Chill hours, how much do they matter?

For the home fruit grower how much do chill hours really matter? I have read stone fruit planting advice from regional universities that commercial orchards should plant 15% of their trees needing 50-100 hours greater than location average chill, and 15% of their trees needing 50-100 hours below their average chill. Giving up to a 200 chill hour spread on commercial plantings. My question is what sort of spread do you think is appropriate for a home grower who cares more about diversity and spreading out harvest dates than actual maximum yield?

I live in western Louisiana on the 8b/9a line and we have highly variable annual chill hours, from under 400 (last year) to over 1,300 in the last decade, using the under 45 to over 32 model with an average of around 600 hours.

Using Get chill our numbers to date (Jan 24th) this year are
Below 45 Model: 622 chill hours
Between 45 and 32 Model: 457 chill hours
Utah Model: 118 chill units
Positive Utah Model: 674 chill units
Dynamic Model: 31 chill portions

As of now I am mostly trying to grow peaches that range from 550 to 750, as well as plums and pluots that range from 350 - 750, though some of those are newer releases with broad estimated chill requirements (ie 300-500 chill hours)

I guess what I am asking is does this chill hour spread seem good, should I try to narrow it down, expand it, is it better to have too few or too many chill hours compared to my average, If I were to expand my range should I go low or high, etc…

It depends greatly on species, cultivar, and location. @fruitnut has written extensively on the seriousness of providing high-chill hours to peaches listed as such. @applenut broke the record books by growing what were thought to be high-chill apples in a low chill area (zone 9a, Riverside CA). Many people will vouch that high-chill sweet cherries will not produce much in a “low chill” (zones 9 and up) areas. I’m in zone 10b and have success with some Prunus that are rated in the 200-350 range.

Chill hours are an estimate. The metric was originally developed in zone 7 and below (places with 10-year average winter lows in the teens … 10F to 20F). The actual chill hour label comes from testing a cultivar for 5 years and then deciding if it works there.

The extension to zones 8, 9, and 10 has been problematic. In fact there are at least 3 methods for measuring chill in those zones. As @alan recently pointed out, cloud cover in these zones can make a huge difference. It would seem that “more cover” → “more chill”. So I think there’s more to estimating fruiting vigor than temperature.

This past year I started measuring chill hours at my site. Two standard methods have come up with 41 hours so far, while the Utah method has come up with -391.5 (negative). And yet, I’m certain that bumper crops of many fruits are coming.

@Isaac-1: your profile states you are in zone 8. I recommend that you view “chill hours” with caution – i.e., a bit of folklore that sometimes is correct for some species cultivars.


My question would be how often do you get spring freeze injury and how severe is it when you do? That would tell me whether I need higher chill material to hopefully lower the damage. Many of us are used to damage most yrs and even nearly every yr.

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A great question.

Typically we only have hard freezes through January or February, though many years we will get a mild late freeze some time in April after 3-5 weeks of warmer weather after many native plants start blooming. Depending on the type of fruit this may or may not significantly effect budding. In about half the years we do not have a freeze before New Years, though in some years our first freeze happens in late October.

That doesn’t sound like a serious spring freeze risk. But it’s hard to say if you don’t have experience with known varieties. Some experience will be a good teacher. But I’d think you could learn a lot by asking around. There must be peaches nearby. Ask them when peaches bloom and how much damage they get.

There are in fact I am in the process of replacing a number of 25-30 year old no longer productive peach trees with younger replacements. Unfortunately I do not know the names of the varieties of the older / non productive trees and few people in the area grow stone fruit. In fact fruit in general is not that widely grown in the area, instead the vast majority of people that grow their own food have vegetable gardens or grow more traditional regional fruits that are less prone to late season mild freeze damage (Blueberries, figs, mayhaw, pears, persimmon etc.)

The usual reason is because they’ve exhausted potash resources in their root zone and nobody has bothered to properly feed them.

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Hardiness zone maps have nothing to do with chill hours. Miami, Los Angeles and San Francisco are in zone 10b. Are the growing climates similar? In zone 9a where I live 25 miles south of SF, 704 chill hours have been accumulated so far. In parts of San Diego, also in zone 9b, 13 hours of chill hours have been accumulated so far this season.I have not been able to find a chill hour map online can someone locate one?


I am somewhat guessing on the age of the old peach trees, though I strongly suspect they date to the mid to late 1980’s. As to the exact cause of decline I can only speculate, though I have read that peach trees typically only live 20-25 year or so. Also these old trees have generally been suffering from severe splitting of the trunks and internal rot.

