Winter Losses

Winter Losses

 

Hostas are among the sturdiest plants I've ever grown, and fortunately, we seldom have any losses.  But all living things can have their problems, and if you are going to lose a hosta, it's probably going to happen over the winter or very early in the spring.

If you've lost one or more hostas over the winter, it's probably easier than you think to determine the cause.  As far as I know, and I've been growing these things for almost 30 years, there are no chronic diseases that will suddenly kill a hosta.  Those few pests and diseases that hostas might contract, like nematodes, fungus diseases, and viruses, are generally an obvious problem with obvious symptoms years before they can kill a plant.  Many of these pests and pathogens lead only to leaf damage and do not kill the plant. 

Obviously there are things like accidental herbicide poisoning, a dog that waters your hostas every day, etc., but aside from these things that would be unique to your experience, there are only two significant causes of winter losses, voles and crown rot. (OK, three if you live in areas where heaving is a problem.  We'll cover heaving at the end)

Voles feed on the plants from underground.  You may think you don't have voles, but unless you know what they are and how they live, and have actually looked for them, don't make that assumption.  Voles are very small, short-tailed field mice that live underground and about the only time you will see one is when your cat brings you a dead, furry present.  They are very common in all areas, but because they burrow and live underground, most people never see them.  Voles consider the starchy rhizome of a hosta a delicious treat, and they can destroy a plant or a whole group of plants in a very short time.  Voles have to find your plants before they can eat them, so you may grow hostas for several years with no damage and then one year, they're gone.  Vole damage is much more common in the winter than at any other time of year.

When we first moved our nursery here a couple years ago, the previous owner of the property had a few mature clumps of hostas scattered around the house.  These plants had obviously been here a while, so I assumed that voles were not a major problem.  We were way too busy that first year to do much planting but I decided to put in a small hosta garden to get started.  In the spring I found the entire area was undermined with vole runs and out of about 25 hostas, not a single plant was undamaged.  I don't know why her plants survived, but the area was infested. We are trying again this year, but only after a serious eradication effort. 

Essentially, voles are just mice that live underground, and control measures generally the same as those for mice anywhere, traps, poisons, or cats.  You just have to be careful that your efforts do not harm pets or other wildlife.  Newer methods, such as noise makers and castor oil sprays are also sometimes recommended, but I don't know if they actually work or not. And lastly, you can use wire cages or other barriers to surround the roots and stop the voles from tunneling to the rhizome. There are a number of web sites that provide information on vole control.  Just use your favorite search engine.

The second possible cause of winter loss is crown rot, and it is a bit more complicated because it can be caused in several ways, but almost always by water, either too much or too little, or by freezing temps at the wrong time, or a combination.  First the water factor.  Usually the problem is too much water in the winter. In heavy soils, without good drainage, the plants often stay too wet in the winter and rot sets in.  Hostas love water when they are actively growing, but not when they are dormant. Occasionally, crown rot can also be a problem if the plants do not get enough water in the fall and are stressed when they go dormant.

The solution to this problem is to insure that your hostas are planted in well drained soil. If your natural soil is heavy clay, you should add as much compost as your back will allow.  If you can't incorporate compost into your soil, you can add compost and topsoil to form raised beds so the water can drain away from the rhizome.  Be careful with raised beds, because piling soil on top of tree roots can smother and kill a tree.

Crown rot can also be caused by a late spring freeze.  Hostas are extremely hardy and can withstand very cold winter temps, as long as they are dormant.  Things are different when the plant starts growing in the spring.  Once the plant begins active growth, a late freeze can damage the plant or even kill it.  Some varieties, like plantiginea or montana 'Aureomarginata', break dormancy very early in the spring and are difficult to grow in areas where late freezes are common.  Virtually all hostas are susceptible to freeze damage, but the early risers tend to be hurt more often.  Blue hostas seem to be especially susceptible to such damage.

If freezing temps occur after the leaves unfold, even a light freeze can turn the foliage to mush.  Plants damaged by a light freeze will usually recover by summer.  A hard freeze can be much more serious, and can lead to crown rot even if the leaves are still tightly furled and only the spear is showing. 

So, if you're not sure what happened, how can you tell why your plant died?  The best way is to dig the plant and inspect the rhizome.  It won't hurt the plant, it's too late for that, and if you have many hostas in the same area, you should find the cause of the problem before you lose them all.

If you dig up the remains and find that the plant is still there but the rhizome has turned mushy and the roots are brown and pull apart easily, the plant has rotted.  On large plants, you may find that some of the rhizome is still firm and if you cut away all of the rotted flesh and dip the healthy part in a 10% bleach solution for a few minutes, you may be able to save something of the plant.  If the whole rhizome is mushy, there is no hope of saving it.

If you find that the rhizome is not there any more, or if part of it has been eaten away, you have voles.  Plants that rot take a while to completely decay and disappear, so if there's nothing left, it's been eaten.  Sometimes you don't even have to dig the plant, you can just stick your finger in the soil where it used to be and you will find the tunnel that the voles used to reach you hosta.

There are limits to what you can do to prevent winter losses..  The problem is, losing plants to things like this is just part of gardening.  You can't control nature.  But there are things you can do to limit the risk. 

Obviously, you cannot stop late spring freezes, but you may be able to protect your plants from damage.  If your hostas have broken through the soil surface and a freeze is coming, you should cover them with an insulating material. Nurseries generally use large cloth or plastic foam blankets specially made to provide several degrees of protection, but for hostas in the garden, covering individual plants or small groups is usually more practical.  Spring freezes often last only a few hours, so if you can provide an insulating barrier between the outside air and the warmer earth and plant, you can usually prevent damage. Anything from loose dry leaves to an overturned  box or bucket with a weight on top is suitable.  Try to keep the cover from touching the foliage, especially if you are using plastic sheets, so that there is an insulating air space between the cold air and the foliage.

If your plants are hit by a frost despite your efforts, there is nothing to do but cut off the mushy leaves and wait for them to regrow.  I suggest waiting a couple days before you cut them back because after a light frost the leaves will sometimes appear to be damaged but will recover after a bit of warmer weather.

Last, there is heaving, which is apparently prevalent in areas that have significant temperature fluctuations that cause the ground to freeze and thaw repeatedly.  I hear it's pretty common in the North, especially in years with little snow cover, but in over 30 years of growing hostas in this area I have never had a hosta heave. Heaving is caused when the ground movement causes the plant to lift from the ground and expose the roots.  Small or young plants are the most susceptible because they are not well anchored. If you have the problem in your area, mulching will be helpful. Be sure to pull the mulch away from the crown of the plant before it starts growing in the spring.