Clover is a cool-season plant. It does its best in mild spring and fall weather, not in summer heat or deep cold. Both extremes can be softened by how short you graze, when you irrigate, and how you feed the soil.1NZGA: Research and Practice Series. Will current rotational grazing management recommendations suit future intensive pastoral systems?2ASSA, CSSA and SSSA. Effect of Fertilization on Winterhardiness of Forages3PubMed Central. Growth and Physiological Responses of Temperate Pasture Species to Consecutive Heat and Drought Stresses Much of clover’s survival is also settled before the stress arrives, by how much energy the plant has stored up.
The Temperature Range Clover Grows In
Clover seed sprouts well, at least 80% germination, across roughly 41 to 83°F (4.9 to 28.2°C).4Agrosystems, Geosciences & Environment. Optimizing Temperature Requirements for Clover Seed Germination Some seed still sprouts outside that range, but far less reliably.5Agrosystems, Geosciences & Environment. Optimizing Temperature Requirements for Clover Seed Germination At the sweet spot of about 52 to 63°F (10.9 to 17.2°C), most seed that will sprout does so within about 1.5 to 2 days.6Agrosystems, Geosciences & Environment. Optimizing Temperature Requirements for Clover Seed Germination In temperate areas, early spring or late summer into fall seeding usually lines up with those temperatures.
Check the soil rather than the air. A cheap soil thermometer pushed 1 to 2 inches deep in the morning gives a reading the seed actually feels, since clover seed is small and sits near the surface. That top inch swings with the weather more than deeper soil does, and soil lags behind warm spells.
A brief warm snap in early spring can mislead you. Seed that sprouts during it may be followed by a cold return, and tiny seedlings with almost no root are the most exposed. Look at the extended forecast and notice whether nights are still near freezing, not just whether one afternoon felt mild.
Germination is only the first step. A seed can sprout at the warm end of the range and still produce a weak seedling if the soil then dries out or heats up. Plan around the following six to eight weeks of weather as well as the seeding day, because the seedling needs that time to grow a root system.
Cool weather also builds a tougher plant. White clover grown in cooler conditions forms runners about a third thicker.7Crop Science. Effects of Temperature and Photoperiod on Stolon Characteristics, Dry Matter Partitioning, and Nonstructural Carbohydrate Concentration of Two White Clover Ecotypes Cool weather slows new leaf production more than it slows photosynthesis, so the plant makes more sugar than it can spend on growth. The extra sugar is stored, with leaves holding about 63% more and runners and roots about 48% more.8Crop Science. Effects of Temperature and Photoperiod on Stolon Characteristics, Dry Matter Partitioning, and Nonstructural Carbohydrate Concentration of Two White Clover Ecotypes
Species and varieties differ. White, red, and other clovers vary in heat and cold tolerance, and breeders are developing red clover varieties for warmer regions. Your local seed dealer or extension office can tell you which varieties have proven themselves in your climate.
What Summer Heat Does to Clover
Under moderate heat and drought, clover closes the leaf pores that release water, which saves moisture.9PubMed Central. The Effect of Combined Drought and Temperature Stress on the Physiological Status of Calcareous Grassland Species as Potential Candidates for Urban Green Infrastructure Under high heat it opens them again even when the soil is dry, trying to cool itself.10PubMed Central. The Effect of Combined Drought and Temperature Stress on the Physiological Status of Calcareous Grassland Species as Potential Candidates for Urban Green Infrastructure In severe heat, then, clover loses water fast and runs out of the ability to protect its leaf machinery. Its proteins also begin to unfold in high heat, and the plant relies on protective “heat shock” proteins to repair them.11PubMed. Metabolic regulation of polyamines and γ-aminobutyric acid in relation to spermidine-induced heat tolerance in white clover
In the field, leaves fold or wilt in afternoon heat and may not recover overnight. Red clover may brown and thin, while white clover runners go dormant. Pale, yellowish growth can be one sign of weakened nitrogen fixation, though other causes are possible. Dead patches often appear first on thin, sandy, or south-facing ground.
