Occultation is a technique where opaque sheets—such as tarps, black plastic, or specially designed opaque films—are placed over soil after harvest. The goal is to block sunlight and trap heat, killing crop residues, weeds, and some soil-borne pests and pathogens. This practice can accelerate soil cleanup and improve conditions for the next planting without the need for tillage or chemical herbicides.

Table of Contents

Why Occultation Is Used

Weed Control: Shading prevents photosynthesis, reducing weed growth and seed production.
Pest and Pathogen Reduction: Heat and darkness can suppress soil-borne diseases and pest eggs.
Residue Management: Helps break down leftover crop material faster.
No-Till Friendly: Prepares beds for replanting without disturbing soil structure.

The Good of Occultation

Reduces Labor: Minimizes manual weeding or residue removal.
Non-Chemical: Provides an environmentally friendly alternative to herbicides.
Soil-Friendly: Protects soil structure and encourages microbial decomposition under the tarp.
Fast and Effective: In warm sunny periods, weeds and residues can die in a few weeks.

The Bad of Occultation

Timing-Sensitive: Needs several sunny days or weeks to be effective.
Cost: Requires purchase of tarps or opaque films.
Moisture Trapping: Can create excessive soil moisture if it rains or irrigation is used.
Incomplete Control: Some perennial weeds or deep-rooted species may survive.
Heat Stress: May negatively affect beneficial soil microbes if soil gets too hot.

Tips for Successful Occultation

Use high-quality opaque material that fully blocks sunlight.
Cover the bed immediately after harvest for maximum effectiveness.
Anchor edges well to prevent wind from lifting the tarp.
Leave in place for 2–6 weeks depending on temperature and sun intensity.
Monitor soil temperature if possible; extreme heat can harm microbes.
Remove the tarp carefully and add mulch or compost before replanting.

My Experience

Over 30 years of vegetable gardening in California’s Sonoma Valley have shown me that occultation is an effective, low-disturbance method for managing post-harvest beds. I’ve tested different tarp types and timing strategies to suppress weeds and accelerate residue breakdown. In my experience, proper occultation can save labor, protect soil life, and prepare beds for successive plantings, but poor timing or material choice can reduce its effectiveness.

Occultation Chart: Opaque Covers for Post-Harvest Soil Management

Cover Type	Recommended Duration	Best For	Pros	Cons / Notes
Black Plastic Sheet	3–6 weeks	Most annual vegetables, cover crops	Affordable, widely available, heats soil effectively	Can overheat soil; must anchor edges; not biodegradable
Opaque Tarp (Polyethylene or Canvas)	2–4 weeks	Fragile crops, seedbeds, soil with high organic matter	Durable, reusable, blocks light fully	Heavy; may trap moisture; can blow away in wind
Biodegradable Opaque Film	3–5 weeks	Organic gardens, short-term residue kill	Environmentally friendly; reduces cleanup	Higher cost; less durable; may degrade prematurely
Specialized Opaque Solarization Film	4–6 weeks	Disease-prone beds, soil pathogen suppression	Maximizes soil heat; kills weeds, pests, and pathogens	Expensive; single-use; requires sunny, warm weather
Landscape Fabric (Black or Dark)	2–6 weeks	Weed suppression in beds, slower residue breakdown	Durable, reusable; lets water through; easy to move	Less heat buildup; may require multiple layers for full occultation
Cardboard / Paper Layers	2–6 weeks	Organic gardens, light weed control, cover crop residues	Biodegradable; adds organic matter as it decomposes	Slower to suppress weeds; may blow away if not weighted; retains moisture
Raised Bed Shading with Opaque Fabric	2–3 weeks	Small beds, sensitive plants	Lightweight, easy to move	Less heat buildup; may require extra sun exposure

Tips for Using Opaque Alternatives

Weight edges down: Use soil, stones, or stakes to keep cardboard or paper from blowing away.
Layering matters: Multiple sheets of cardboard or overlapping landscape fabric improve weed suppression.
Moisture monitoring: These materials trap some moisture; ensure soil doesn’t stay soggy.
Post-Occultation: Remove or cut through landscape fabric carefully; cardboard/paper can stay to decompose and add organic matter.
Combination: You can layer cardboard or paper under plastic or tarps for faster weed kill and residue management.