Urban Gardener: It’s cool to garden this season

The recent cool weather has reminded me that winter is my favorite time of year for gardening. The cool temperatures make it a lot more enjoyable to be working outside. Many of our veggies like it better as well. While some parts of the mainland have had their first snows, this is our time to get excited about gardening.

With our relatively warm temperatures, tropical vegetables such as taro, or perennials such as moringa (marunggay) do very well year-round. Some vegetables — such as corn, beans and squash — prefer our summers, with their warm temperatures and long days. But many vegetables more attuned to Asian and Western cultures are adapted to temperate areas and prefer cooler weather. Vegetables that will grow better during our winter include cabbage, kale, broccoli, lettuce, celery, carrots, cilantro, beets, spinach, bulb onions, tomatoes, potatoes, chili peppers and peas.

Reasons for winter growing preferences vary. In most cases, the intense heat and solar radiation of summer can be stressful for vegetables with tender foliage. While collards are a little more tolerant of the heat, kale may wilt and growth may be stunted in the summer, as the plant struggles to cool itself. Kale may develop bitter flavors as it manufactures compounds to defend against the heat and sun.

We can grow many varieties of lettuce year-round here, especially Mignonette types such as Manoa lettuce. However, when stressed by summer heat, even these tend to develop bitter flavors. Due to their inability to keep up with water demands, they may experience tip burn, or dying leaf edges. The stress may cause them to bolt early, or send up a stalk of flowers in an attempt to reproduce at the expense of growing tasty leaves. Cilantro is another vegetable prone to bolting in warm temperatures.

Sometimes flowers are affected. Tomatoes and chili peppers may grow nice foliage through summer, but warm temperatures, especially nighttime temperatures above 75 degrees, can reduce flower production and fertility, and lead to blossom drop or early fruit drop.

When the plants are stressed by heat and struggling to drink enough water to cool themselves, large fruits may develop unevenly and appear misshapen or have other disorders. Small-fruited varieties, such as Hawaiian chili peppers or cherry tomatoes, may be easier to grow than larger fruits. The seed program at the University of Hawaii at Manoa sells a grape tomato called Komohana that grows well here, along with the Hawaiian chili pepper.

Broccoli may grow through much of the year, but high temperatures when buds are forming can lead to uneven and poorly-shaped heads, as well as yellowing, which is unattractive. Cauliflower prefers even cooler temperatures, and won’t even form heads if it’s too warm. It might be too late to sow seeds, but you might try transplanting in hopes of a harvest. UH sells a variety called Puakea.

Green onions grow well year round here. Bulb onions, on the other hand, will make the largest bulbs if seeds are planted between September and January. The short days encourage development of larger base plants that will form larger bulbs when days get longer in the spring. UH sells an excellent green onion called Koba, and a bulb onion called Awahia.

There are always exceptions to the rules. In higher elevations such as Kula, Maui many cold-loving crops can be grown all year. Even on Oahu, cool-season crops may thrive in Wahiawa, or may have fewer problems on higher ridges or in valleys that get cool downdrafts.

Many cool-season crops have heat-tolerant varieties. Head cabbage, for example, grows well all year at all elevations, though typically the flat-head varieties do better during the hot summers, while round-head varieties may struggle to form nice tight heads. Check with the UH Seed Program for varieties adapted to our climate, and for information onlocal pests and diseases. When looking through seed catalogs or websites, try to find varieties that are heat-tolerant, as well as resistant to diseases such as powdery mildew or viruses.

Be inspired to get out and garden this winter! Remember, if you have questions or problems with pests and diseases, help is available. Contact a local master gardener through our website, ctahr.hawaii.edu/UHMG.

Kalani Matsumura is a junior extension agent with the University of Hawaii’s College of Tropical Agriculture and Human Resources and coordinates the UH Master Gardener Program on Oahu.