We have had a pretty nice fall, the first that I can recall in recent years. I know that open falls have occurred in recent memory, but the last 2 were so awful as to obliterate all memory of a better example. Despite the good run that we have had, the cold weather will eventually arrive, which means that we need to be thinking about protecting our sensitive perennial crops from extreme temperatures (as well as fluctuations in temperature) and desiccation.
Which plants require protection?
When I think about protecting perennial horticulture plants, I do not typically worry much about the woody trees and shrubs, provided that they are well-established and have received adequate care leading up to winter. I mostly think about strawberries and garlic, as they can suffer some injury in some years, if snow covers are thin, if temperatures are severe, and temperature fluctuations are extreme. Depending on what you are growing, some other crops might benefit from a layer of protection, such as a cloth cover, or some other type of barrier, but generally, it is just strawberries and garlic that we talk about putting protection over.
Crops that are not actually in the soil, such as container nursery crops, or perennial plugs that have been started and are being overwintered in trays, are another story, for another day.
Why protect?
Even the crops that are adapted to a temperate growing region can be sensitive. This might be because they are not entirely hardy in the colder zones, or because they do not handle temperature changes well, or because they need to be forced to stay calm and dormant for their own good (i.e. particularly in the spring-confused, a.k.a. seasonally challenged, prairie environment).
Regardless, a layer of protection is most often used to insulate the delicate growing points of the plant, protecting them from the extreme cold that we can experience. It is also used to keep air and soil temperatures more stable underneath, despite what is happening on top of the protection. Covering layers can also protect the crop from the cold, drying winds and low humidity conditions that are common in our climate. Having a covering layer will also trap more snow, adding to the insulating effect.
What to use?
You can theoretically use many different things to insulate and protect a crop during winter. However, depending on the nature of the crop, some covers work better than others. People use fabric covers of different thicknesses, leaves, bark mulch, as well as different types of straw.
Characteristics of a cover
In my opinion, the most important characteristic of a protective crop cover is the ability to maintain an insulating layer of air, both above and around the crop, as well as within the cover itself, while still allowing some air and water flow, as required. A covering layer that compresses will provide much less benefit than a layer that stays “fluffy” and air-filled. Also, any cover that creates layers of moisture that freeze should be avoided.
Straw Mulch
We typically use straw to insulate strawberries and garlic through the extended winter period. Straw is lightweight, fairly inexpensive, and is easy to dispose of through incorporation after it is done insulting. The straw can be simply moved off into the alleyways after the winter is over. It then provides insulation from extreme heat and captures and conserves moisture. It may also provide a straw layer under the plants to keep berries clean and makes picker movement more all-weather than it would be without.
It should be noted that straw is not simply straw. There are differences between the types of straw that might be commonly available, with some better than others. Regardless of the type of straw that you use, it should be as weed free as possible. Volunteer crops are a big problem in many straw sources, as there are always seeds that remain after combining. Having the combine process the straw a couple of times can help cut down the volunteers but adds expense and effort that might be prohibitive. There are also different post-baling treatments to control weeds, but they can be a bit finicky.
The best straw to use is generally considered to be wheat straw, as it holds its structure and shape for extended periods, while having plenty of air spaces. It is also fairly common and easy to find. Oat or rye straw are also acceptable, if they can sourced. Barley straw can be used but is a bit heavier and tends to flatten out and compress, reducing the benefit of its application. Flax straw will hold up for a long time.
Timing
The timing of application of a protective cover is critical, however it can be a bit challenging to pin down each year, especially since every year is different. It would be great if you could calendar your straw mulch applications, but nature does not typically cooperate. As a general rule, you can apply mulch in mid to late October, but there are years where we will see much colder temperatures much earlier than that, resulting in some winter injury that shows up the next spring.
For strawberries, they will start to shut down and acclimate once days become shorter, daytime temperatures dip below 10°C and nighttime temperatures drop below 0°C. Once they are hit with 3 to 5 frosts of -5°C, they will fully acclimate. Once plants have been hit with a hard frost, they will start to turn red and flatten down.
Strawberries will be damaged if temperatures reach temperatures of -7°C or colder, with the level of damage increasing as temperatures go lower.
Straw should not be applied to non-dormant plants, as this can result in less cold hardy plants, since the plant’s natural hardiness will not have fully kicked in. I guess that the concept is that if you protect it too early, you essentially are protecting a weaker plant.
For things like garlic, mulch is applied after the crop has been planted in the fall once it has had time to establish some roots. The mulch should be applied once the soil cools down a lot, generally in late fall or early winter (e.g. late October or early November). Avoid applying the mulch too early, as this can create warm and wet conditions that result in bulb rotting.
Amounts
For strawberries, mulch should be applied over the dormant rows in a layer approximately 4 inches (10cm) thick, followed by another 2 inches of straw over the entire area, including the between-row spaces. This layer anchors everything down.
Garlic requires about 6 inches (15cm) of straw to protect the crop.
In order to get 4-6 inches (10-15cm) of straw over a field, you will need about 2-3 tons of straw/acre, or 4.5-6.5 tons/hectare. This translates into about 120 small square bales, or 10-12 large round bales, per acre. For smaller scale situations, that translates to about a 1/4 to a 1/3 of a small square bale per 100 square feet.
Conclusions
You do not have to apply mulch to your sensitive crops, but the use of a mulch can improve plant survival and productivity, and will go a long way towards protecting the plants from the increasingly variable (and potentially damaging) fall/winter/spring conditions.