Be weary of weeds

By Steve Reiners, Professor and Chair, Horticulture Section, Cornell University, Cornell AgriTech. This is part of a series of columns that he wrote about vegetable gardening during the pandemic that ran Spring-Summer 2020. Read more articles from 2020 | Read later articles in this series.

May 15, 2020

Ralph Waldo Emerson described weeds as plants whose virtues have not yet been discovered.  With all due respect to Mr. Emerson, I might describe weeds a little differently.  I know the virtues of a pepper plant and an oak seedling, but don’t want either growing next to my carrots.  For me, a weed is a plant growing out of place, robbing water, nutrients, and light from my vegetables.  It’s a problem that dooms more gardens than anything else.

Gardeners, especially new ones, worry about hungry insects or threatening diseases, but weeds are usually the biggest problem.  Fortunately, with a little planning and work, it’s a problem easy to control.

Disturbing the soil as we prepare our gardens awakens long slumbering weed seeds, some of which may lay dormant in soil for up to 50 years.  For some seeds, just a flash of sunlight as the soil is turned is enough to start germination. 

Weed removal

If you have the time, one way to manage weeds is making a stale seedbed.  Prepare the ground in the spring then wait a couple of weeks before planting.  As the first flush of weeds emerge, lightly rake the ground killing the seedlings.  Do this a few times and you will reduce the weed seed reservoir in the soil.

Your best friend in the garden is a hoe.  Every time you go in the garden, have one at your disposal.  It takes a fraction of the time to kill weeds when they first emerge, compared to chopping and pulling them out one by one later.

I wouldn’t recommend using an herbicide in a vegetable garden due to selectivity.  Selectivity refers to the herbicide’s ability to control weeds without damaging the desired plant.  The problem in a vegetable garden is the diversity in a small space.  An herbicide that might be fine on peppers could kill the broccoli in the very next row.  So, what options do we have?

Mulch

Your best bet is mulch.  In an earlier column I talked about using black plastic mulches to warm the soil to get plants off to a fast start and conserve moisture.  The other big advantage is weed control.  A weed can’t grow due to the lack of sunlight under the plastic.  Poke some small holes in the plastic to plant your vegetables and you are set.  Stay away from clear plastic, as this is the perfect “greenhouse” for weeds. 

There are a couple of drawbacks with plastic mulch.  First, it needs to be picked up and land filled at the end of the season.  It also lets no water through, requiring a trickle irrigation system underneath to water plants.

Some biodegradable mulches are available today.  These warm the soil like plastic but can be turned into the soil at the end of the season.  Unfortunately, they still shed water. They typically cost more than plastic but are much easier on the environment. 

Some gardeners use landscape fabric.  This is typically applied where bushes and perennials are planted but can also be used in a vegetable garden.  The advantage is that the fabric allows water to penetrate.  The disadvantage is that it does not warm the soil and also must be removed at the end of the season.

Organic mulch

Organic mulches include straw, grass clippings, old leaves, newspapers, paper bags, shredded paper and bark --  anything that can be turned under at the end of the season.  These add organic matter to your soil as they breakdown, something plastic cannot do.  You need to apply straw 3 to 4 inches thick to prevent weeds. 

And yes, newspapers are safe to use as a mulch.  Only 2-4 sheets are needed.  Unfortunately, they don’t look very nice and can blow away.  Solve that by laying straw over the paper, but only enough to hide the newspaper, about an inch.  That will make your bale of straw go much further too.

All organic mulches will keep the soil cool.  For that reason, it’s still a bit too early to apply these mulches.  Wait until the soil warms, about June 10, before applying organic mulch.  Use what’s handy and cheap.

One word of caution if using straw or grass clippings.  Only use them if you know they have not been treated with an herbicide. Once spread on the garden, the chemical could be released  killing your vegetables.  Look for organic straw bales to ensure that no herbicide was used.  If using clippings from a neighbor, ask if any weed killer was applied.

Keep planting those cool season crops!

Some gardeners have planted some warm-season crops already, even though it is too early to do so without protection.  These late frosts are killing unprotected crops like tomatoes, cucumbers and squash. There’s still time to plant cool-season crops such as potatoes, onions, herbs, broccoli, cabbage, carrots, chard, endive, lettuce, and beets.  And work the ground to prepare for cukes, squash, melons, peppers, eggplants, and beans, all warm season crops that can be planted around June 1.