The Pakistan Mulberry, botanically known as Morus macroura or sometimes sold as Morus alba ‘Pakistan’, is desired by gardeners for its long, sweet fruit and rapid growth habit. Its berries can reach lengths of up to six inches, making them a unique addition to the home orchard. Since this variety originated in subtropical and warm temperate regions of Asia, growing it in cooler climates requires specific winter preparation. The primary challenge is ensuring the tree’s delicate wood and buds survive the cold.
Understanding the Cold Hardiness Limits
The Pakistan Mulberry is rated for USDA hardiness zones 7 through 10, meaning its long-term survival is expected in areas where winter temperatures do not drop below 0°F. This overall hardiness rating primarily describes the resilience of the established root system. The tree’s above-ground structures, including the trunk’s cambium layer and the fruit buds, are considerably more sensitive to cold exposure.
These delicate parts can suffer damage when temperatures dip below 28°F for extended periods, making the tree frost sensitive. A particular vulnerability is its tendency to break dormancy quickly during mid-winter warm spells, known as precocious bud swell. Once the buds swell and sap starts flowing, a subsequent drop in temperature, even into the low 20s Fahrenheit, can kill the newly active tissues. This causes severe dieback or even the death of the entire tree above the graft. Younger trees are especially susceptible and require protection during late spring frosts.
Seasonal Preparation for Dormancy
Successful winter survival begins months before the first frost, focusing on cultural practices that encourage the tree to “harden off” and enter dormancy naturally. Stop applying high-nitrogen fertilizers by mid-summer, typically around July. Continuing to apply nitrogen promotes tender new growth that is poorly lignified and highly vulnerable to damage from early cold snaps.
Adequate soil drainage is a prerequisite for winter hardiness, as waterlogged soil combined with freezing temperatures can lead to fatal root rot. Selecting a planting location that offers a sheltered microclimate, such as a spot near a south-facing wall or a large boulder, can provide a slight temperature buffer. As autumn progresses, gradually reducing the frequency of irrigation encourages the tree to shed its leaves and transition fully into its dormant state.
Essential Physical Winter Protection Methods
For Pakistan Mulberries grown in marginal zones, physical protection is necessary to insulate the wood from extreme cold and temperature fluctuations. Protect the root zone by applying a thick layer of organic mulch around the base of the tree after the ground has frozen. A layer of straw or wood chips, six to twelve inches deep, helps stabilize the soil temperature and prevents the heaving that can damage shallow roots. Leave a small, donut-shaped area of bare ground immediately surrounding the trunk to prevent moisture buildup that can lead to collar rot.
Protecting the trunk and main scaffold branches from windburn and direct freezing preserves the cambium layer. Wrap the trunk with specialized tree wrap, burlap, or insulated blankets for the duration of the coldest months. Wrapping helps to moderate the temperature of the wood, reducing the risk of sun scald on warm winter days and mitigating the damage caused by rapid temperature drops at night. For young trees, creating a temporary structure, such as a wire cage wrapped in frost cloth, offers a protective barrier.
For trees grown in containers, which offer no natural insulation for the root ball, the most reliable method is to move them into a protected environment for the winter. An unheated garage, shed, or cool basement that remains consistently above 20°F provides adequate protection. If the container tree must remain outside, the pot should be insulated by surrounding it with straw bales or burying it in the ground. The canopy should still be covered with a breathable frost cloth during any anticipated deep freezes.