Timing/Soil Temperatures for Full Repot of Japanese Maples

peapod13(8 South Sound WA)February 21, 2011

New poster and new to keeping trees in containers. I have been a lurker here for some time and have tried to do my due diligence.

My goal is to keep some (narrowed list down to 10 that I absolutely want, currently have 3 of those, list may grow as time goes along) Japanese Maples in containers long term. Through root pruning, size of container, pruning, partial defoliation, wiring, etc, I want to keep these trees semi dwarfed and shaped (however, I don't want full on bonsai, just small versions of what I've seen in Japanese gardens I've visited in the Puget Sound area).

I've choosen cedar containers that are approximately 7.5-8 gallons. I'm going to use the 1,1,1 gritty mix (currently have approximately 30 gallons screened and mixed up waiting on the right time).

Posts I've read regarding the process and timing or full repots indicate that the best time is during the quiescent period. I've also read that this period begins when the tree roots reach a certain number of thermal units, and that this can be estimated when the soil temperature is above 45 degrees. I've also read in my research about Japanese Maples in the Pacific Northwest, that Japanese Maples can have problems with root rot and fungus when roots are pruned too early, due to the cooler, wet spings we tend to have.

Air temperatures here have been in the mid 40's to mid 50's during the day, dropping to the upper 20's to low 30's at night. I used a meat thermometer (don't tell my wife) to try to measure the soil temperature in the 2 gallon nursery containers the three Japanese Maples are in and got an unexpected result. The soil temperature in the pots measured approximatley 50 degrees.

So now finally to my questions. 1) How accurate would using a meat thermometer be at measuring soil temperatures? 2) If the soil temperature I've measured today is reasonably accurate, should I go ahead and perform the full repot, or wait until either night time temps remain above freezing or until buds begin to noticably swell (The buds still look very small compaired to the buds we saw on some Japanese Maples in a garden in south Seattle/Tukwila yesterday.)?

Thanks in advance for any help given,


Thank you for reporting this comment. Undo
tapla (mid-MI z5b-6a)

Roots never really go 'dormant' like buds do. They will continue to grow as long as soil temperatures are above freezing. The best time to do root-pruning & repots on Japanese maples is just as buds begin to swell.

Just after Father's Day, decreasing day length (it's actually increasing night length, but that's a technical issue) stimulates the plants to start preparing for dormancy. As fall approaches, deep chill seals the deal. Once dormant, plants require a certain amount of chill to release buds from dormancy. Once the chill requirement has been met, plants pass into a quiescent (quiet) state in which soil temperatures prevent the top from growing. Several warm days can trigger the onset of growth, after which time your plants will need protection from freezing temperatures because almost all cold-hardiness will have been lost.

Good luck, Blake! You'll really like using the gritty mix. Be sure to get all the old soil off when you repot.


    Bookmark   February 21, 2011 at 9:14PM
Thank you for reporting this comment. Undo
peapod13(8 South Sound WA)

Hi Al,
First, thanks for responding and for all the knowledge you share in an effort to help novices like me (and others too) grow plants. Your reply, above, not only provided the direct answer to my questions (i.e. be patient a little longer and look for buds to begin swelling to do the full repot) but also provided information to clarify my understanding of deciduous plant's growth cycle. Thank you.

I do have one point I'd like you to clarify please.

When you refer to chill requirement, I think you're saying after short days and cold roots/soil send the plant into dormancy, the soil/roots have to warm to a certain point to release the buds from dormancy.

I'm also assuming based on other posts I've read, that this "chill requirement" occurs when soils temperatures are at or above 45 degrees for a certain amount of time.?

So to put it all together, once soil temps reach a certain point the buds are released from dormancy however, the roots don't start sending sap to rehydrate the cells in the above ground portions of the tree and start buds swelling until longer days (or shorter nights as the case may be) and warmer temps indicate to the roots that the tree isn't likely to freeze.?

Thanks again for your help and information.


    Bookmark   February 22, 2011 at 12:11AM
Thank you for reporting this comment. Undo
tapla (mid-MI z5b-6a)

In general:

Increasing night length + chill = dormancy.

Once dormant, it is the chllling that actually releases plants from dormancy. Chill is measured in units, which can be roughly converted to hours. Different plants require different periods of cold temperatures between approximately 32-42* to release them from their dormant state. Once the chill requirement is met, plants pass into a quiescent period (unnoticed) where they will remain until soil temperatures rise high enough for long enough to stimulate the onset of growth. You could keep plants quiescent for a very long time if you keep them chilled. This is why you often receive bare-rooted material in late May or even June that is still not in leaf or is just starting to put on a flush of growth.

