Fall Repotting

by Brent Walston

Introduction

Fall is a good opportunity for repotting. I have been an advocate for fall repotting in mild areas (zone 8 and warmer) for many years. However, there are some pitfalls. In this article the physiological state of the plant and the environmental factors in autumn are discussed so you can decide if fall repotting is for you.

The Advantages of Fall Repotting

There are some really good reasons for fall planting and repotting. The most important is that transpirational stresses are reduced. Fall is a time of falling air temperatures but residual warm soil (earth) temperatures. Deciduous plants will be pretty much done with their leaves so transpirational losses are minimal. Evergreens will still have active leaves, but the activity is reduced due to cooler temperatures and shorter days (lower light intensity).

This means that you can manipulate and prune roots without much fear of throwing the tree into shock. Additionally, fall is a time of massive root activity. Only the top of the tree is going dormant, the roots have a full complement of stored food from a whole season of activity. They will continue to grow as long as daytime temperatures are above about 50F, independently of the light and other conditions. For roots, soil temperature is everything.

Deciduous plants repotted a few weeks before leaf drop will become partly established in the new pot and soil before winter sets in. You need about six to eight weeks of warm daytime temperatures to achieve this for minor root work and repotting.

Contrary to popular belief, you should feed your plants 20-20-20 during this period. The nitrogen will not stimulate bud break once the buds have fully set. The roots will use this feed to speed their activity and build reserves for spring.

The Bad News

Now for the bad news, as I unfortunately discovered the hard way. I have been suspicious for some time that much of my failed fall transplants were due to killer freezes and not fungal activity in our mild very wet winters and springs. Now I can say that I am pretty sure that this is the case.

Undisturbed roots that grow in spring and midsummer will achieve a fair amount of cold hardiness due to lignification by fall and winter if they remain undisturbed. This is not the case in fall transplanting. Root pruning and repotting activity will stimulate root growth late in the season as pointed out above. These new white fleshy roots are quite fragile and have very little ability to withstand sub freezing temperatures. The degree of hardiness they obtain prior to winter is dependent on several factors.

Factors Determining Root Cold Hardiness

The biggest factor is the genetic trait of the individual species (sometimes cultivar as well). This is a matter of testing to find out how much cold an individual species can tolerate. There is precious little scientific work on the cold hardiness of roots in this juvenile state. Andy Walsh wrote a post about this a couple of years ago. He also wrote an article on freeze damage.

Two species that do have low thresholds for freeze damage to new roots are the less hardy Cotoneaster species and Japanese maple, Acer palmatum. Both of these have great economic importance to me, especially Acer palmatum. It is A. palmatum that I have watched very closely.

It is generally accepted that many woody species will sustain freeze damage to new roots below 20F (soil temperature). Above 28F most species, even a lot of tropicals will suffer little root damage. A number of important species will suffer damage to new roots between 20F and 28F Andy cited studies that showed some Cotoneaster species were damaged in the mid 20's.

The Evidence

Several years ago, I did a lot of shifting of seedling A. palmatum into 2 3/4 inch pots from seedling flats, hoping that we would have a mild winter since I didn't have room to protect them from substantial cold weather. Of course we had the coldest winter in a decade, so I learned some valuable lessons.

The newly potted seedlings grew very well in the fall. No top growth (they were fertilized), but substantial root growth nearly filling out the containers before Christmas. The roots were nice white succulent roots as new roots should be. I checked the roots each week during the winter to see how they were progressing. We had very heavy early winter rains, no root rot, no signs of damage. Then we had an arctic express. Temperatures fell to around 16F for four nights running. About two weeks later the results were in, massive root damage.

Fortunately, I didn't use all the seedlings but left several flats undisturbed. These were from the very same seedling batch. In one case there was half a flat left, the others were potted up. The seedling flats were left out in the open with no protection. Virtually all survived and were potted in early spring with a success rate of about 90%. The roots were all brown and fully lignified at the time of transplant, no root or stem damage from the cold whatsoever.

The potted seedlings suffered both root and stem damage. The stem damage didn't become apparent until later in the spring when the bark became mushy. Some of these seedlings survived, but I lost about 80% of them. There were a few potted seedlings in the shade house which stays about 2 degrees warmer due to the reduced air circulation. These did a little better than the potted seedlings left out in the open.

There were similar losses of other species. I lost the entire new crop of Corokia cotoneaster, the old plants were unaffected, only the newly repotted ones froze. Nearly the same for Sophora tetraptera 'Nana', 90% loss for newly repotted plants in the open, 50% loss for those under shade cloth, 0% loss for older plants. Apples, Malus species were unaffected, and did even better than the previous year, same for Chaenomeles. Both Scots and Black pine (Pinus sylvestris, P. thunbergii) totally unaffected. In fact they grew better than ever that spring (probably due to improved soil mix), completely filling out their pots by mid spring.

There were practically no losses to older plants in the nursery, even in exposed 2 3/4 pots, confirming my general rule of thumb that most temperate climate woody plants can survive temperatures down to 15F in containers without protection. The normal percentages survived except for Prunus mume, which suffered heavy losses of even older plants. The losses here appeared to be of fungal origin.

And finally

The conclusion? I still recommend fall planting, transplanting, repotting, and minor root pruning, but with the following cautions. For most woody temperate species:

It should be done in zone 8 and higher or in other areas where the winter storage temperature can be kept above 28F. In zone 8 some species will suffer in an abnormally cold winter and should be protected in the event of sub 20F temperatures. In zone 8 and higher, ground planting should be perfectly safe for recommended species. The difference of only a few degrees marks the difference between survival and death for some species.