Heat and drought often arrive together and worsen each other. Dry, crunchy soil with wilting points to water first. If irrigated clover still struggles, heat is likely a major factor.
A hot stand can look worse than the air temperature suggests. Bare soil and thin stands can run far hotter than the air above them, so roots and nodules may be under more heat than a weather report implies. A dense, leafy stand shades its own soil, which is why thin or recently grazed clover often shows damage first while a thick stand next to it looks fine. Dark, dry, sandy soils also heat faster than moist loams, so two fields in the same weather can behave very differently.
Heat reaches the roots too. Clover gets nitrogen from bacteria living in its root nodules, but soil heated to about 104°F (40°C) kills many of those bacteria.12Queensland Journal of Agricultural and Animal Sciences. Effect of High Soil Temperatures on Rhizobium spp.
Once the heat breaks, plants that wilted but have living crowns usually send up new leaves once cooler weather and moisture return, often within a couple of weeks. Crowns that stay brown and dry after rain are dead, and gaps there will fill with weeds or grass unless you reseed.
Livestock feel it as well. High temperatures lower forage quality and increase nutrient losses in legumes, and heat combined with drought cuts protein, fiber, and mineral content and makes clover less digestible.13Journal of Oasis Agriculture and Sustainable Development. Influence of drought and heat stresses in fodder legumes productivity and nutritive value Animals grazing heat-stressed clover may need supplement or a move to better pasture. If stock are grazing clover heavily but gaining poorly or milking less, weaker forage is a likely reason.
How to Protect Clover Through Heatwaves and Drought
The plant height left behind after grazing or mowing matters most in hot, dry spells. Leaving up to about 70 mm (2.75 inches), almost double the usual recommendation, may improve survival through heatwaves and droughts.14NZGA: Research and Practice Series. Will current rotational grazing management recommendations suit future intensive pastoral systems? Raise the mower deck and move stock sooner during hot spells. Taller cover shades the soil, which helps protect the root bacteria from the heat. Leaves left behind keep making sugar, so a taller stubble means the stand can keep feeding its roots during stress.
Watering pastures cools the plant canopy about 2 to 5°F (1 to 3°C) below air temperature,15PubMed Central. Growth and Physiological Responses of Temperate Pasture Species to Consecutive Heat and Drought Stresses so irrigation helps clover beyond just supplying water. Water early in the day before peak heat. That lets leaves dry before evening, which lowers the disease risk of foliage staying wet through a warm night. Water that soaks in deeply encourages roots to go down, while frequent shallow watering keeps them near the surface where heat reaches them. Apply it before the plant is badly wilted, since a collapsed stand may not recover from water alone.
When a heatwave is forecast, pull livestock off clover before the heat peaks. Avoid close grazing or mowing during and right after hot spells, and don’t fertilize with nitrogen or spray herbicides on stressed stands. Then let the stand recover with a longer rest once temperatures ease.
When grazing resumes, start at the taller height and lower it gradually only as the stand thickens again. If large bare patches remain, plan to overseed in late summer or early fall, when soil temperatures return to the germination range.
For clover lawns and food plots, mow at 3 inches or higher in summer and water deeply and less often rather than sprinkling lightly each day. Accept some summer dormancy in white clover, since the runners often regrow in fall.
How Clover Survives Winter
As fall cools, red clover hardens off by building protective proteins, some rising up to 30-fold.16PubMed Central. A proteome analysis of freezing tolerance in red clover (Trifolium pratense L.) Once temperatures drop below 20°F for a few nights, clover goes dormant for the winter, with little growth.17MU Extension. Clover Temperature Tolerance and How to Manage It Forage species also differ in how well they tolerate deep freezes and the smothering effect of ice sheets that cut off oxygen.18Canadian Journal of Plant Science. Winter damage to perennial forage crops in eastern Canada: Causes, mitigation, and prediction
The sequence of cold matters. Plants that cool gradually in fall are ready for a freeze, while a sudden deep freeze before they have hardened can injure them badly. A warm spell in midwinter can partly undo hardening and leave plants less protected if hard cold returns.
