Winter Care of Bamboo

Needmore Bamboo Co.

The reality of Growing Bamboo in Zones 5 & 6

     There are many bamboo enthusiasts who live in Zones 5 & 6 and we want to grow varieties of bamboo that are not really hardy to our climate zones. We all want to believe that the hardiness data for a given species may be conservatively understated or that the combination of our micro-climates & our extra care in winter will add a few degrees of warmth and thus protect our bamboo from the ravages of extreme cold. I do not know if this is true for folks with a passion for plants other than bamboo, but I suspect that ornamental gardeners worldwide all try to 'push' the growing range of plants. Just look at me - I am growing bananas, hardy citrus, cacti, agave, and hardy orchids to name a few - all here in Indiana!

     In my experience, I have found that extra winter protection allows me to grow plants rated a zone warmer, but the reality is that some may survive but will not prosper. And I've tried many things to protect the culms - deep mulching of the rhizomes; covering them with blankets; burying them in snow, leaves, and mulch; venting the dryer towards them; and asking for divine intervention. And to some extent all of these protection attempts have helped. The problem is that as your groves expand it is no longer possible to protect more than a few culms. I still employ some of these methods by selecting culms of my prized varieties for 'wintering over' (see the photo of Moso below) and I am having the most success with burying entire culms in mulch, blankets, snow etc. when the forecast first calls for subzero temperatures (for me this is usually late December or early January) and leaving them that way until daytime highs are back above freezing. My goal is to save a few culms of the species so that there can be active photosynthesis during the spring shooting period.

      I've recently come to terms with my 'zonal denial' and I accept that most springs I may have to clear cut nearly all of the previous year's above ground growth (see the photos below). I live on the edge of zones 5b/6 and in an average winter the temperature will fall to at least -10F. At these temperatures nearly every species of my bamboo collection will likely suffer total leaf loss and in all most cases, total top kill of the culms. I know that many other bamboo web sites suggest that Atrovaginata, the Aureosulcata cultivars, Bissetii, Nuda, Rubromarginata and the Fargesias will hold green leaves beyond -10F and perhaps it has been their experience, but it has not been the case for me. As my groves mature they will probably begin to exhibit increased hardiness but I would encourage all new bamboo growers to accept that for at least the initial 5 years after planting you should be prepared to lose all above ground growth when temps/wind chills dip much below zero. The lone exception to this is my native Indiana Arundinaria Gigantea,- this species amazes me with its ability to retain some green and some moderately burned leaves to -19F and I suspect that it would be the only bamboo to emerge from some mild Zone 4 winters with some green foliage. Even the Fargesias look pretty ratty and usually defoliate in winter so I no longer expect them to hold green leaves despite their reputation for being hyper-hardy.

      There is a category of bamboo that can be grown with little regard to its hardiness rating - the ground covers and in particular the non-arborescent Pleioblastus species. Even when grown in warmer climate zones the ground cover bamboos tend to look ratty in the spring. Since these species are grown mainly for their foliage and culm height is not important it is recommended that they be mowed to the ground so that the new spring foliage will not be muted by the previous years dead brown leaves. In this case you are acting like winter by mowing them to the ground and as long as they are mulched they'll come back in the spring looking fresh and beautiful. I have seen species of Pleioblastus considered to be hardy to only 10F that were buried in snowfall emerge from -20F with no damage. I recommend mowing them down in February and immediately following the mowing, mulch them deeply before the new culms appear so that they can go into the next winter with some mulch protection.

      I have reached the conclusion that despite how excited I am at the prospect of planting a newly acquired species, the best strategy is to container grow the bamboo until it has really filled up a large pot with new rhizome growth.   I am presently up-potting new plants until they are well established in squat 7-gallon pots before planting them out. I believe that this extra rhizome mass will help the plant to endure cold temperatures and also to have much greater energy for producing new canes in the spring, particularly if there is top kill.   This means that one must over winter the bamboo indoors for at least 1 full growing season which may not be possible for everyone.   If you must plant out smaller plants then mulch very deeply and be prepared to cover the bamboo with blankets during extreme cold snaps, leaving it buried until daytime temperatures are back above +32F.

