Quick answer: For truly dry hay (under ~18% moisture), sealing it in airtight plastic film usually does more harm than good. There is not enough sugar and moisture to ferment, so instead of preserving, the film acts as a vapor barrier: moisture in the bale condenses on the cold plastic at night and feeds a 1–2 inch layer of surface mold. The right tool for dry hay is open net wrap (mesh) to shed rain and hold shape, plus a tarp or shed for cover. There are a few honest exceptions — covered outdoor storage with no barn, or "tough" hay at 18–25% — but if your hay is genuinely dry, wrap it in net, not film. (Wrapping wet hay to ferment is a different question — see our wrapping wet hay guide.)
"Can I just wrap my dry hay in plastic to store it outside?" comes up constantly, and it is easy to see why — you have a wrapper, you have film, and you want to protect bales without building a barn. But wrapping dry hay in airtight silage film is one of those moves that feels smart and usually backfires. The physics work against you.
This guide explains why dry hay molds against plastic film, what actually happens if you do it, the narrow cases where it can make sense, and the storage approach that beats it for most operations. Importantly: if your goal is to ferment wet or rained-on hay, you are on the wrong page — head to our guide to wrapping wet hay instead. We pulled the real-world results from producers on HayTalk's forum and checked the fermentation science against university-extension guidance.
Why dry hay and plastic film are a bad combination
Baleage works because wet forage sealed away from oxygen ferments — bacteria convert plant sugars into acids, the pH drops, and that acidity preserves the bale. Dry hay cannot do this, and that single fact explains the whole problem.
Dry hay is still "breathing" — but it cannot ferment
According to Penn State Extension, the optimal moisture for baleage is about 45–60%, with fermentation possible but increasingly risky from 30–70%. The reason matters: fermentation is "the conversion of sugars to acids after oxygen is eliminated," and the more vegetative (wetter) the forage, the more fermentable sugar is available. Hay cured to under ~18% has shed most of that easily-fermentable sugar and moisture. Seal it and you get the anaerobic conditions — but no acid drop, so nothing inhibits mold.
The condensation cycle: the "double-pane window" effect
Even hay at 12–15% moisture holds enough water to cause trouble once it is sealed. A producer on HayTalk described the mechanism precisely: on a warm, sunny day, heat evaporates water out of the hay into the bale's internal air space; at night, when the surface drops below the dew point, that vapor condenses against the cold plastic. The hay touching that condensate wicks it up and stays perpetually damp — "very much like the process in a double-pane window with a broken seal." That persistent wet zone, with no fermentation acidity to stop it, is exactly where mold grows.
Fewer wraps does not fix it
It is tempting to think "I will just use less film so it can breathe." It does not work — fewer wraps simply add pinholes that let outside moisture in, while still trapping enough vapor to condense. You end up with the worst of both.
What happens if you wrap dry hay anyway
Producers who have done it report a consistent set of outcomes:
- A 1–2 inch layer of surface mold. Guidance from the University of Kentucky Forage Extension, quoted by producers on the forum, says plainly that "a 1–2 inch layer of white mold may develop on the outside of each bale" — though the loss is still less than leaving bare dry bales fully exposed outdoors. Forum members describe the same thing as a "fuzzy marshmallow" or "chalky, dusty, moldy-looking" outer skin. Cattle usually eat through it; horse-hay buyers will reject it.
- Worse results the drier and the warmer it is. One operator summed it up: "usually for me, the drier the [hay], the worse the mold issues." Row orientation matters too — bales need sun on both sides to burn off condensation, so north–south rows outperform east–west.
- The "sweat" trap. Freshly baled hay keeps respiring and giving off heat and moisture for weeks. Seal that in and you guarantee mold. Producers who wrap dry hay successfully insist on a sweat period of 2–6 weeks first, sometimes confirmed with a temperature probe. The catch: you need covered, dry space to let bales sweat — and if you have that space, you may not need to wrap at all.
From the forum (2024): A Pennsylvania horse-hay producer asked whether wrapping would have prevented ground-moisture mold. The answer from an experienced wrapper: "Dry bales, wrapped, are likely to develop mold all around the wrapped portion of the bale. No hay is zero percent moisture, and the wrap will act as a vapor barrier… moisture tends to condense on the plastic and damage the hay touching the plastic." (HayTalk, "wrapping dry bales")
When wrapping dry hay might make sense
It is not never. There are honest exceptions — just go in with eyes open and the right protocol.
Outdoor storage with no barn or tarp option
If your only alternative is leaving dry bales fully exposed to weather, wrapping can be the lesser evil. The University of Kentucky Forage Extension guidance (as cited by producers) is specific: use only enough plastic to cover the bale — usually 2 layers with about 20% edge overlap — and choose black plastic rather than white, because it is cheaper, draws heat, and helps evaporate internal condensation. In a tube, make a vertical slit between bales at the bottom for ventilation. Expect the 1–2 inch surface mold, and only do it with hay below ~20% moisture; above that, mold gets much worse.
Humid-climate horse-hay producers (a commercial niche)
Some producers in hot, humid regions wrap retail hay to protect color and quality between baling and sale. Results are genuinely mixed — several report moisture issues even with black film — so treat this as a producer-reported niche, not a default recommendation, and budget for surface loss.