RichSV, I am not sure I understand the point of your reference to chill hours and hardiness zones here. As to a chill hour map of current chill hours try

In reality they live and produce in their native environment for hundreds of years, and a few cases over a thousand. But if you take them out of that nutrient stream and don’t compensate there will be problems.

Agreed. This is my main complaint about comparisons between “southern states” and the “southwest”, incl. Mexico.

Well, we’d need to have at least 3 maps to account for different chill hour metrics. Further, the map resolution for “32F < chill hour < 45F” and for “Utah method” would have to be 100 meters (10k m^2) for my region.

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Now would be the time to take Scions from any remaining new growth. Give you a little chance to clone proven winners.

That may be worth doing, though I suspect most of these older trees were higher chill hour varieties given what was available when they were planted. For my new peaches I have picked all LSU peach releases, which were developed at the research station in Clinton Louisiana, that is about 120 miles due east of where I live as the crow flies, with nearly identical climate, about the same distance in from the coast, and has very similar native soil series according the state soil survey, etc.

I have found in our area you have more success with lower recommended chill hour fruits than higher chill hours. If you do not get enough chill hours, the tree will not produce. But if you get 300 more hours than needed, you can still get a crop if the fruit does not freeze. With plums I did have problems with cool wet weather preventing bees from pollinating blooms. I have only been planting fruit trees in the area for 5 or 6 years so it is a limited evaluation time.

The extension service did advise to add 200 hours for west coast varieties. They said we lose chill hours with our warm, wet winters.

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Gary, I know you are somewhere nearby in SE Texas, but I think you have told me you are somewhere closer to the coast than I am. (I am about 80 miles inland from the coast). If so you may have more temperature mitigation due to the waters of the gulf.

I was about 35 miles from the gulf but on far north side of town. On cold still days I was 5 degrees colder than on the south side of town, according to my temp gauge on my pickup. I have just moved and about the same miles from gulf as you are. Still had wild swings in chill hours, according to getchill.

The Gulf series plums do flower early, but did a good job the last two years. Not sure how they will do this year with all the extra cold. I really liked the Gulf rose flavor, had to grow gulf beauty for pollination. Really pushing it since the are rated at about 200-250, but worked last 2 years. I grew 2 tropic snow peach in containers and would bring them in under my attached carport iif needed. Work but was a fair amount of trouble. I guess the squirrels were happy, since they stole all but 2 or 3 of the 3 or 4 dozen they produced. The first year squirrels did not get them and they tasted very good.

Years of testimony suggest stone fruits are subject to the rules of chill, apples no. I remember reading decades old Nafex stuff of people growing apples with essentially no chill and having good results. I remember one guy who had a productive apple orchard who received very little chill in CA. He died and his trees succumbed not because of chill, but because no one was there to irrigate them in the desert summer conditions of his climate.

Stone fruits are different. You have to pay attention to chill hours. If I were in LA, I’d probably look at low chill cultivars. Florida has several, as well as some developed in LA.

In terms of the actual percentages of various low chill cultivars you should plant, I’m afraid probably no one is qualified to answer such specific questions on this forum (or even in practice). I’ve had different, but equally specific questions I’ve had to try to discover answers on my own.

Let’s take Mid-Pride Peach as an example. It has:
Estimated Chilling Requirement
250 hours below 45°F

I’ve had bumper crops 3 years straight and going on a 4th. I get about 50 chill hours by the “below 45°F” method and negative chill hours by the Utah method. The temperature hasn’t hit freezing here since 1992.


Back to one of these side topics, I went out to inspect one of the old dying peach trees today, which is located on the other side of the house about 250-300 feet from where I am planting my new stone fruit, and found one of the limbs that was bigger around than my arm was rotted so much that I was able to break it off easily with pressure from one hand. The interior of the limb of it was full of ants, and I assume the main trunk is the same way. Having said that there are about half a dozen 2-3 ft high shoots coming up apparently from the root stock all around the trunk.

I plan to cut away as much of the old tree and stump as possible, perhaps tomorrow weather permitting.

My question is what to do with the shoots that are coming up, were peach trees sold in the 1980’s typically grown on their own roots or on some form of edible fruiting root stock? These shoots are close to the main trunk so I will probably have to cut down some of them to get the dying tree out. If I leave them, how many should I leave?

One important thing that has been left out here, fruit quality can be greatly reduced with some plants if there is not enough chill hours. People in warm climates like Los Angeles have that problem with a lot of varieties of pomegranates.