Protecting Cacti from the Cold

Unless they’re lucky enough to live in the hottest and driest regions, all cactus growers fear extreme cold and freezing weather! Here’s some advice on protecting cacti from the cold. This is vital when cultivating all such plants, as short or sustained periods of frost are likely to seriously harm – or even kill – our precious cacti. With this in mind, here’s a collection of materials and structures which we find helpful for reducing the risks of cold damage to our precious cactus collection over the winter months. Illustrating quite how serious such damage can be, in 1987, a prolonged cold snap in Barcelona is estimated to have reduced the genetic diversity of the historic Jardins de Mossèn Costa i Llobera by approximately forty-percent!


Useful Materials for Insulating Cacti

Trichocereus cactus next to some cold ice!
Trichocereus cactus keeping cold with some ice!

There are many materials which are useful practical tools for minimising the effects of cold on individual plants of any sort. Here’s some of those we’ve tried out in our nursery as of the time of writing. Feel free to send us an email if you have any suggestions that you believe we should add here.

Things to consider when protecting cacti from the cold using any type of sheet material:

  • The material should be large enough to cover the plant or plants completely, without touching the ground (else moisture can wick up, potentially causing rot).
  • Check the plants for damage before covering them. Dependent on the weather conditions, they may benefit from watering at this point too. Clear any organic or other debris from the soil’s surface; this avoids unwanted pockets of moisture building up beneath them.
  • Cover the plants carefully and evenly with your chosen sheeting, without drawing the material too tight. Clothes pegs/pins, twine and wire are all helpful for securing it.
  • Covering any plant with opaque or semi-opaque materials for extended periods of time is potentially problematic, due to them partially or completely blocking out sunlight (required for photosynthesis and general good health). In such cases, it’s especially important to regularly remove the material during daylight hours.
  • Remember to regularly check all of the material covering your plants for damage or loosening due to harsh weather conditions, reinforcing and replacing as necessary.
  • Once the warmer months return, remove all protective coverings slowly and carefully, paying close attention for any signs of damage to your plants, which may require further action (such as sulphur or similar treatments, or even surgery).
Agave americana coated with frost in cold ground
Agave americana coated with frost in cold ground.

Bubble wrap

Plastic bubble wrap (as used in packaging breakable goods, parcels and shipments) can be used as an insulating wrapping to protect cacti and other plants from the cold. We recommend removing it periodically to check for unwanted squatters, such as snails and slugs, or damaged spots. Bubble wrap is also really useful for plugging up gaps in greenhouses and suchlike – whether caused by breakage or by sloppy workmanship!

Burlap

Burlap – also called hessian and jute cloth – is a coarse fabric made by spinning the fibre from plants of the genus Corchorus (collectively known as ‘Jute’ plants). Burlap is a useful addition to any garden arsenal, used to wrap plants and as groundcover for soil beds and plant pots, protecting against cold, soil erosion, weeds and wind.

Horticultural fleece

Horticultural fleece is a type of lightweight, porous fabric that is often used in gardening and horticulture to protect plants from harsh weather conditions, such as frost and wind. Both breathable and durable, it works by protecting against cold air and moisture, while still allowing heat and water vapour to escape – creating an ideal microclimate for delicate plants and seedlings. Such fleece – often used for the inner linings of commercially-available mini-greenhouses – is commonly made from polypropylene (or a similar material). Drape it over individual or grouped plants, or even to protect entire beds from frost damage.

Pipe lagging

Pipe lagging (used to insulate water and other pipes in buildings and elsewhere) is a handy gardening tool when cut to length and slit down one side, enabling it to be used as a self-attaching insulation layer (or several) to wrap around columnar cactus species. This material blocks out much of the plant’s available sunlight, but usually without long-term harm (at least based on our past experience using it in the coldest months). We recommend occasionally checking for snails and other issues by removing the lagging from time-to-time.

Plastic sheeting

Similar to the other sheet materials discussed on this page, transparent plastic sheeting of various thicknesses may be used for protecting your cacti from the cold, as it traps warm air within its area of coverage, while letting light pass through. This reduces the chances of frost forming on the plant or its substrate, but does not eliminate the risk totally. Due to its comparatively poor properties as an insulator and tendency to disintegrate into micro-plastic particles over time, we see this material as more of a last resort for protecting our plants from the cold, but it’s still useful to keep around!

Styrofoam cups

Covering the growing tips of columnar cacti with Styrofoam cups during the winter months is a good way to minimise, or even completely avoid, cold damage to this delicate part. This is especially important with these plants, as this type of damage can cause the stem to terminate, meaning it will either no longer grow, or else start forming a new pup (similar to when a cactus stem is cut). Larger plants can be covered with larger Styrofoam containers, although these may be harder to source in small quantities.


