Know the #1 Cause of Weeds

Depending on what kind of weeds are common to you area, you might have thousands to millions of weed seeds lurking in every garden bed. Yet only a fraction of them sprout at any given time. And some never do.

Do you know why all the weeds don’t germinate at once?


The conditions for germination haven’t been met.

Every seed comes programmed by nature with an on button. Until that button gets pushed, seeds stay dormant or in a resting mode, waiting for a nudge to wake up.

Seed Germination Triggers

The conditions necessary to turn on a seed can range from simple to incredibly complex. In some plants, those conditions can even be different in every seed a plant sets.

Simple Triggers

Seeds that stay in resting mode, rather than going dormant, usually germinate when some simple conditions are met. A certain soil temperature, a bit of water, or exposure to light or dark might be all it takes.

Weeds that happen all at once after a good rain, a few warm days, or in fall when temperatures start to cool off fall into this category.

Complex Triggers

Seeds with hard outer shells, or those that do go truly dormant and can stay that way for years, have more complex requirements to power up and turn on.

These may need exposure to wide temperature swings. In some cases, the outer shell needs to be worn down in the soil (natural scarification) or soaked by a deep saturating, long-standing rain to soften protective coatings. Sometimes, specific chemical reactions or nutrient influxes may be the magic that wakes the germplasm from its slumber.

In stable soil, these complex triggering conditions may never occur. So, those seeds stay dormant. Over a period of years, and sometimes decades, then seed viability declines. Eventually the seeds die and decompose.

With that background in mind, here’s the #1 reason why weeds keep happening in your garden.


Big Changes to the Soil

I bet that’s not the secret answer you were expecting. Most people guess bare soil or tilling. Those are perfect examples of events that cause big changes to the soil. But there are lots of things we do to improve our soil that also trigger simple and complex weeds.

Let’s take a closer look at some examples.


Tilling oxidizes carbon and other nutrients stored in soil, causing them to aerosolize and leave the soil. That changes the soil composition and disrupts soil life networks. Those two things set off a bunch of chemical reactions in the soil that break down the outer coating of long-dormant seeds allowing them to germinate.

Of course, that disturbance also moves some of the seeds with simple triggers to the surface so they see light or feel the warmth or cold they need to germinate.

Temperature/Moisture Fluctuations

Likewise, bare soil is more susceptible to rapid changes in temperature and moisture. Those changes are often the trigger those simple annual weeds to start growing. Also those rapid changes impact soil life causing those secondary chemical reactions that trigger long-dormant seeds.

Other Soil Changes

Basically any big changes to soil can trigger weeds. And when you consider that even in ideal conditions, nature only produces a fraction of an inch of new soil in year, most of the stuff we do as gardeners constitutes a “big change” in soil.

Here are a few more examples.

Soil pH

We often use things like lime or sulfur to raise or lower our pH. Those pH changes, which happen over a long period of time, will continue to trigger different kinds of chemical reactions — and new weeds — at different pH levels along the path to the pH you want.

Expect new weeds to continually emerge until the pH stabilizes at your target zone. Then, if you can keep it it in a stable range, pH-related weed pressure will decline.


Fertilizer, especially the fast-acting high nitrogen stuff used for heavy feeders in the vegetable garden or on lawns, is epic in terms of its impact on the soil. Frankly, if new weeds don’t germinate within a few weeks of applying fertilizer with a number higher than “3” in the “N” position of the N-P-K line up, I’d be worried.

Nature treats nitrogen as an on-demand kind of nutrient rather than something it stockpiles in the soil. So, whenever there is too much of it, chemical reactions will wake up weeds to clean up the excess nitrogen in your garden. Even an influx of naturally occurring nitrogen, such as from a lightning strike or a deep-saturating rain, triggers new weeds to germinate.

Big influxes of nitrogen always set off weed reactions in healthy soil. That’s why applications of well-aged compost, or on-demand feeding of plants with low dose fertilizers like compost tea, is a better strategy for weed control than periodic applications of lots of fertilizer at once.


Putting several inches of compost on your garden bed, especially if it hasn’t been long-aged, triggers lots of new weeds too. It just doesn’t do it immediately.

When first applied, compost acts as a mulch regulating the temperature and moisture in soil. That reduces weeds. However, as the soil life and natural factors like rain, roots, and more work that compost into the soil profile, it sets off those chemical reactions that germinate dormant seeds.

The good news about compost, though, is that because there’s a delay between application and weed eruptions, your chosen plants get time to grow in before that happens. When the weeds start waking up, the leaves and roots of your mature plants will crowd and shade them out and many will die without ever seeing the light of day.

The key with compost is to keep your beds planted after you apply. Also, if you need a break from the gardening such as in winter, fill your compost rich soil with cover crops or be prepared to have lots of weeds show up to act as nature’s cover crops.

Manage Soil Change

The best way to prevent weeds is to aim for soil stability. But sometimes we have to wake up a few weeds to get there.

New Garden

In a new garden, we usually want to make big soil changes in a hurry so that soil will be fertile enough to support cultivated plants. While that is happening, expect lots of weeds and receive them as a welcome sign that your amendments are being successfully incorporated into your soil structure.

Yes… you still have to disturb those new seedlings so they don’t root deeply. If you miss the early weeding window, instead cut them the ground a few times until they stop trying or cover them with cardboard weighted with a rock to starve them of the light.

Just be careful not to disturb the soil too much while weeding or you’ll cause more big change that leads to new weeds.

Mature Garden

As your soil improves you can taper off on the big changes and start to focus on smaller incremental changes. Wean off fertilizer by using cover crops and light layers of mulch and compost applied regularly rather than in large quantities all at once.

And when you do have eruptions you didn’t expect, backtrack through your garden journal to figure out the most likely cause. That way you can adjust your methods or make a mental note to expect weeds the next time you do that thing again.

Weed-Free Gardening

Weeds aren’t punishment and they aren’t arbitrary. They are the storytellers of big changes that happen in your soil.

For more weed-free tips and tidbits, check back weekly or sign up for email notifications of new posts. Also, if you want lots more Weed-Free Gardening information at your fingertips, consider buying a copy of my book.

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