How do seed vendors determine in which months the buyer can start sowing and in which months the buyer can expect to start harvesting?

Question

When I buy a seed package at the local gardening store it usually has the following information on the back (this is an example):

1. Can sow from march-may
2. Flowers from may-june
3. Harvest starts in june-august
4. Days until maturity 85-90 days
5. Sun exposure full sun

I assume that for each different region, they make a different sow plan. The above may work for buyers living in the Netherlands, but I take it that the sow instructions have completely different months and perhaps even a different maturity day indication when the same seeds are sold in Australia for example.

My question is: On what data do the vendors base their instructions on the back of the package? Is it the USDA zone of a specific region? Or do they base it on the average monthly temperatures of a region?

Also, I don't understand how they come up with the 'days until maturity' estimate. Some months have a lot of lower temperature averages than others so it can't be the same number or the same range like 85-90 days for every month. For instance, I grew some field lettuce once, and on the package it said it would take 2 months or so to grow, but I decided to grow it from autumn through winter. The lettuce grew fine but it took an extra month I believe for it to reach maturity.

So I'm really curious how the vendors determine when a certain region can start growing a certain crop.

Answer 1

Plants have a natural annual cycle. For example, seed will only germinate when the soil temperature is right. Flowering and fruit production is often controlled by day length.

The "instructions on the packet" are a guide to allow the plant to do what it would do naturally. For example if you planted the seeds earlier, most likely the soil would be too cold for them to germinate, or the air temperature would be too cold for them to grow well if they did germinate. Since you paid good money for the seed, you don't want it sitting in the ground for months before it germinates, or more likely it provides food for birds or rodents and never germinates at all!

Similarly if you sow the seed later than the recommended time, the plant will probably grow but it won't reach maturity and flower before the weather conditions are unsuitable for it.

The instructions you quoted leave some room for local climate variations. For example in the UK you could probably sow the seed successfully in March if you lived in the south-west (which has the mildest winter weather conditions) but if you lived further north or at a higher altitude, you could expect overnight frosts even in early May so it might be more sensible not to sow the seed until mid-May, not in March.

If you usually buy seeds from the same supplier, you learn by experience how to interpret their recommendations for your particular conditions. You can't reduce gardening to a set of instructions like "do this on January 1, this on January 2", and so on through the whole year!

Answer 2

Trials. For example I noticed the Digger's Club conducted trials in my state on many vegetable varieties before commercialising them.