Interesting math. Unnecessarily complicated and confusing, even for
non-newbies? Maybe just start with what DTM is, or what your definition
of days to maturity is. There are all sorts of variables just for germination time.

"Unnecessarily complicated and confusing" is right.

Rodney

what you call unnecessary and complicate is what makes the calculations accucate. Also the reason for starting this thread is I am sick and tired of posters both newbies and those still leaning whining that there plants are not producing harvest when they say it should compared to DTM. Also most poster don't allow enough time expect results to soon.

I'm not sure how you can trust DTM without figuring in at least the temperature. DTM is one of those odd numbers listed on seed packages that don't include any environmental caveats. I plant peas in December, and I don't expect to get a harvest until March. The DTM on those peas is 60 days. The variation of DTM with temperature completely overwhelms the other numbers you add to it in your math, so unless you have some climate variable in your math it's really pretty useless.

I haven't seen anybody whining about their stuff not producing in an allotted amount of time.

Agree with Dave (dave_f1) that the germination times can vary widely due to temps.

Also agree with Dan and Dave (digdirt) that it would have been easier to say that the DTM are a very general guideline that is calculated under ideal conditions. And that it takes longer for things to mature in the early spring and late fall due to lower temps.

Another thing that you haven't taken into consideration is the amount of sunlight. Shorter days in early spring and late fall makes things take longer. Not to mention stuff (trees/other plants/buildings/whatever) causing shade. There are just too many different things that come into play when trying to calculate the total amount of time it takes for a crop to sprout, grow, and mature.

Rodney

I'm not even sure what a "none fruit crop" is. Is this supposed to be "non"?

OP themselves states "extreme heat or cool spells may throw off the the numbers by several days to weeks". If the figures can be out by weeks there doesn't seem much point in being so specific and double checking the maths. A newbie trying to use these formulae could go into a tail spin when their plants refuse to follow the rules.

Bit puzzled by 'transplanted post-frost transplanted fruit crops' too.

Well, my winter peas don't often undergo "extreme cool spells". Just pretty normal winter. So there is nothing "extreme" about the temperatures my peas see. I guess if everyone is going to do the traditional "plant peas by St. Patricks Day" then the published DTM might have some meaning to some. But following that tradition would result in no crop down here.

For me really the only useful thing with DTM numbers is comparing varieties within a particular crop. And then even with that, the numbers would have to be published by the same source. Maybe I'd like to grow a variety that's a week earlier or later than the one I have grown already. The numbers are all relative.

I think that's right, about using DTMs to compare varieties. The problem with mathematical exercises like this is that you're going to get newbies excited about starting their harvest for a particular crop on, say, July 26. But if you're just wanting "sooner" rather than "later" such DTMs can be helpful.

what do when you don't have DTM to compare. if you lived in Uk you be out luck since they do not use that terminology even by same variety and same seed company selling seeds in USA

daninthedirt - I'm not sure if my meaning was clear. I was agreeing with you. Grid put in a caveat saying temperature could throw the calculations way out. My point was that if that's the case the calculations aren't very useful.

I have read Grid's last post a few times and I'm not clear what it means. Grid uses DTM in his/her formulae but then says they are no use in the UK?

floral you incorrect i did not say DTM was no use in the UK. I said DTM is not used in uk in general.

For example:

Thompson & Morgan a well known seed company in UK uses DTM on there USA seed packets but does not use it on there packets in UK.

Growing Degree Days is a much better system for determining DTM. But most seeds packet will list only expected DTM. GDD account for temps which will speed up or slow down DTM

I have been trying to find a chart for veggies with GDD or sometimes called GDU (growing degree units) The best I could find says tomatoes will taken around 1330 GDD.to ripen.

jnjfarm_gw thats true; but Growing Degree Days has flaw as well in it's use which you can not use it as plan a head method.

While it is true you can't plan ahead using GDD since you don't know ahead of time what the weather will be like. The same applies to DTM.

I have been gardening in my own garden since 1959 and well before that. Plants mature + or - a few days from year to year, yet very little overall.

If I plant Ambrosia sweet corn on April 15, it will mature about July 3. ....about 79 days for a 76 DTM corn. If I plant a second planting on May 2nd, it will mature about July 10...69 days. If I plant another planting on June 5, it will mature about August 9....65 days...etc

I think it's those "ifs" that we're pointing at. The OPs math is more correct if you replace the term "DTM" with "DTM (to the extent you believe it)".

GDD is a nice concept, and if you plant at a particular date at a particular venue, I guess that GDD translates to a particular DTM.

I harvest when stuff is, well, ripe. The time that takes has too many variables to really predict with a reasonable degree of accuracy, especially in an arid climate. I would guess that the OP's system would be useful by larger scale growers in that growing area who would need to plan ahead for a harvest to schedule logistics and shift resources. DTM, when taken with a very large grain of salt and supplemented by knowledge of the unique characteristics of the growing season in my area, is more useful for determining planting date than harvest.
As far as newbies and harvesting, it seems more helpful to describe the physical characteristics of a crop that is ready for harvest than to give them a day on the calendar to circle. Also, there seem to be more posts on the forum regarding when to plant than when to harvest.

