established when spring arrives, managing them in the fall can greatly reduce weed problems come spring.

Preventing seed formation

Regardless of whether the weeds are perennials, biennials, or annuals, it’s important to prevent them from producing and spreading seeds. One of the reasons many weeds can be so problematic is that they can produce an incredible number of seeds. Additionally, many seeds can remain viable for years or even decades in the soil.

So even if it’s not possible to remove the entire plant, removing the flowers and seedheads can go a long way in reducing weed populations. Avoid putting seedheads in compost piles, as backyard composting usually does not get hot enough to kill seeds.   

Managing weeds in the fall

Like other times of year, weeds can be managed by hand pulling, cultivation, mulching, cover crops, or herbicides. When using herbicides, apply them while weeds are still actively growing and follow all label directions. Mulch can help suppress weed germination, and minimizing soil disturbance during cultivation can help prevent bringing weed seeds to the soil surface.

Good Growing Tip of the Week: When using herbicides (or any pesticide), make sure to read and follow all label directions. The label will tell you which weeds will be controlled, when to apply, how to apply, how much to apply, what personal protective equipment is needed, and more. 

By Ken Johnson, Horticulture Educator, University of Illinois Extension