Snow cover insulates plants from deep cold. Ice sheets are dangerous because they suffocate crowns, and good drainage reduces the ponding that freezes into ice.
Stresses also compound. Mild cool weather before infection makes red clover more resistant to clover rot, a winter fungal disease, but freezing before infection makes it much more vulnerable.19PubMed Central. Winter survival in red clover: experimental evidence for interactions among stresses A hard freeze followed by wet, mild spells is a risky combination. Very low temperatures also reduce helpful root fungi that aid nutrient uptake.20Soil Biology and Biochemistry. Arbuscular and ectomycorrhizal root colonisation and plant nutrition in soils exposed to freezing temperatures
Clover rot works under cover, often beneath snow or in wet, mild periods when the plant is dormant, so damage is typically discovered only in spring. Stands that went into winter weak or thin lose more plants than thick, well-fed ones.
To check for damage, tug on plants in spring. Dead crowns pull out easily, with brown, mushy roots. Low spots where ice sat often show the worst loss, and white fungal growth or rotting crowns suggest clover rot. Thin stands may need overseeding in early spring.
Fall and Soil Management for Winter Survival
After mid-September, let clover stands rest so they can rebuild reserves for winter.21MU Extension. Clover Temperature Tolerance and How to Manage It Plants enter winter living on stored energy. During rest, leaves keep photosynthesizing and send sugars down to the roots and crown, which are the stores the plant lives on all winter. Grazing or cutting in this window removes the leaves just as the plant is trying to bank energy, and the cost shows up as winter loss or a weak spring start. The date is a guide for temperate regions, so growers in much milder or colder areas should adjust it to their own first-frost timing.
Red clover can be safely grazed once it is dormant, but graze it soon after dormancy begins.22MU Extension. Clover Temperature Tolerance and How to Manage It A dormant plant is not storing energy, so removing top growth does less harm than during active fall growth. Growth left standing can be flattened by snow and become a surface for disease. Graze lightly enough to leave some cover rather than taking it down to the crown.
Soil fertility matters as well. Liming acid soils and adding phosphate improved red clover’s winter hardiness.23ASSA, CSSA and SSSA. Effect of Fertilization on Winterhardiness of Forages Potassium, and sometimes phosphorus, are known to improve winter survival, possibly by strengthening roots.24ASSA, CSSA and SSSA. Effect of Fertilization on Winterhardiness of Forages Soil test in late summer and correct pH, phosphorus and potassium before fall. A test shows your pH and nutrient levels, so you add only what is short. Lime works slowly, so applying it in late summer gives it time to adjust acid soil before winter, and a spring application helps less for the first winter. Hay fields lose potassium with every cut, so they are the most likely to run low and are worth retesting regularly.
Cold Soil and Nitrogen Fixation in Spring
In cool spring soil, clover formed effective nodules even without added inoculant.25Grassland Research. Selection of rhizobial strains differing in their nodulation kinetics under low temperature in four temperate legume species Other legumes, like sainfoin and birdsfoot trefoil, benefited from carefully chosen strains.26Grassland Research. Selection of rhizobial strains differing in their nodulation kinetics under low temperature in four temperate legume species Inoculant matters more on a field that has never grown clover or related legumes, where no suitable bacteria are likely to be in the soil. It costs little compared with seed and establishment, so it is a sensible choice if you are unsure.
To check nodules, dig a plant and gently wash the roots. Nodules that are pink or red inside when cut are actively fixing nitrogen, because the color comes from a compound that helps the process. White or greenish nodules suggest little fixing, and very few nodules suggest the bacteria are poorly established. If plants on new ground have few nodules, use inoculant at the next seeding.
Check in spring or early summer when plants are growing actively, since old or dormant plants can show misleading roots. Look at several plants in different parts of the field, because one plant may not represent the whole stand.