Alicia Silverstone is no garden-variety actress — she also has an impressive green thumb.

The ardent vegan recently Instagrammed a picture of her 8-year-old son, Bear, holding a homegrown zucchini considerably wider than his shoulders.

“It was so huge,” the “American Woman” star told The Post at the Christian Siriano show, which she attended along with Lucy Liu, Ashley Graham, Jennifer Coolidge and Sarah Michelle Gellar. “It was by accident, I swear. You don’t want them that big,” she added, insisting that she didn’t use fertilizer.

The colossal courgette popped into her field of vision one day. “I was standing in the kitchen washing dishes and I looked out the window and was like, ‘What happened?’ I was gone for a week and I came back and it had turned into an arm. I had two like that, actually.”

What does one do with a summer squash of unusual size? “It took a while to finish it,” said the cookbook author, who attributes her son’s good behavior to his vegan diet. “I did so many different things with it. We made these amazing muffins and a really amazing dish from my book, ‘The Kind Mama.’ I made it for my dad’s birthday. I just kept making things.”

After all, the time was ripe.