Winter Damage

      Bamboo is damaged by extreme temperatures, cold dry winds, and prolonged periods of below freezing daytime highs. All of these conditions are even more damaging if they occur when the bamboo is exposed to direct winter sun as the sun light adds to the other environmental factors that desiccate the bamboo leaves. When the ground freezes the rhizomes can no longer take in water and they respond as they would in drought conditions - they curl up their leaves. If this condition continues for very long the leaves will ultimately dry up and fall off. As long as the leaf buds on the culm branches are undamaged, new leaves will appear in late spring.
     Bamboo displays cold weather stress beginning with the leaves, which will start to show signs of cold-burn, starting at their tips and moving towards the culm, eventually drying out and falling off. This is known as leaf burn. Culm damage comes next and the culm will begin to die from the top down until total topkill occurs.
     The most critical winter protection that you must provide your bamboo is to deeply mulch the rhizomes. Even if total topkill does occur, as long as you have deeply mulched the rhizome system, the plant should continue to live and produce new shoots in the spring, however, the ensuing culms will be smaller than the prior years’ growth because the energy reserves of the colony have been stressed.   Exceptions would be the species growing on the extreme fringe of their hardiness will often begin to lose vigor after a few years of this cycle and may die off.
      Another way to minimize winter damage – particularly if you have selected a marginally hardy variety – is to be aware of any microclimate protection that a planting site may offer. These sites would provide shelter from winter sun & winter winds, and may also offer a few degrees of radiant heat from a house or other structure – even your clothes dryer vent directed into a temporary tent structure can provide some extra warmth on those subzero nights.
     In my location our coldest subzero temperatures usually occur as a result of snow covering the ground. The snow cover reflects daytime sunlight back into the atmosphere preventing the ground from receiving solar warmth and it also blocks the existent ground warmth from radiating back upward to the atmosphere. When these conditions are present, for additional protection I usually will bury my most sensitive bamboo in deep piles of snow. This will greatly help to protect the foliage from the air temperatures by several degrees. I am always amazed to see how the shrub & ground cover species that were totally buried in snow, will emerge looking fresh and undamaged by even the coldest air temperatures. The worst situation is for the subzero spell to occur with no snow cover at all, followed by sunny days.

      Bamboo can be amazingly resilient even after the coldest of winters. Although the culm size may be compromised in the year following the cold damage, the grove usually will recover and once again provide a beautiful grove or mass planting by mid-July. Below are some photographs of Phyllostachys Viridis that illustrate the cycle of winter damage & grove recovery in the spring.

After a severe cold snap 'peaking' at -20F in late January of 2004 the Viridis was totally top killed and turned tan by mid-March. I then cut down all of the dead culms leaving only the few that were buried in snow during the cold spell. This first photo was taken around April 1st of 2004.

This photo, taken from the same spot in mid-July shows no signs of the damage as a flush of new culms were produced. For reasons beyond explanation other than the extreme vigor of this particular clone, the Viridis has continued to increase in size each spring despite suffering total top kill each of the winters that it has been in the ground. A contributing factor may be that the Viridis is planted directly on top of my septic field and this may provide added ground warmth & moisture - not to mention some extra nitrogen!

This next photo was taken from the same spot on January 17, 2005 and it shows the results of -5F on 12/24/2004 & -12F on 12/25/2004 (some Christmas present eh?). This cold snap was preceded by a 17-inch snowfall and the remaining green leaves are on culms that were completely buried in the snow. The forecast for the night that this photo was taken, again calls for a subzero temperature of -7F with only 1-inch of snow cover and I anticipate more damage to the remaining leaves.

This photo shows leaf drop within a week of the -12F when temperatures shot into the 50's & 60's and stayed there for several days.

This photo of Moso - Phyllostachys Edulis - was taken on January 3, 2005 after the -12F a week earlier. I had covered the culms with a blanket and the 17-inch snowfall buried and further insulated them. They emerged from the deep freeze with no visible damage but they will experience subzero temps again in January with only the blanket cover. On the right is 1, 3 year-old division and on the left 1, 2 year-old. Note the deep mulching with a horse manure/sawdust mix.