Hello all. Today in PA is quite chilly, and although this is an anomoly for August, it has started me thinking about the fall and winter. I believe I am going to overwinter my three little trees (unknown orange, key lime and Meyers lemon rooted cuttings) in my attic, which is insulated, finished, but not heated. It does not normally get below freezing. I have a small southern exposure window, and plan on using Christmas lights to keep my pots warm. I do have access to a small heater if there is an exceptionally cruel cold spell that could threaten the trees. The main living area of my house is just so dry that I think the trees would suffer. My 2 year old may be even a worse fate than the dry air, and I don't think I can keep her in the attic! I understand that they will likely not see any growth using this method.

That is actually just a set-up to my question, however any opinions on the info above would be welcome. My question has to do with fruit ripening. I know that I have read about required "heat units" for ripening of different citrus, however I tried to do a search and was unable to obtain the information that I remember browsing previously. I do not have any fruit yet, so this does not concern this year's winter, but I am wondering about future winters. Is there a chart or other reference that is available that provides information on the needs of different citrus types to ripen? Also, a description of the "heat units" and how one could estimate there environment's potential in terms of "heat units" Please excuse my terminology, I am using these terms from a vague memory of browsing a thread. If memory serves, I believe Millet was the source of information...

If my little guys survive (hopefully thrive!) the winter, and I expand my tree collection or play with grafting (my husband may have me committed if he sees that!), I would like to choose wisely for my environment.

Thank you for any input you may have. Kind regards, Barbara

It is hard to even think of cool weather down here! But, I think your idea may work and you may even get some growth with the southern exposure. Just remember, exposure to direct sunlight requires that the roots are heated to at least 60, preferably 65 F.

I do not know anything about the heat units required for ripening, but I remember a discussion earlier where it seems that cool winter temps are part of what makes the fruit sweet, the summer heat helps develop the flavor.

As for actual temps, the key lime does not like to be exposed to temps below 40 F -- the others can go lower.

Boy, I had to laugh when I saw the subject line, it has been in the 90s here in TN all month (at least, probably longer)...
Somewhere there is a chart telling how many cold hours are needed for various citrus to set ?? buds, I think. I remember seeing it and thinking uh, oh, as I had intended on keeping our greenhouse quite warm this winter!
I know I set it as a bookmark, but can't find it now.
Maybe misting would help with humidity?

GENERALLY for a citrus trees to differentiate buds from vegetation into flower buds, the tree should receive between 650 to 850 hours at temperatures below 68F, depending on the previous years crop load. According to research conducted by Purdue University, hand misting of indoor trees provides practically no benefit at all to the tree. A humidifier, of course, works well. - Millet

Hi All!

Susan, I did feel a little bit like Chicken Little when I wrote the subject line! I almost repeated the phrase twice, but I thought everyone may think I was a little off. Sometimes you wonder how personality flows through your written humor.

I did Google "heat units" and "citrus" last night and did come up with some Australian sites which discuss this subject and they do provide a calculation for "heat units" based on temperature. I do not have the link, because we lost electric in the storm last night and I have not "re-Googled" (I love new verbs!) but if memory serves, it was something like the average temp in C minus 13. I don't know if I can write it in a formula, but I'll try:

HU (heat units) = (Tmax + Tmin)/2 - 13, where T is in degC.

Apparently a negative result is meaningless in this calculation. I just was not able to determine in my short time online if this was a daily calculation and the resultant number is a cummulative sum beginning from bloom to maturity, or some other handling of the information. And I did not find various HU requirements for various citrus types. As I write this, I also wonder if this is a reasonable approach for a containerized citrus owner from PA who does not document all of this daily information for her citrus' environment! I guess the engineer in me wants to apply too much logic on these little guys. In general, though, I would like to pick future citrus types appropriately, so knowing the general HU requirements as these relate to different species would be helpful.