Commercial growers will use GDD to estimate which week they will harvest a particular variety. GDD from transplanting or from a particular growth stage of the crop (e.g., from first flowering) It's not true that you can't use GDD to plan ahead. Of course you don;t know the exact weather that will occur. But a GDD model only uses AVERAGE daily temp, along with a base (or minimum) temp, which can be different for different crops. It can be further modified with a max temp and other variables. No model will tell you the exact day of harvest 2 or 3 months out! You can keep an eye on the accumulated GDD and tweak your harvest a few weeks out and forecast using that and predicted temps. It's amazing to me that a gardener will want to be told when they will harvest their tomatoes soon after they put out transplants or sow seeds. If you really need to know...take careful notes on a calendar the first year you grow a certain variety. After the season, you can look back at min/max temperature records and see how many GDD occurred in your area between your dates for seeding/transplanting and your first harvest. Then the following year keep track of the accumulated GDD and then you can zero in on a harvest date. I'm not sure why a backyard grower who is not selling produce would really need to do this.

DTM really shouldn;t be printed on seed packets since they cause more confusion then anything else. There isn't even a standard within the seed industry as to how DTM is calculated. Also when do you consider a crop mature? For fruiting veggies, mature and ripe can be different dates. Commercial growers often focus on maturity, certainly when shipping is involved. As gardeners we're more likely to harvest a tomato when fully ripe.

Who's this grid ? A translating machine or what, he talks real weird ! However I got it, gardening is hard work and no wages, on top of that now, no whining allowed ;-)

For pros there are several sites offering a chart that will give the GDD of the day every evening, for groups of plants. For instance today here was 1.5 for white clover and 0 for bermuda grass, since clover grows at lower temps than BG. Yes I know I'm obsessed with BG ;-) I check every evening and that's the good side of winter, every day is GDD 0 for BG !

Wayne your posting on sweet corn is a perfect example of how temps affect growth. GDD are mainly used for corn production and used in insect predictions. as a market gardener, I plant my crops and hope to have a supply ready for market all season. In a given area if my tomatoes are growing fast or slow everyone's are doing the same. that why I have a hoophouse.

knowiing when sweet corn is done is easy you count 21 days from when silk appears.

Actually, I think one of the first lessons you should learn as a gardener is when a crop is harvestable. That lesson certainly doesn't come from counting days. For non-fruiting crops, harvestability can't entirely be determined by when you want to eat the crop, but also by the maturity of the plant, and how much more I'll want from it. If I want to eat my lettuce when it's a few inches high, I won't get much from it, and I won't have lettuce anymore! That's a matter of judgement.

Was this really a question that has appeared again and again? Don't think so. Also, I'm not sure what the lecture on patience refers to. That's a good point, but I saw no evidence of any impatience with new gardeners here. .

I think usage of GDD as opposed to the "traditional dates" in all facets of gardening is starting to catch on. I am wondering though, if GDD could be used not only to predict harvest windows, but maybe also to pinpoint the earliest day you could set out transplants/direct sow seed? I am also wondering what it would take to have a program mine the max/min temp data for you from your closest weather center and update some form of spreadsheet....Probably a ton of code writing.

That's a good point. It wouldn't take a lot of cleverness (though it would take a lot of effort) to make a data product that would take as an input your location, a planting date, and a crop identifier, and output a nominal harvest date. It's not a simple equation, but all the data is out there. I'll bet commercial producers have that data product in some form or another already.

hokie, GDD would normally only be used as a measure of growth and development of a plant or insect, So with plants you could use them to determine when a seedling will likely reach transplant size or to flower, or to ripen fruit. The best time to set out transplants (assuming they have reached that stage) is more determined by soil temps and air temp forecasts. And maybe some other things.

Its easy to determine your GDDs from just high and low daily temps. And knowing your base temp, which I think is 50F for tomatoes for example. A day with a high of 90F and a low of 70F would give you a mean temp of 80F and 30GDD (80-50) for that particular day. Just keep adding those GDDs up from your day of transplanting or direct seeding. There are more complicated models out there based on hourly temps but there's really no need for our purposes.

There are several free websites that will automatically calculate GDD accumulations for you. Cooling degree days are really the same as GDDs and these are used for building energy use calculations....cooling costs. Just select your nearest weather station (enter zip code), calendar dates you want and the base temp and it'll spit out a table including each day's total and the cumulative total for your date range.

Thanks Dave, did not know there was something I could tinker with out there already.

There are a lot of such tools out there. See this for example. Of course then you have to come up with your own GDD-to--harvest, which will be different for different crops. You can easily go online and find those numbers for various stages of corn, and I think I saw them for tomatoes.

P.S. In that last link I gave, GDD=GDU. Same thing, different terminology.