A hay tarp, used correctly, can cut outdoor dry matter losses by 10–20 percentage points compared to uncovered bales. Used incorrectly — particularly if sealed against the ground without airflow — a tarp creates a moisture trap that accelerates mold and results in worse outcomes than no cover at all. This guide explains the difference.
When Tarping Helps
Tarping provides meaningful benefit in: high-rainfall climates (Pacific Northwest, Gulf Coast, Southeast), storage periods over 90 days, high-quality hay where nutritional preservation justifies the investment, and sites where a gravel base is in place (solving the ground-moisture problem separately).
Tarp Types That Actually Work
Woven Poly Agricultural Tarps
The standard choice for farm use. Look for 6–8 mil woven poly with reinforced grommets at 18–24 inch spacing, UV stabilized, rated for outdoor agricultural use. These last 3–5 seasons under typical conditions. Brands like Tarps Now, FarmTuff, and heavy-grade poly tarps sold at agricultural suppliers are appropriate. The thin blue poly tarps from hardware stores typically shred within one season of outdoor exposure.
Individual Bale Bags
Stretch-over net bags sized for 4×4, 5×5, or 5×6 bales provide excellent individual protection. More expensive per bale ($15–$30 each) but more secure in high-wind areas, easier to handle solo, and reusable for many seasons. Best for high-value hay or farms where wind is a major problem.
Silage Film / Stretch Wrap
Round bale silage wrap (typically 4–6 layers of stretch film) creates an anaerobic environment that ferments the hay into haylage rather than curing it dry. Haylage is excellent feed but requires different handling — it must be fed within days of opening a bale, and improperly fermented haylage can develop harmful molds. This is a separate product category, not interchangeable with tarps.
How to Anchor Tarps Against Wind
This is where most outdoor tarp setups fail. An unanchored or poorly anchored tarp becomes a sail in a windstorm — tears, relocates, and exposes the hay it was protecting:
- Sandbag anchor systems: Fill heavy-duty bags with gravel or sand, clip to tarp grommets, and lay at the base of the bale row. Reposition after any significant wind event.
- Ratchet strap lacing: Thread rope or strap through grommets on alternating sides of the tarp, anchored to ground stakes or the bale row itself. Ratchet to firm tension.
- Old tires: Tires laid on the sides of a tarp-covered row are a low-tech but effective anchor, particularly for row-style storage.
The Critical Airflow Rule
Never seal a tarp completely against the ground on all sides of a bale or row. The tarp must allow air movement at the edges. A fully enclosed tarp creates a humidity trap — moisture from the bale cannot escape, condensation forms inside, and mold develops on the tarp contact surface. Drape the tarp over the top and down the sides, but leave the ends open and raise the base edges slightly off the ground.