Clover (Trifolium species) is a legume known for its ability to fix atmospheric nitrogen, provide high-quality forage, and serve as an effective cover crop. These functions enrich the soil and support livestock. The growth and long-term survival of clover are governed by its tolerance to temperature fluctuations, which dictate its geographic range and seasonal performance. Managing the plant’s response to both heat and cold extremes is necessary for maximizing its benefits in cultivation.
Impact of High Temperatures and Drought
Clover, a cool-season plant, experiences physiological stress when ambient temperatures exceed its optimal growing range of $65-80^\circ\text{F}$ ($18-27^\circ\text{C}$). Once temperatures rise above $85^\circ\text{F}$ ($29^\circ\text{C}$), the plant shifts energy away from growth and toward survival mechanisms. This heat stress is often compounded by drought, forcing the plant to increase transpiration to cool its leaves, which rapidly depletes available soil moisture.
The upper thermal range for many clover types is between $95-105^\circ\text{F}$ ($35-40^\circ\text{C}$), where foliage integrity is compromised. Exposure to these temperatures causes wilting and visible sunburn on leaves, indicating internal cellular damage. This damage, known as photo-oxidative stress, occurs when excessive light and heat overwhelm the plant’s protective mechanisms. Prolonged exposure forces the plant into summer dormancy, halting above-ground growth to conserve resources and protect the crown.
If high temperatures and drought persist, cellular damage can lead to plant death as root energy reserves are exhausted. The plant’s ability to recover from summer dormancy depends on the duration and severity of the stress period, and the availability of moisture when cooler conditions return. White clover, with its prostrate growth and stolons, is better equipped to survive summer dormancy than the more upright Red Clover, as its growing points are closer to the cooler soil surface.
Mechanisms of Cold Hardiness and Winter Survival
Clover prepares for winter through cold acclimation, triggered by decreasing temperatures and shorter daylight hours in the autumn. This preparatory phase develops tolerance to survive sub-freezing temperatures. During acclimation, the plant accumulates non-structural carbohydrates, such as sucrose, which act as cryoprotectants within the cells. These sugars lower the freezing point of the cellular sap, functioning as a natural anti-freeze to protect tissues.
The accumulation of carbohydrates in the roots and crowns is linked to the plant’s final level of freezing tolerance, a process that can continue even under snow. A primary cause of winter mortality is freeze damage, which involves the formation of ice crystals inside the plant cells, leading to a rupture of the cell membranes. This is distinct from frost damage, which typically affects only superficial leaf tissue and allows the plant to recover.
Successful overwintering involves changes in cell membranes, specifically an increase in the unsaturation of membrane lipids, which maintains fluidity at low temperatures. Red Clover varieties exhibit high cold hardiness, overwintering in regions down to USDA hardiness Zone 4. White Clover can survive short cold snaps down to $15^\circ\text{F}$ ($-10^\circ\text{C}$). For perennial clover to enter and maintain dormancy, winter temperatures ideally need to drop below $32^\circ\text{F}$ ($0^\circ\text{C}$), preventing premature growth flushes susceptible to subsequent hard freezes.
Comparing Thermal Tolerance Across Clover Types
The three most common cultivated species—White, Red, and Crimson Clover—exhibit distinct differences in their tolerance to thermal extremes, influencing their suitability for various climates. White Clover (Trifolium repens) displays superior heat tolerance compared to Red Clover, due to its prostrate growth habit. Its growing points are protected near the soil surface, aiding survival through summer dormancy. White Clover is moderately cold-tolerant, hardy in Zone 4 to 8, but less resistant to deep, sustained freezes than some Red Clover varieties.
Red Clover (Trifolium pratense) is a hardier winter survivor, with some varieties tolerating deep freezes into Zone 3 and 4. This cold tolerance is coupled with a reduced ability to withstand intense summer heat and drought due to its upright growth and deeper taproot. Crimson Clover (Trifolium incarnatum), typically grown as a winter annual, demonstrates a greater tolerance for heat and drought than Red Clover. While it germinates best in warmer soil temperatures of $65-75^\circ\text{F}$ ($18-24^\circ\text{C}$), it is the least cold-tolerant of the three, making it unsuitable for areas with harsh, prolonged winters.
Cultivation Practices for Temperature Mitigation
Mitigating Heat Stress
Proactive management can protect clover stands against the damaging effects of temperature extremes. During heat waves where temperatures exceed $80^\circ\text{F}$ ($27^\circ\text{C}$), ensuring adequate soil moisture is essential, as evaporative cooling from the leaves is the plant’s primary defense against overheating.
The use of a misting system can lower ground temperatures and reduce the plant’s reliance on transpiration, conserving water. For small plots, temporary shade cloth can be deployed to limit sun exposure, reducing the risk of photo-oxidative stress. Adjusting mowing height is a practical method for heat mitigation, as leaving the canopy taller provides self-shading for the lower leaves and the vulnerable crown.
Preparing for Cold
In preparation for winter, applying a layer of organic mulch, such as straw or hay, provides insulation to the soil and plant crowns. This layer helps regulate soil temperature, reducing the impact of rapid temperature drops and protecting growing points from soil heaving caused by repeated freeze-thaw cycles. Protecting first-year seedlings with mulch or horticultural fabric is important, as they lack the established root reserves of mature stands.