To answer your last question, the plant has usually long been 'released' from dormancy by the time soil temperatures start to rise in the spring. In most cases, it would only have been low soil temperatures that offered the protective mechanism that prevented plants from flushing too soon & being exposed to killing low temps.

Mother nature has a little 'cush' built in to her plan. She knows the plants need chill, but she also knows she can't let her plants start growing in the middle of winter, so she decided to program plants to only grow (above ground) when rising soil temperatures stimulate the chemical messengers that tell plants cold injury is unlikely.


    Bookmark   February 22, 2011 at 9:03AM
Thank you for reporting this comment. Undo
greenman28 NorCal 7b/8a

Hey, guys!

As of now, all of my maples have been re-potted.

When I noticed the first of the buds begin to move, I got started.
Thankfully, most of the maples due for a re-potting had been in 4-inch and 1-gallon
containers, and so I didn't have to make a large volume of potting mix - just 4-gallons.

I move the containers back to the north side of the house to keep the pots out of the
direct sun. As Al has said, 'cool soils, low in initial fertility, are conducive to

Two of the maples are actually opening minute leaves...and, wouldn't you know it, this
past weekend we got a few inches of snow. I checked the maples, but I don't see any
sign of frost-burn...fingers crossed! ;-)


    Bookmark   February 22, 2011 at 11:32AM
Thank you for reporting this comment. Undo
peapod13(8 South Sound WA)

@Al, That makes perfect sense. Thanks for the further information.

@Josh, That is one of my concern's. While I don't want to wait too long and leaves start to open, I don't want to repot too soon either. Right now my three trees are in 2 gallon nursery pots. They are fairly easy to move into the garage if I need to. For example, the forecast is for snow here Wednesday into Thursday and then the temps are supposed to drop into the mid 20's this weekend. The larger containers would have been harder to move into the garage and, Al's post on repotting indicates moving the trees around or windy conditions, for that matter, can lead to damage to the new roots. Although, I do plan to use soft yarn to secure the trees after repotting, I figure the less moving them around the better.


    Bookmark   February 22, 2011 at 3:56PM
Thank you for reporting this comment. Undo
tapla (mid-MI z5b-6a)

Use something that doesn't stretch, like sisal or jute. If you're growing in nursery containers, you can melt holes with a hot nail near the pot rim at third or quarter points points (every 120* or 90*) around the pot rim & secure your plant to the pot with 3 or 4 strings. I very often use something like this, employing small spring clamps so I can quickly/easily tighten the strings without retying anything:

You're only going to have the strings on for about a month (unless you want to be EXTRA sure the newly root-pruned plant won't topple), so there is no need to worry about using 'soft' string or yarn. The coarser, rougher jute/sisal won't bite into the bark in such a short time.

If your trees haven't yet been repotted, just put them somewhere cold and watch carefully for the onset of budswell - repot at that time. I can't imagine it would be cold enough where you live to do any harm to trees not in leaf. Light is not an issue to dormant trees, so a cold shed or garage is fine.

BTW - you mentioned in your original post that you were surprised at how warm the soil was. Composting is an exothermic process, producing heat as it progresses. This is just one of the reasons that ingredients that break down quickly should be avoided in container media. Soils that contain significant amounts of unfinished compost, mushroom compost, sawdust ...... can cause soil temperatures to rise as much as 30-40* above what they would normally be if the ingredients were more resistant to break-down. During the hot months, even an extra 10* can have a very significant impact on root growth/function/health/metabolism.


    Bookmark   February 22, 2011 at 4:31PM
Thank you for reporting this comment. Undo
peapod13(8 South Sound WA)

The trees are actually in the garage now. Supposed to get into the high 20's tonight with possibility of 3"-6" of snow in the next 24-36 hours. Based on your advice, to others who live in colder climates, to put their trees in a garage with a cardboard box over them, I figured it wouldn't hurt to put the trees in the garage until this cold snap passes.

I haven't repotted yet, so the trees are still in their 2 gallon black plastic nursery pots.

I will be repotting into 7.5 - 8 gallon cedar containers. (They look like small wine barrels.) I was planning on sticking a 3/4" sheet rock screw into the outside of the container and tyeing off to the screw, but I like your idea of using spring clamps.