"Tough" hay at 18–25% moisture
This is the most common honest case, and the key is timing. Hay at 18–25% is below reliable baleage range but too wet to store safely as dry hay. If you already have a wrapper and film: wrap it promptly while it is still wet enough to start a mild fermentation (the wetter end of that band is better), rather than letting it dry to 20% and then trying to dry-wrap it. If you would rather keep it as dry hay, a propionic-acid hay preservative inhibits mold without requiring fermentation. For dialing in that moisture call, see our baling-moisture guide. (This borderline forage is also where "sweet hay" — lightly fermented, low-acid hay some horses prefer — comes from; it is a different product from both baleage and dry hay.)
What to do instead for dry-hay storage
For genuinely dry hay, the proven, lower-cost, lower-risk approach is net wrap plus cover — not film.
Net wrap + a tarp
Net wrap is open polyethylene mesh: it holds the bale's cylindrical shape and sheds rain off the rounded surface, but it is breathable, so vapor escapes normally and there is no airtight film for moisture to condense against. Add a quality tarp and you protect the stack without ever creating the condensation trap. See our hay-bale tarp selection guide and DIY bale covers.
Net wrap + a shed or hoop building
Covered storage is the gold standard. Net-wrapped bales stored under roof keep their color and feed value with minimal dry-matter loss. For the full rundown on protecting bales kept outside, see storing net-wrapped bales outside and our general round-bale storage guide.
Why this wins on dry-matter loss
The reason net wrap is the dry-hay default in the first place is weather shedding: full-surface mesh coverage means net-wrapped bales typically lose far less dry matter outdoors than twine-tied bales, where water runs in between the strings. You get the storage protection you wanted from "wrapping" — without sealing moisture in. For that comparison, see net wrap vs. twine.
Baleage vs. dry hay: the film decision at a glance
| Moisture at baling | What happens | Correct approach |
|---|---|---|
| Under ~18% (dry hay) | No fermentation; film traps condensation → surface mold | Net wrap + tarp or shed. Don't seal in film. |
| ~18–25% ("tough") | Too wet to store dry, marginal to ferment | Wrap promptly as baleage or use propionic acid and store dry |
| ~40–60% (baleage) | Ferments and preserves under an airtight seal | Net wrap inner layer + silage film — see below |
If your hay is in the baleage range, you want the right two-layer system: net wrap to hold shape, film to seal. We cover that in the first-time baleage guide and make the case for net wrap as the inner layer in net wrap under plastic film. And if you are wrapping wet or rained-on hay to save it, our wrapping wet hay guide is the one you want.
Frequently asked questions
Can you wrap dry hay in plastic to protect it outside?
You can, but expect a 1–2 inch layer of white surface mold where the film contacts the hay. University of Kentucky Forage Extension guidance suggests 2 layers of black plastic, a vertical ventilation slit between bales in a tube, and only doing it with hay below ~20% moisture. The loss is less than leaving bare bales exposed, but net-wrapped bales under a tarp or shed are usually cleaner and cheaper.
Why does dry hay mold when you wrap it in plastic?
Even at 12–15% moisture, hay holds enough water to cycle as vapor. Daytime heat drives that vapor into the bale's air space; cold nights condense it on the plastic, and the hay touching the condensation stays wet. Because dry hay does not ferment, there is no pH drop to inhibit mold, so it grows on that perpetually damp surface.
What is the minimum moisture for baleage fermentation?
Penn State Extension puts the optimal range at 45–60%, with fermentation possible but riskier from 30–70%. Below about 30% there is not enough sugar substrate and moisture for bacteria to produce preserving acids. Truly dry hay at under ~18% is far below the practical threshold, which is why it molds instead of fermenting when sealed.
I have tough hay at 20–25% moisture — should I wrap it?
If you have a wrapper ready, wrap it promptly while it is still wet enough to begin a mild fermentation rather than waiting for it to dry further. Propionic-acid preservative at the labeled rate helps in this borderline band. If you do not have a wrapper, treat it with preservative and store it as dry hay — do not let it dry to 20% and then try to dry-wrap it in film.
What is the difference between net wrap and silage film for dry hay?
Net wrap is breathable open mesh that holds the bale together and sheds rain without trapping vapor — the right outer wrap for dry hay. Silage stretch film is airtight, made to exclude oxygen for fermentation; around dry hay it becomes a vapor barrier and causes condensation-driven mold. Net wrap for dry hay, film for wet hay you are fermenting.
The bottom line
For genuinely dry hay, wrapping it in airtight plastic film usually trades a weather problem for a mold problem. Without enough moisture and sugar to ferment, the film just traps condensation, and you lose the outer inch or two of every bale. Reserve film for forage you are actually fermenting as baleage, handle "tough" 18–25% hay promptly (wrap-as-baleage or preserve), and store your dry hay the proven way: net wrap to shed weather and hold shape, plus a tarp or shed for cover.
That breathable, weather-shedding outer layer is exactly what net wrap is for. Explore XES® Extreme bale net wrap for dry-hay bales that hold their shape and shed rain, and compare options in our best bale net wrap guide.
Sources: Penn State Extension, baleage moisture and fermentation guidance (extension.psu.edu/baleage, accessed June 11, 2026); University of Kentucky Forage Extension guidance on wrapping dry hay for outdoor storage (paraphrased from a producer-quoted passage; original page not independently verified); HayTalk.com producer forum, threads "dried wrapped hay question" (#100523), "wrapping dry hay" (#89522), "wrapping dry bales" (#101674), "wrapping dry hay?" (#36977), accessed June 11, 2026.
Featured photo: Round hay bales in covered farmyard storage by Tozina, licensed under CC BY-SA 4.0, via Wikimedia Commons.
