How much compost does i need to block weed seeds

Yard and Garden News

Landscape fabric also works well to suppress weeds. It is most appropriate for transplanted vegetables, since holes can be cut into it to plant seedlings.

Mulch can be applied at any time, but it is generally a good idea to wait until late spring, once the soil has warmed. Vegetables need warm soil to grow, and applying mulch too early will keep the soil cool for longer into the spring, potentially slowing the growth of vegetables.

In particularly wet years, mulching too early can also create habitat for slugs that chew on vegetable leaves.

2: Throw away or burn mature weeds – Do not compost them.
If left to grow, one common lambsquarter plant can produce over 70,000 seeds on average. Photo: Annie Klodd

The best way to control weeds is to not have them in the first place. Fortunately, there are ways we can help reduce the amount of weed seeds that get into our garden’s soil.

The most important tip is to never leave mature weeds in the garden, otherwise they will deposit their seeds into the garden soil and lead to more weeds in the future. Did you know. weed seeds survive in the soil for several years, and some common species can produce hundreds of thousands of seeds per plant?!

That’s why you should always remove weeds before they are mature enough to flower and produce seeds. But if weeds do grow to maturity, remove them as soon as possible and either dispose of them or burn them – do not leave them in or near the garden, and do not put weeds in the compost pile if they have flowered or have seeds on them.

Composting can only kill weed seeds if all parts of the pile reach at least 140 degrees F for an extended period of time. While some commercial farms and composting facilities achieve these temperatures in large piles that are turned regularly, ideal conditions are challenging to achieve in a home garden setting.

3: Use the hoe and tiller wisely!
Tilling a garden on May 8, 2018, about three weeks before planting warm-season vegetables.
Photo: Annie Klodd

Have you ever noticed big flushes of new weeds after you till your garden soil in the spring? When we till, hoe or rake the soil, that disturbance does uproot existing weeds, but it can also lead to new weeds. This is because tilling stimulates buried weed seeds to grow by exposing them to the sunlight and warm temperatures that they need to thrive.

Using tillage strategically, and understanding a bit about how weeds grow, can help reduce weed problems before even planting the vegetable garden in the spring. A tillage technique called a “stale seedbed” aims to do just that.

Try the stale seedbed technique

In the stale seedbed technique, the soil is tilled or hoed just once, about 2-4 weeks before planting. This purposefully forces weed seeds to emerge early, before the vegetables are planted. The soil is then left undisturbed until it is time to plant.

Right before vegetable planting, the surface of the soil is raked or hoed again to kill the emerged weeds. After planting, try not to disturb the soil again, in order to discourage new weeds from coming up. A mulch or landscape fabric can be laid down at this time in order to suppress further weed seeds from emerging.

It helps you begin the season with a head start on the weeds, and what gardener doesn’t like that?!

How to Kill Weed Seeds in Compost

Ideally, you wouldn’t add weeds that are in seed or even in the late part of their blooming cycle to the compost pile. Thus you can avoid the problem of their seeds germinating in the garden when you later use the compost you produced. But sometimes, you have little choice: perhaps the most easily available compostable material (horse manure, hay, etc.) contains seeds or else the endless sorting of weeds according to their “seediness” would just be too complicated. Or, like me, you just feel that everything organic should be composted.

Fortunately, there are other solutions.

A Big, Hot Pile

A compost pile that gives off water vapor is working hard to kill weed seeds. Source: Anatomy of Living,

In general, the bigger the compost pile, the more heat it produces … and heat kills seeds, even weed seeds.

After a week at 130 ° F (55 ° C)*, most weed seeds will be dead, but it takes a month at 145° F (63 ° C) or more to kill the most resistant ones. Curiously, most common weeds actually produce seeds that are fairly easy to kill and they’ll die at relatively low temperatures. That’s the case with dandelions (Taraxacum officinale) and Canada thistle (Cirsium arvense), for example.

*Note that such temperatures will also kill any weed roots and rhizomes placed in the compost. Two birds with one stone!

Heat-resistant weed seeds requiring treatment at 45° F (63 ° C) include:

  • Bird’s-eye speedwell (Veronica persica)
  • Broadleaf dock (Rumex obtusifolius)
  • Common groundsel (Senecio vulgaris)
  • Common lambsquarters (Chenopodium album)
  • Field bindweed (Convolvulus arvensis)
  • Ladysthumb (Polygonum persicaria, now Periscaria maculosa)
  • Round-leaved mallow (Malva pusilla)
  • Spiny sowthistle (Sonchus asper)
  • Wild buckwheat (Polygonum convolvulus, now Fallopia convolvulus)

To find out if your compost pile heats up enough to kill weed seeds, simply insert a compost thermometer into it and note the temperature. If you don’t have a compost thermometer, try sinking your hand into the pile. If it’s so hot you to feel uncomfortable, it’s heating up enough.

Do not forget to return the pile regularly, not only because that helps to oxygenate it and thus stimulates microbial life, leading to and maintaining higher temperatures, but also so the ingredients on the outside of the pile, where it’s cooler, can also get their full heat treatment.

Note too it may be necessary to water your compost pile from time to time. Compost heats most efficiently when it is neither dry nor wet, but moderately moist.

When the Pile Is Not Heating Up Enough

The compost bins commonly sold generally can’t hold enough material to ensure high temperatures. If you’re using one, you’ll have to resort to other methods if you want to kill weed seeds in your compost.

Bury compost to prevent weed seeds from germinating. Source:

It’s important to understand is that weed seeds* can only germinate when exposed to light. If you are concerned that your compost might contain viable weed seeds, simply bury it when you use it, covering it with soil or, if you apply it to the surface, cover the compost with mulch. Problem solved!

*Warning: unlike annual and perennial weed seeds, a few tree seeds, especially nuts, will germinate when covered with soil or mulch.

You can also kill the seeds at the end of a composting cycle by solarization. To do this, spread the compost on a very sunny surface and cover it with a sheet of transparent plastic, holding the plastic in place with rocks or bricks. That will quickly create a greenhouse effect and very high temperatures. Even if there is some germination at first, the heat underneath the plastic will be such that it will soon kill both the seedlings and any remaining seeds, leaving you with weed-free compost you can use as you want.

With these methods in mind, you can dare to add weeds at any stage of their life to your compost pile.