I'll bet your right about the compost in the nursery soil causing the soil temps to be above what I expected. The nursery where I bought the trees recommended a soil mix that included: small pine bark, pumice, loam, and composted yard wastes. They tried to sell me on the benefits of their soil. I politely declined.

Thanks again for all the information you so freely give.

PS - I'm going to be taking pictures of the process as I repot. (I take pictures of all my hobbies so I can follow progress.) I'll post pictures here or in a new thread if anyone is interested.


    Bookmark   February 22, 2011 at 9:42PM
Thank you for reporting this comment. Undo
jojosplants(9/ Tucson, Az.)

HI Blake,
Pictures are always welcome and enjoyed here! :-)

Looking forward to seeing them.

    Bookmark   February 22, 2011 at 9:57PM
Thank you for reporting this comment. Undo
tapla (mid-MI z5b-6a)

Oh yes! Please do post pictures. Others can learn from both the pictures and subsequent comments. We'll look forward to them.


    Bookmark   February 22, 2011 at 10:48PM
Thank you for reporting this comment. Undo
tapla (mid-MI z5b-6a)

Oh yes! Please do post pictures. Others can learn from both the pictures and subsequent comments. We'll look forward to them.


    Bookmark   February 22, 2011 at 10:50PM
Thank you for reporting this comment. Undo

Blake, as someone who lives in your area and grows a lot of maples in containers, maybe my experience will be helpful to you.

Late winter/early spring weather in the Puget Sound area can be very unpredictable - mild and balmy one day with freezing temps and snow the next. That said, I repot maples pretty much year round......it just depends on the reasons and when I've purchased them. A routine maintenance repotting (and one that may involve root pruning) I generally reserve for just before bud break, although I've done it in late fall as well. Hard to say exactly when that may be, as it seems to vary both with weather and the tree in question. After examining the repotting candidates for this year recently, I'd guess I'd start this process in about 2-3 weeks or mid to late March - the buds on my trees are looking very good and healthy and starting to show signs of coming to life. These also tend to be some of my larger maples, so working with the plant while it is still pretty much dormant is both easier on me and on the tree. But it is quite possible to repot JM's succcessfully when in leaf as long as they are small enough to handle easily and you can provide some additional specialized care to get them over the transplant hurdle.

I also don't pay too much attention to soil temps at this time -- that doesn't seem to have too much bearing on container soils, as they don't hold the moisture as avidly as Puget Sound inground soils do and they do tend to warm sooner as well. FWIW, I wouldn't necessarily consider planting a new JM in the ground at this time of year as the soil is simply too wet, in fact it's too wet to plant much of anything and you can damage the soil if you attempt to work it in this condition.

I also don't provide any special winter attention unless it is a very small plant - 2G or less. Mine are out there year round and have gone through some seriously cold temps without any problems. They are in pretty good sized pots, however - repotting is a lot of work and I tend to use larger containers (15G at least) just because they seem to fit the scale of the plant better and don't require repotting attention quite so often.

I addition to the excellent and helpful information you find here, you might want to visit the Maples forum too. There are a number of us that grow JM's in containers that post there regularly and lots of good info about growing these trees, both in containers and in ground.

    Bookmark   February 24, 2011 at 9:34AM
Thank you for reporting this comment. Undo
peapod13(8 South Sound WA)

Hi gardengal48,

I'm quite sure your experience will be helpful. Before actually posting here, I've done many searches and reading on both tapla's (Al) and your posts. (To others reading and contributing to my post, please don't be offended. Just seems like gardengal48 and tapla were involved in many of the posts involving the specific information for which I was searching.)

Regarding size/cold protection, my trees are actually all 2 gallon (just bought them this December) and my Fjellheim has brown/black tips on many of the new shoots (which I assumed was either frost or wind burn), so I started putting all three in the garage when the forecasted temps drop below freezing. (When I bought the Fjellheim the shoots were maroon/purple all the way out to the tips.) I move them back outside to get rain/sun when forecasted temps are above freezing. So far this seems to have worked as the black tips haven't gotten any worse.

Regarding container size, I chose the 7.5-8 gallon container hoping/assuming I would get some additional dwarfing by keeping the trees root pruned to be healthy in the smaller container.

Thank you for sharing your knowledge of Japanese Maples in the PNW. It's the so freely given information, gathered from experience, that keeps me coming back searching and reading, trying to give myself and my trees the best chance at suceeding.


    Bookmark   February 24, 2011 at 8:02PM
Thank you for reporting this comment. Undo
greenman28 NorCal 7b/8a

"All gardening is local..."

