Weeding the Perennial Garden

Every gardener's least favourite task is weeding. It's a chore much like house cleaning - never done and universally disliked. If you want to reduce or eliminate weeding read on...

Weed types

There are 2 types of weeds and understanding how each enters the garden and how each type reproduces helps to guide your control efforts most effectively.

Annual and Perennial Weeds

Annual weeds live out their entire life cycle in only one season, so theoretically, if they can be prevented from reproducing, once removed, they can be eliminated from the garden. Removal is a simple matter of pulling them out of the ground due to the shallow roots system of most annuals. But typically new gardeners diligently remove all of the weeds from a plot, only to get a new crop within weeks. So what is going on?

In a word - seeds.

There are vast quantities of seeds hiding in the soil just waiting for the right conditions to germinate. By disturbing the soil during weeding and bringing the seeds up near the surface you inadvertently improve the conditions for germination providing the light, moisture and warmth they were waiting for.

Seeds come into the garden on the wind, in bird droppings, in purchased plants as hitchhikers, and in soil brought from other locations. Additionally seeds are almost certainly already in your soil (some remaining viable for many years). It is difficult to prevent seeds from entering the garden and impossible to remove all of the seeds from the soil.

Germination
So the trick to control is to prevent germination. All seeds need moisture and sufficient warmth to germinate, and many also need light to begin the process. Most annual weeds will only germinate close to the soil's surface where all of these requirements are met.

Disturbing the soil
So to prevent the seeds that are now deeper in your soil (where they cannot successfully germinate) from being moved up to the surface, the soil must not be disturbed. Digging, tilling, cultivating, planting, and even weeding should be kept to a minimum.

Farming versus Gardening
For many people this is counter intuitive and even blasphemous! After all don't farmers till the soil before planting to control weeds and cultivate or hoe to weed during the season? Don't they know best? Well actually they don't - they are producing an annual crop for profit - not a perennial garden for pleasure, and the techniques and priorities are not the same. Your goal is to enjoy a garden full of beautiful mature plants that persist from year to year with as little effort as necessary and profit is not a concern. Conversely the farmer must raise a crop of seedlings to maturity and successfully harvest it, all in one season, and profit is the primary concern. So what works best for perennial gardeners is quite different from standard farming techniques.

Covering the Soil
Bare soil is an invitation for weeds. Any seed on or near the surface of bare ground will germinate and grow at its earliest opportunity. By not disturbing the soil we keep the deep weeds seeds from coming to the surface and germinating, but what about newly deposited seeds? We can prevent seeds from reaching the soil surface, and/or bury seeds newly brought to the surface by covering the soil's surface with a weed free layer. Mulches prevent weeds in this manner. For perennial gardeners I cannot stress enough the importance of mulch. Mulch is your best friend!

Ground covering plants cover the soil as well and also may crowd out any seedlings that do manage to germinate.

Perennial Weeds

Perennial weeds may start out as seeds, but, once established many, are harder to remove from the garden due to their more extensive roots systems. Weeds like Canada thistle, burdock, and goutweed have deep (as much a meter!) horizontal roots that produce shoots that can come up meters away from the original plant. Pulling these weeds does not result in removing the roots and so these weeds return quickly no matter how diligently you try to "get the tap root". The best organic control technique for these types of weeds is to starve or exhaust them to death by repeated removal of the top growth. Covering the soil after pulling the shoots with a thick or impermeable layer requires the plant to spend more of its resources to grow new shoots around or through the covering. If it can never "recharge" by reaching the sun and photosynthesizing, it will die. By the same token cutting the new shoots off at ground level consistently over time will starve the plant out. This is more effective than digging, or tilling as any small bit of root left in the soil will grow - in effect - starting a new plant!

16 Tips for Easy Weed-free Gardening:

Identify your weeds. Many new gardeners have trouble identifying weeds and may let them grow until they become quite a problem in the garden thinking that they may be a "good" plant. A useful web site by the OMAFRA or the Ontario Ministry of Agriculture and rural affairs can help: http://www.omafra.gov.on.ca/english/crops/facts/ontweeds/weedgal.htm
Cover the soil with a weed free layer 1-2 inches deep at least once per year - preferably twice. Mulch, compost, manure, peat moss and even raw plant cuttings from pruning the garden may be used for this purpose. For more details see the article on Mulches.
Use cardboard, or layers of newspaper (free for the taking on recycling days) under a layer of mulch on pathways and around larger plants for even more protection against weeds.
When starting a new garden bed improve the soil with weed free soil amendments such as compost, manure, peat moss and the like. Be sure that the manure is well rotted and free of weed seeds (test it first before spreading it on a large potion of your garden if you are not sure. If you bring in new soil such a the triple mix, often purchased for this purpose, be sure to get it from a reputable dealer who will take pains to limit the amount of weed seeds in his product.
Pull weeds after a rain. It is much easier to get more of the root out when the soil is wet since wet soil is softer and the roots will slip out of the soil intact rather than breaking off.
Remove weeds before they go to seed to reduce the number of new weed seeds entering the garden. Even if you cannot spend the time to do a really good weeding, get into the habit of picking off or cutting the flowering tops of weed plants as you pass by.
Do not put weeds seeds (tops of the plants that have gone to seed) into the compost heap. Unless high temperatures are reached during correct hot composting techniques, the seeds will remain viable in the compost and will reinfest your garden as you use the compost. Do not agonize over this though; it is better ecologically and for your garden to compost everything, rather than nothing, due to a fear of contaminated compost.
Do not water pathways and other areas where you do not want seeds to germinate.
Rather than difficult and time consuming digging, cut off perennial weeds at the soil level as often as possible, to starve them out.
Plant closely and use ground cover plants between larger plants to eliminate bare soil areas and to crowd out weeds.
Reduce the amount of weed prone areas in your garden. Pathways, patios, lawns, and unused areas can be weed havens. By reducing the size of these areas (i.e. planting them) you can reduce your weeding chores. This may seem paradoxical. But trust me when I say that I spend more time weeding my pathways than I do my actual garden areas.
The unused area between closely spaced houses is a common weed haven. Plant an appropriate ground cover (see the ground cover section of this web site for ideas) keep it weeded well for the first year - then you can safely forget it except for a once per year check up. You won't even have to mow it!
Paths and patios made of gravel, pavers or stone on aggregate, can be difficult to keep weed free with out chemicals. Weeds and self-seeding annual garden plants love the nooks and crannies found between the stones in these areas. Either set the pavers or stones into concrete, opt for a wooden deck, reduce the size of these areas or...
- Use a torch weeder to burn SMALL weeds before they get large. These burners do not work well for larger weeds or during wet seasons.
- Try corn gluten to prevent weed seed germination. Timing of application is crucial and test it on a small area first as it may discolor the surface.
- As a last resort for deep-rooted perennial weeds use Round-Up (glyphosate) which is still available in ready to use spray bottles at many nurseries and country coop stores. As with all chemicals - use sparingly and very carefully and never pour any leftover down the drain, sewer, or into any body of water.

The good news is that as your garden matures, if you follow the above practices you will have fewer weed problems and more time to enjoy your garden.

Written by Candace Carter, for Flowers in the Front Yard, July 2010, all rights reserved.