Creating a Stale Seed Bed with Solarization
Stale seed beds are beds in which weed seeds have been allowed to germinate, and then are killed before direct-seeding or transplanting your crop. Creating stale seed beds can save you time cultivating by getting rid of the weeds before you direct-seed or transplant your crop, and at Good Heart Farmstead this is one of our main weed control practices.
Most weed seeds germinate in the top 1-2” of soil. When creating a stale seed bed, it’s important to minimize soil disturbance below 2”once the weeds have germinated—if you till after this, more weed seeds could be brought to the surface.
Stale seed beds can be created using clear or black plastic, and in conjunction with flame weeding or a power harrow. Each application is detailed below, and they all start with preparing the beds for planting.
Step One: Prepare the Beds
Prepare your beds as if for planting, 2 days - 4 weeks before your seeding or transplant date (the length of time depends on what type of cover you choose: clear or black plastic).
At Good Heart Farmstead, we use a permanent bed system, and we prepare the beds by broadforking, and then running the power harrow attachment on our BCS walk-behind tractor over the beds to create a smooth, even surface. Aeration from the broadfork creates an ideal environment for soil life and activity, while the power harrow creates a consistent firm surface for effective seed germination.
This even surface is important for two reasons: If you’ll be flame weeding following weed emergence, dips in the soil create spaces where weeds can hide from the full heat of the flame, and therefore continue to grow. An even bed surface will also allow you to go right into planting without excess soil disturbance, which could bring up more weed seeds.
Step Two: Cover with Black or Clear Plastic
The choice of black or clear plastic will determine how much time the plastic will stay on the bed.
Black Plastic Silage Tarp:
In spring and early summer, when we’re just starting to plant out the fields, we use a 6mm black silage tarp to cover blocks of 6 beds at a time. Silage tarps warm up the soil, encouraging weed seeds to germinate. Once the weeds germinate they quickly die without light. If there are established weeds, they’ll die within 4-6 weeks of being covered.
Meanwhile, the warm, moist environment under the tarp encourages worm activity, which helps break down the established weeds. When left on long enough, silage tarp is effective at smothering grasses and perennial weeds, and can be an effective method of creating new growing space without tilling.
Clear plastic heats up the soil more than black plastic does, and because of this, it’s important to consider what your goals are when using clear plastic.
Most weed seeds are killed at 160º-180ºF, and while these temperatures kill plant pathogenic bacteria and fungi, the high heat also kills beneficial soil life, including worms and nematodes. While this can be an effective route to dramatically lower your weed seed bank and kill soil-borne diseases, if you’re concerned about beneficial soil life, it’s not always appropriate.
Rather than using clear plastic to kill weed seeds, bacteria, fungi and other soil organisms, it can be used for a shorter period of time and in conjunction with flame weeding or power harrowing.
To use clear plastic in conjunction with a flame weeder, begin by preparing your beds as normal. If it’s dry, irrigate before laying down plastic to ensure a moist growing environment. Once the first flush of weeds emerge, remove the plastic and run a flame weeder over the bed to destroy the weeds. From there, you can rake off the debris or plant directly into the bed. This process is ideal for beds that you’ll be direct-seeding into.
Alternatively, you can use clear plastic in conjunction with a power harrow attached to a BCS. At Good Heart Farmstead, we use this clear plastic/power harrow combination when we want to flip a bed quickly in summer from one crop to another.
Our process goes like this:
- Flail mow the outgoing crop.
- Lay clear plastic on the bed for up to 2 days — this kills the roots of the mowed crop and encourages quick germination of weed seeds.
- Power harrow to incorporate debris lightly into the soil and re-create an even surface.
- Add any necessary amendments.
Because we mow the previous crop, rather than pulling the roots or tilling them in, this process is better suited for beds you’ll be transplanting into.
We time this process to coincide with sunny days in order to take full advantage of the clear plastic’s solarization effect. In order to minimize loss of soil life, we keep a close eye on the air and soil temperature. The hotter it is outside, the faster the plastic comes off. In general, we shoot for a 36-hour application of clear plastic on 80º days.
Using black silage tarp for 4 weeks in the spring to prepare beds, and clear plastic for a couple of days during the summer to flip beds has helped us decrease the overall need for cultivation — which means less weed pressure, and more time for planting and harvesting.
Kate Spring is an organic farmer, mother, and chief inspiration officer at good heart farmstead in Worcester, Vermont.