[asteroidz]

Personally, in my locale March is usually when the crocuses start peeking out and the robins return... first signs of spring. But just 100 miles east the landscape might still be a winter wonderland well into April.

If I had to hazard a guess, this article was probably written with a British audience in mind, where "spring" loosely corresponds with March through May.

[7952]

For planting people develop more nuance than just a binary season change. They look at the condition of soil, temperature, likely weather looking forward etc. Right now it is spring where I live. The soil is a bit wet and clumpy. We may still get a frost but probably won't.

And in the natural world things can be really spread out. Plants have different niches based on being early or late. Shoots will come up in the winter. And some plants will be dormant until much later.