Useful Structures for Protecting Cacti from the Cold

For protecting cacti from the cold – whether you have several plants or even the largest of cactus collections – it’s possible to improvise or build several different types of semi-sealed containers and roofed structures. For maximum results, these can then be further insulated using any of the materials described above.

Greenhouse being set-up.

Cloches

Cloches are covers for smaller and/or individual plants, which are made from glass or plastic. Used to create a warmer, sheltered microclimate, they’re a practical solution when protection against frost and/or wind is needed, but where a greenhouse or polytunnel is overkill for the specific situation. The most common style of commercially-available cloche is bell-shaped, and usually made from glass. However, we always prefer to recycle materials we already have to hand, hence tend to repurpose the five or seven-litre plastic drinking water containers commonly available in our local area. Choose the right size cloche for your needs, allowing sufficient air circulation and avoiding the plant touching its container. This reduces the chance of mould via condensation soaking the plant and soil. Be careful with any glass containers during the coldest months, as they may shatter if left outside unattended.

Cold frames

Typically constructed from glass or plastic, cold frames are a great way to extend the growing season by providing protection for plants during harsh winter months or in areas with cooler temperatures. Functioning similarly to greenhouses, but on a smaller scale, cold frames trap heat inside, helping to regulate both soil and air temperatures. Remove their lids in the daytime to encourage air circulation, placing them back on in time to trap warm air in time for the cooler night-time periods. Understand that it’s often necessary to cover such frames with further insulation in order for them to protect your cacti from the cold during more severe frosts.

Fish tanks and vivariums

For those who have them to hand unused, repurposed glass or plastic fish tanks and vivariums can work well as improvised greenhouses. Note, however, that such containers require regular fanning (or other ventilation tactic) to avoid stagnant air causing problems for the plants. As with glass cloches – and the plastic boxes we describe further down the page – take care not to shatter this type of container, whether through negligence or sustained exposure to extremely-low temperatures.

Greenhouses

Greenhouses, most commonly constructed from various thicknesses of glass or polycarbonate plastic, are generally the most durable cultivation structures available to the average gardener, nursery, or research facility for protecting their plants – including cacti – from the cold. When installed and furnished sensibly, greenhouses of any type provide the perfect environment for plant growth by regulating humidity, light, temperature and other environmental factors. This – combined with a properly integrated maintenance and pest management routine – ensures your cacti have everything they need for optimum growth. Additional options to consider to enhance the cold-protection offered by a specific greenhouse include improving insulation (for example, by replacing polycarbonate sheets with a greater thickness, or plugging holes with bubble wrap or thermal blankets), installing specialist greenhouse heaters and supplementing existing ventilation by adding vents or windows.

Polytunnels

Polytunnels (also known as high tunnels and hoop houses) are a great way to extend the season, inhabiting a niche somewhere between a large cloche and a fully-featured, permanent greenhouse. Generally, a more affordable, yet much less structurally secure than a proper greenhouse, they’re commonly made from plastic or polyethylene film, installed over a metal or wooden frame, and are available in a wide range of sizes. Some designs offer the option of double-layered coverings for extra insulation, along with sturdier frames, aiming to increase the polytunnel’s durability against harsh conditions.

Besides the now-familiar caveats regarding proper ventilation, we must stress here how important it is to secure polytunnels as strongly to the ground as possible. We’ve learned this ourselves through bitter personal experience, when two of ours were completely reduced to twisted metal and ripped plastic sheeting, after failing to endure a comparatively short twenty-minute gust (albeit an extremely heavy one) of wind which appeared out of the blue at our nursery one autumn day! All of this occurred despite our following the manufacturer’s installation instructions to the letter – further adding a number of breezeblocks as weights and burying the bottom part of the frame in the ground.

Transparent lidded plastic boxes

Pereskiopsis succumbing to frost
Pereskiopsis succumbing to frost.

When there’s nothing else to hand when the temperature drops, we’ve found the type of generic transparent lidded plastic boxes common to many households a valuable tool for protecting cacti and other plants from the cold. While no good against more extreme temperatures (due to the thin plastic they’re usually made of), such boxes do offer some protection against milder, shorter periods of cold, as well as from slugs, snails and wind. One great use for these containers is as a “second-skin” inside greenhouses or polytunnels. For example, we’ve been trialling them to hold small Trichocereus seedling grafts on Pereskiopsis spathulata, which are much more tender than more mature cacti on their own roots (or a hardier root stock).

However you use these boxes in your own garden, make sure to regularly ventilate them by removing their lids and fanning them for a while – else you’re creating an ideal environment for fungi and mould. Furthermore, untreated plastics which were never intended for outdoor use do break down comparatively quickly when exposed to the elements (especially cold temperature and intense sunlight) – meaning that they become brittle and less clear over time. Keep this in mind when handling them after a long season outside, else it’s easy to cover your garden in tiny shards of plastic – as well as potentially dropping all the plants you were keeping safe!


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