I thank you for your interest and responses. BTW, it is cold this morning in PA also! Kind regards, Barbara

The actual term is Heat Index. One of the most common methods of calculating the heat index for a particular area is by the summation of remainders. The summation of remainders creates an index of the total summation of all the mean daily temperatures throughout the fruiting season that are either above or below the vital temperature for citrus of 55F. For example, if on the first day of the fruit growing season the average mean daily temperature was 56.6F, than the heat index for day one would be 56.6 minus the required vital temperature of 55F = 1.6 unites. The mean daily temperature is kept for the entire season. Another example: on March 12 the mean daily temperature was 51F therefore, the heat index remainder for that day would be negative or -4 unites ( 51F minus 55F). By keeping a running total for every day throughout the fruiting season one comes up with the "Heat Index" for that area. Following are the heat index summations for the fruiting season (March through November) for some areas in California: Imperial Valley, CA = +6,078 degrees, Oakland, CA = +1,186 degrees, Riverside, CA = +3,203 degrees, Santa Ana, CA = +2,728 degrees. Santa Ana may be taken as superior to any of the four location for lemon production, having a heat index of 2,728. So you can see that citrus trees grow poorly in the region of Oakland due to the lack of sufficient available heat to promote normal fruit development. Sweet oranges, grapefruit trees planted in Oakland grow very slowly. On the other hand grapefruit would grow well in the area of the Imperial Valley of California. For further reading and study see The Citrus Industry Vol. 1 Chapter 11. - Millet

Very interesting! We had such a warm winter I started keeping track of the daily temperature on a calendar. Wish I would have kept it up!

Thanks for the info...

Thanks Millet. It appears to be the same formula, only in degF, so thanks for the explanation for how to handle the information. It was similar to what I was thinking, however knowledge on individual citrus' needs would definately be necessary to use the information. I believe I found the reference you indicated online, however only chapters 1-4. Chapter 4 did provide some hit-and-miss heat index requirements for some citrus types, so it is a place to start if I persue this method. I do not think that PA can get anywhere close to the requirements for grapefruit, and I now even question whether I can adequately support the needs of my key lime. I think my Meyers lemon will do okay, though. Possibly a Tahiti lime would be a better choice for obtaining fruit in my situation.

The information is very interesting, and makes me realize how very indepth you can get if you really want to understand what makes these trees thrive! Thanks again. Barbara

Barbara, PA should have no trouble accumulating heat unites for any variety of citrus, because container trees growing in the Pennsylvania area, would be outdoors all summer and then in a warm home or building all winter. - Millet

Millet, This winter will be my first close monitoring of the attic temperature, however I would be optimistic to expect above 55 degF consistently as a daily mean temperature. Future winters I may be able to change my arrangement and allow the trees downstairs, but I am afraid my 2 year old may not respect their needs as much as the rest of the family would. I may modify my thoughts on the use of the attic to introduce some consistent heat and humidity, even if it is in only the one room. I still have some time to brainstorm. So I will wait and see...

I am having fun learning about these trees! Barbara

Placing your tree in the attic that generally maintains a temperature of 55F, which is absolute zero for citrus, could possibly be ok, as long as the foliage is not setting directly in front of the direct rays of a southern sun. In such a situation, if the sun shown very long on the foliage, the sun's rays could easily heat the surface of the leaves to temperatures near or above 100F. In such a case the foliage would call for water from the root system to cool down, but the root system would not be able to help, and the tree would then drop all of its leaves in an attempt to get into balance. Between the root system and the foliage, everything is balance. - Millet

Millet, I will use Christmas lights around the pots at a minimum, and I hope to get even more creative and try to provide a better than "just adequate" environment if time and resources allow. More land and a greenhouse would be nice... but those are long term goals/dreams. Barbara

Recently, I was in your area of Pennsylvania visiting my niece who lives in Chadsford, PA on the Brandywine River. I will again be back in PA working the next Philadelphia Flower Show. - Millet

Millet, I am only about 20 minutes from Chads Ford! What a small world! I, sadly, have not been to the Phila. Flower show in 13 years (pre-children). What will you be doing at the Flower show? Do you visit Longwood Gardens when you visit your niece? I expect that may be a silly question!

I went to high school in West Chester, which is a right on Rt202 on your way to Chads Ford from the airport. I am currently about 15 miles past Chads Ford on Rt 1 and various back roads on your way into Amish country. I used to "tube" on the Brandywine river in the summer, which back then meant jumping in the river with a bunch of friends and truck intertubes and floating, swimming, jumping from hanging ropes all day long and getting picked up down river at the end of the day by somebody's parent or older brother/sister. Chads Ford was close to the picking-up point. As a parent in today's world, I don't know how my parents stomached it! Boy have things changed. I wonderful memory to revisit, though. Barbara