Choosing a walnut variety is about more than yield and nut quality. Local winter conditions can have a major impact on how well a walnut tree grows, produces, and survives.

A study conducted at the Ataturk Central Horticultural Research Institute in Yalova, Turkiye compared ten local and international walnut cultivars to understand two important characteristics: how much winter chilling they require and how well they tolerate freezing temperatures. The cultivars included Chandler, Howard, Hartley, Franquette, Pedro, Sebin, Bilecik, Maras-18, Yalova-1, and Yalova-4.

Not Every Walnut Needs the Same Amount of Winter Chill

Walnut trees need a certain amount of cool winter weather before they can properly break dormancy and begin growing in spring. The study found considerable differences among cultivars:

Walnut Cultivar	Chilling Requirement
Yalova-1 & Yalova-4	879 hours
Howard, Hartley, Maras-18, Bilecik & Sebin	1,098 hours
Chandler, Pedro & Franquette	1,241 hours

These values represent hours below 7.2 C (45 F). This matters because a variety planted in a climate that cannot provide enough winter chill may experience irregular budbreak, reduced productivity, and other growth problems.

Cold Hardiness Changes Throughout the Winter

One of the most interesting findings was that a walnut tree’s tolerance to cold is not constant throughout the winter.

The cultivars were most vulnerable near the beginning of dormancy in December. As winter progressed and temperatures decreased, their cold tolerance increased. The highest level of cold hardiness was observed in February.

In other words, the same freezing temperature may not have the same effect on a walnut tree in early winter as it does in the middle of winter.

The study also found meaningful differences between cultivars. For example, Yalova-1 became considerably more sensitive than other cultivars when temperatures dropped below approximately -5 C (23 F), particularly with longer exposure.

What Does This Mean for Walnut Growers?

There is no single walnut variety that is automatically the best choice for every location. A successful orchard starts with matching the cultivar to the climate. Growers should consider:

• Winter chill accumulation
• Minimum winter temperatures
• Timing and severity of frost
• The chilling requirement of the cultivar
• Its tolerance to prolonged freezing temperatures

This is particularly important when introducing a successful variety into a new growing region. A cultivar that performs extremely well in one climate may behave differently somewhere else.

The Bottom Line

The research showed chilling requirements ranging from approximately 879 to 1,241 hours among the ten walnut cultivars studied. It also demonstrated that cold hardiness changes throughout winter and varies significantly among cultivars.

Don’t choose a walnut variety based on popularity alone. Choose it based on how well its biological requirements match your local climate.

Research basis: Erdal Orman, Ismail Tosun, Mehmet Emin Akcay, Veli Erdogan, and Yasar Akca. Published in Bahce, Vol. 46, Special Issue 2, 3rd National Walnut Symposium, 2017, pp. 313-324.