Nature signals to plants to begin the ripening of fruits and vegetables. These signals may vary with different types of plants. The major causes of ripening, or plant maturity, include: genetic characteristics, plant stresses, dry soil, hot weather, especially hot direct sunlight, and the length of day. Many onions, for example, begin the bulbing process after the days are 12 hours long. Other plants are signaled to mature as days grow shorter. Often, a combination of factors, prompts a plant to ripen its fruit.
For most plants, the best way to slow down the ripening process, is to provide ideal growing conditions. Most importantly, keep your soil moist, at all times. Providing shade cover over fruits like pumpkins and tomatoes, is very effective. This keeps the fruit green and its skin more pliable, slowing down the maturation process.
To prolong the harvest period for many vegetable plants, it is important to harvest frequently and often. Don’t allow fruits and vegetables to mature completely on the vine. This encourages the plant to produce more flowers, and ultimately more and bigger vegetables.
For leafy vegetables and many herbs, pinch back the plant or prune frequently.
There’s nothing worse than spending an entire season growing a pumpkin or a watermelon, only to find it still green as frost approaches. Very often, fruit can be ripened quicker, even after it is off the vine.
If it is still on the vine, cut back any leaves that block the sun from shining directly on the fruit. Also, stop providing water. Note, that this can have an adverse effect on producing drier, less juicy fruit.
If the fruit is off the vine, place it in a warm, sunny location. Given enough time and heat, and a little luck, it will ripen. A sunny deck is a great place to ripen fruit. This is akin to ripening green tomatoes in late winter. A green tomato is taken out of its cool, dark storage location, and placed in a sunny window. After a couple of weeks, it ripens.
Also see: How to Ripen Green Pumpkins