为什么此FAB无法更新状态？

I have followed as best I can this question: Adding new ListTile item on click

尽管它不一定具有可复制性，但我创建了可以编译但不能按预期运行的文件：

import 'package:flutter/material.dart';

void main() => runApp(SeedBank());

class SeedBank extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      theme: ThemeData(
        primaryColor: Colors.lightGreen,
      ),
      home: Seeds(),
    );
  }
}

class Seeds extends StatefulWidget {
  @override
  SeedsState createState() => SeedsState();
}

class SeedsState extends State<Seeds> {
  var _seedSection = List<Widget>();

  Card seedSectionMethod(String title, String subtitle) {
    return new Card(
      child: ListTile(
          title: Text(
            title,
            style: TextStyle(fontWeight: FontWeight.bold),
          ),
          subtitle: Text(subtitle),
          trailing: Icon(Icons.more_vert)),
    );
  }

  void initState() {
    super.initState();
    _seedSection.add(
      seedSectionMethod("Leek", "Bulgaarse Reuzen - Lincoln"),
    );
    _seedSection.add(
      seedSectionMethod("Kale", "Jardin mix"),
    );
    _seedSection.add(
      seedSectionMethod("Sunflower", "Helianthus Red"),
    );
  }

  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Seed Bank'),
      ),
      body: ListView(
        children: this._seedSection,
      ),
      floatingActionButton: FloatingActionButton(
        child: Text("+"),
        onPressed: () {
          setState(() {
            _seedSection.add(
              seedSectionMethod("New Seed", "With notes"),
            );
          });
        },
        backgroundColor: Colors.lightGreenAccent,
      ),
    );
  }
}

所需的行为是：

• on app startup, that the three 'cards' with the seeds are populated.
• on button press, a new 'card' is added with the values "new Seed" and "with notes".

我希望我错过了一些有关设置状态的基本知识（仅从昨天开始），但是我对下一步该去哪里了解更多。