For many gardeners in cooler climates, the question of how to successfully overwinter purple verbena is a topic of great interest and occasional frustration. While this plant is a hardy perennial in its native range, it often struggles when faced with the prolonged freezing temperatures and wet soils of a typical northern winter. However, with a bit of planning and the right techniques, it is entirely possible to help your plants survive the cold and return with renewed vigor in the spring. Whether you choose to protect them in the ground or move them to a more sheltered location, understanding the principles of dormancy is the key to success.
The cold hardiness of purple verbena can vary depending on the specific cultivar and the local environmental conditions of your garden. Generally, the plant is considered hardy in regions where the winter temperatures do not regularly drop below minus ten degrees Celsius. If you live in a colder area, you should treat the plant as a tender perennial or a high-performing annual. Knowing your local climate zone is the first step in deciding which overwintering strategy will work best for your specific situation.
The condition of the plant as it enters the winter months will also play a significant role in its ability to survive. A plant that has been well-cared for throughout the summer will have stronger roots and more energy reserves to carry it through the dormant period. You should avoid any heavy pruning or late-season fertilization that might encourage soft growth that is easily damaged by frost. Instead, let the plant naturally slow down and prepare itself for the change in seasons as the days get shorter.
Understanding cold hardiness
Cold hardiness is not just about the absolute minimum temperature, but also about the duration of the cold and the moisture levels in the soil. Many plants can survive a brief frost, but they will struggle if the ground remains frozen for several weeks at a time. Wet soil in the winter is often more damaging than the cold itself, as it can lead to root rot and ice crystals forming within the plant tissues. The purple verbena is particularly sensitive to “wet feet” during the dormant season, which is why good drainage is so important.
The microclimate of your garden can also make a big difference in how well your plants handle the winter. Plants that are located in a sheltered spot, such as near a south-facing wall or among larger evergreen shrubs, will be protected from the worst of the cold winds. These locations often stay a few degrees warmer than the rest of the garden, which might be just enough to ensure survival. When you are planning your garden layout, keep these “warm spots” in mind for your more tender species like the verbena.
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Snow can actually act as a very effective insulator, protecting the dormant crowns from the fluctuating temperatures of the air. A thick layer of soft snow keeps the ground temperature relatively stable and prevents the cycles of freezing and thawing that can push plants out of the earth. However, if the snow is heavy and wet, it can also cause physical damage to any stems that you have left standing. Understanding the local weather patterns will help you decide when to intervene and when to let nature take its course.
If you are unsure about the hardiness of your plants, you can always take a “belt and braces” approach by protecting some and taking cuttings from others. This ensures that even if the winter is exceptionally harsh, you will still have a way to replenish your garden in the spring. Gardening is often a process of trial and error, and learning what works in your specific location is part of the fun. Over time, you might even find that your plants become more adapted to your local conditions with each passing year.
Protecting plants in the ground
If you decide to leave your purple verbena in the garden over the winter, providing a protective mulch is one of the most effective things you can do. A thick layer of organic material, such as bark chips, straw, or even fallen leaves, will help insulate the root zone and the crown. This layer should be about three to four inches deep and spread out around the base of the plant once the ground has started to cool down. Be careful not to pile the mulch directly against the stems, as this can encourage rot in the damp autumn air.
Some gardeners prefer to leave the dead stems of the verbena standing until the following spring. These stems can provide a bit of natural protection for the crown and also offer winter interest and a home for beneficial insects. The hollow stems of some varieties can even serve as nesting sites for solitary bees that are looking for a place to wait out the cold. In the late winter or early spring, you can then cut the old stems back to the ground just as the new growth begins to show.
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In particularly cold regions, you might want to use a more intensive form of protection, such as a cold frame or a horticultural fleece. These materials can be placed over the plants during the coldest weeks to provide an extra layer of warmth and protection from freezing winds. Make sure that any covering you use is well-secured so that it doesn’t blow away during a winter storm. It is also important to remove the covers during sunny spells to prevent the plants from overheating or becoming too damp underneath.
Watering the ground thoroughly before the first deep freeze can also help protect the roots from damage. Moist soil holds heat better than dry soil, and it also prevents the roots from dehydrating during the winter months. However, this only applies if your soil has excellent drainage and won’t become waterlogged for long periods. Once the ground is frozen, you can stop all watering until the spring thaw arrives and the plant begins to wake up again.
Container overwintering strategies
If you are growing your purple verbena in pots, you have a few more options for helping them survive the winter. Containers are more vulnerable to the cold than the ground because the roots are not insulated by the large mass of the earth. The smallest pots can freeze solid very quickly, which is often fatal for the delicate root systems of the verbena. One simple solution is to move your containers into a more sheltered spot, such as an unheated garage, a shed, or a cool greenhouse.
Before moving the pots indoors, you should cut the plants back by about half to make them more manageable and reduce the amount of moisture they lose. You should also check the plants thoroughly for any pests that might try to hitch a ride into your winter storage area. Once they are in their new home, the plants will enter a state of dormancy and will require very little attention. The goal is to keep them just above freezing and provide only enough water to prevent the soil from drying out completely.
If you don’t have an indoor space for your pots, you can try “heeling” them into a garden bed for the winter. This involves digging a hole and placing the entire pot into the ground, then covering the top with a thick layer of mulch. This method takes advantage of the earth’s natural insulation to protect the roots inside the pot. It is a great way to save space in your garage while still giving your container plants a much better chance of survival.
Another option for container plants is to wrap the pots in bubble wrap, burlap, or specialized pot insulators. This adds a layer of air that helps to slow down the rate of temperature change inside the soil. Grouping several pots together also helps them retain heat more effectively than if they were left standing alone in an exposed spot. Whichever method you choose, remember that consistency is the key to preventing the stress of rapid freezing and thawing cycles.
Transitioning back to spring
As the days start to get longer and the temperatures begin to rise, it is time to start thinking about bringing your verbena back into active growth. If you have protected your plants with mulch, you can gradually start to pull it away from the crowns as the danger of deep frost passes. This allows the sun to warm up the soil and encourages the first new shoots to emerge from the earth. Be prepared to put the protection back if a sudden late spring frost is predicted in your area.
For plants that have been stored indoors, you should start to gradually increase their exposure to light and water. Move the pots to a brighter spot and give them a light watering to signal that the growing season is about to begin. You might see some pale, spindly growth starting to appear, which is normal for plants that have been in a dark place. Once the weather is consistently mild, you can start the hardening-off process to move them back into their permanent outdoor positions.
Spring is also the ideal time to perform any necessary maintenance on your overwintered plants. You should cut back any dead or damaged stems that you left standing during the winter to make room for the fresh new growth. This is also a great time to refresh the soil in your containers or add a bit of compost to your garden beds. Providing a light application of a balanced fertilizer will give the plants the boost they need to start the new season with plenty of energy.
Don’t be discouraged if some of your plants don’t make it through a particularly tough winter. Even professional gardeners lose plants from time to time, and it is all part of the natural cycle of the garden. The survivors will be even more precious, and you can always use the space to try something new or expand your collection with cuttings. With each passing year, you will become more skilled at predicting what your garden needs to survive and thrive through all the seasons.
📷 Auckland Museum, CC BY 4.0, via Wikimedia Commons