No offense taken, Blake! Local advice is good advice!
And GardenGal's advice is Sound! (Pardon the pun) ;-)


    Bookmark   February 24, 2011 at 8:36PM
Thank you for reporting this comment. Undo

Good one, Josh, LOL!!

With overnight temps here right around 20F or less, stashing those maples in the garage was a smart idea. Roots tend to be more sensitive to cold than does topgrowth and can be badly damaged at around 24-26F. 2G plants with a small amount of soil surrounding the roots just don't have benefit of enough insulation to keep them warm and toasty out in the weather.

FWIW, I'm not sure the tip discoloration on the Fjellheim is necessarily wind or frost burn. This is a dwarfed sport of Sango Kaku, which is well known for its propensity to pseudomonas, a bacterial stem blight. Pseudomonas is usually diagnosed by black or dark purple lesions or branch tips and IS aggravated by cold damage, however, so your precautions are sound. Dead wood will actually show as gray.

    Bookmark   February 25, 2011 at 10:22AM
Thank you for reporting this comment. Undo
peapod13(8 South Sound WA)

I did a search on Pseudomonas. Doesn't sound like a good thing.

Here's a picture: http://farm6.static.flickr.com/5215/5476763375_595a68377a_m.jpg
Does this look like Pseudomonas?

I did a google search for Pseudomonas. One site said there's "no cure" and not to prune in the winter but spraying with a copper solution would help control its spread. Another site said prune the entire effected limb, then spray with a copper solution. Any consensus on the best course of action? Where to get the copper solution and application rates?

Thanks, Blake

Here is a link that might be useful:

    Bookmark   February 25, 2011 at 3:49PM
Thank you for reporting this comment. Undo
peapod13(8 South Sound WA)

Let's try the photo again.


Here is a link that might be useful:

    Bookmark   February 25, 2011 at 3:54PM
Thank you for reporting this comment. Undo

Blake, I'd hate to confirm pseudomonas based on just one, rather small photo :-) It does look like a possibility, tho.

IME, while it can be disfiguring, pseudomonas is seldom fatal. Generally the treatment I recommend is just to remove the affected area, pruning down well below the symptomatic tissue (at least to the next bud union, maybe the second). Be sure to sterilize your pruners after each cut.

I have never needed to follow up with a copper spray but should you choose that route as well, it is available at any good nursery or garden center - it is a very common dormant spray ingredient. And you DO need to apply while the tree is still dormant or before bud break. Japanese maples are very picky about anything sprayed on their foliage.

FWIW, my experience with Japanese maples over the years leans towards pseudomonas being an affliction of mostly quite young trees. Establishment and/or age seems to offer the abiity for the tree to grow out of the problem. And the only trees I have seen that were seriously affected - to the point where their healthy was compromised - were those where the problem was completely neglected or ignored or were suffering from stress due to poor planting issues.

IOW, prune away the affected tissue and I think your tree will be fine :-)

    Bookmark   February 26, 2011 at 10:27AM
Thank you for reporting this comment. Undo
peapod13(8 South Sound WA)

Sorry about that first picture, the second one is much bigger and shows the problem better. However, if the treatment is to trim the effected branch back a couple of bud sets, I'll trim them, and be very happy it isn't worse.

Thanks again for your help.


    Bookmark   February 26, 2011 at 2:27PM
Sign Up to comment
More Discussions
new container gardener
I have decided to plant in containers this year, I...
mary grether
Culinary Herbs?
I need to buy some culinary herbs. Where is the best...
anything you wanted to talk about vii - prolly mostly ot
I guess I didn't realize the last thread was about...
tapla (mid-MI z5b-6a)
Summer Squash
Last spring/summer, I planted a summer squash that...
fertilizing cuttings ?
Hello - I have some fig cuttings in 60% perlite/40%...
Sponsored Products
Babyletto | Mercer 3-In-1 Convertible Crib
Windham Hotel-Style Thermostatic Shower With Hand Shower - 1/2" IPS
Signature Hardware
Furniture of America Wall-mount Mirror with Jewelry Holder
MaxLite MLFP22DS4035 Direct Lit LED Flat Panel, 3500K
PERFECTA Custom Single Wall Mounted Bathroom Vanity CaesarStone(TM) Countertop -
Modern Bathroom
Key Tall Module - Walnut - Housefish
$594.00 | HORNE
People viewed this after searching for:
© 2015 Houzz Inc. Houzz® The new way to design your home™