An apple tree’s roots are mostly shallow, but a few can reach much deeper than people expect. The bulk of the roots that drink and feed stay close to the surface, near the trunk, while a handful of deep roots act as a reserve tap for dry spells. What you planted matters as much as how long it has grown, because the roots under most apple trees come from a separate rootstock chosen to control the tree’s size.
Where Most Apple Tree Roots Actually Are
The roots that take up water and nutrients are thin feeder roots, not the thick woody anchors. On young dwarf apple trees, 61 to 74 percent of all roots were in the top 20 cm (8 in) of soil.1Academic Journals (Kenya). The effects of different irrigation methods on root distribution, intensity and effective root depth of young dwarf apple trees Between 78 and 95 percent were within the top 40 cm (16 in), and very few were found between 60 cm and 1 m.2Academic Journals (Kenya). The effects of different irrigation methods on root distribution, intensity and effective root depth of young dwarf apple trees For the first three years, a dwarf tree’s effective root depth is about 40 to 45 cm.3Academic Journals (Kenya). The effects of different irrigation methods on root distribution, intensity and effective root depth of young dwarf apple trees
Most roots form a bowl about 0.5 m (20 in) across and 0.4 m deep, centered under the trunk.4Academic Journals (Kenya). The effects of different irrigation methods on root distribution, intensity and effective root depth of young dwarf apple trees Root numbers drop off both with depth and with distance from the trunk.5Academic Journals (Kenya). The effects of different irrigation methods on root distribution, intensity and effective root depth of young dwarf apple trees So the soil right around a young tree’s trunk matters most for water and feeding. The thick woody roots mostly anchor the tree and carry water and sugars between the feeders and the trunk. Grafted apple trees on rootstocks grown from cuttings usually don’t form a classic taproot. Their strength comes from a spreading network of shallow roots.
The bowl above belongs to a young tree, and it is a starting size, not a permanent one. As the tree ages, the roots spread outward, often well beyond the edge of the canopy, so a fixed spot at the trunk stops matching where roots actually are. Older trees also grow deeper roots than young ones, though the densest feeder roots still stay in the upper layers.
Dwarf Versus Full-Size Trees
Most apple trees are grafted: the fruiting variety sits on a rootstock, a separate root plant chosen to control size. On the popular dwarfing rootstock M.9, about half of the feeder roots were in the top 20 cm.6HortScience. Dwarfing Effect of Apple Rootstocks Is Intimately Associated with Low Number of Fine Roots On a vigorous rootstock, 55 percent of feeder roots were 1 to 1.5 m deep.7HortScience. Dwarfing Effect of Apple Rootstocks Is Intimately Associated with Low Number of Fine Roots The reason is that dwarfing rootstocks make far fewer roots, and root number is a main difference between small and large root systems.8HortScience. Dwarfing Effect of Apple Rootstocks Is Intimately Associated with Low Number of Fine Roots9Scientia Horticulturae. Root architecture characteristics of differing size-controlling rootstocks and the influence on the growth of ‘Red Fuji’ apple trees
Trees on vigorous rootstocks have deep, widespread, large root systems, while dwarfing ones keep roots in a small area.10Journal of the American Society for Horticultural Science. Seasonal Growth and Spatial Distribution of Apple Tree Roots on Different Rootstocks or Interstems Trees grafted with a dwarfing middle piece on a vigorous base end up with roots between the two.11Journal of the American Society for Horticultural Science. Seasonal Growth and Spatial Distribution of Apple Tree Roots on Different Rootstocks or Interstems The shape differs too: shallow-rooted M.9 roots grow at a flatter angle than those of deep-rooted stock, a trait the tree’s genes control.12Scientia Horticulturae. Root growth angle: An important trait that influences the deep rooting of apple rootstocks
To find out what your tree is on, start with the nursery tag or purchase receipt, which often names the rootstock, such as M.9, M.26 or MM.111. If the tag is gone, look for the graft union, a knobby bump or slight bend a few inches above the soil line where the variety meets the rootstock. Size is a rough clue: a mature tree that stays around 2 to 3 m tall is probably on a dwarfing rootstock, while a much taller one is probably on vigorous or seedling stock. Pruning, soil and variety also affect size, so treat height as a hint and not proof.
A shallow dwarf root system has practical consequences. Shallow roots anchor poorly, so many dwarf trees need a stake or trellis for their whole lives. They also dry out faster in a dry spell, because they can’t reach water deep in the soil, so they depend on you for watering. The same compact roots make dwarf trees a better fit for small yards and spots near paving than vigorous trees.
How Deep the Deepest Roots Can Go
Root density falls sharply with depth, but at every depth older trees have more roots.13Catena. Soil water and root distribution of apple tree (Malus pumila Mill) stands in relation to stand age and rainwater collection and infiltration system (RWCI) in a hilly region of the Loess Plateau, China In deep loess soil, a deep, loose wind-blown silt, roots were found below 8 m in orchards older than 12 years.14ELSEVIER SCIENCE BV. Soil water and root distribution of apple tree (Malus pumila Mill) stands in relation to stand age and rainwater collection and infiltration system (RWCI) in a hilly region of the Loess Plateau, China In the same kind of dryland soil, 22-year-old trees reached about 23 m.15Hydrological Processes. Water mining from the deep critical zone by apple trees growing on loess These extremes come from very deep soils in dry regions, not typical garden ground.
Your yard tree probably won’t get anywhere near those depths. In a watered garden the water is near the surface, so roots have little reason to go deep. Bedrock, heavy clay, hardpan or a high water table can stop roots long before 8 m, and dwarf rootstocks rarely root deeply even in good soil.
Where deep roots exist, they work as a drought lifeline. In dry areas, trees take water mostly from the top 2 m early in the season, then about 60 percent from below 2 m in late summer and fall.16Journal of Hydrology. Research papers Links of apple tree water uptake strategies with precipitation and soil water dynamics in the deep loess deposits When trees were cut off from soil below 2 m during an extreme drought, water use fell 36 percent, yield 11 percent and fruit quality 15 percent.17Journal of Hydrology. Quantifying the importance of deep root water uptake for apple trees’ hydrological and physiological performance in drylands Once deep soil water runs out, though, deep roots no longer help and the tree cuts back on water use.18Agricultural Water Management. Growing deep roots has opposing impacts on the transpiration of apple trees planted in subhumid loess region
Soil and Water That Stop Roots
In a soil with a hardpan, a dense layer below the surface that roots can’t easily pass, apple roots stayed mostly above it.19Journal of the American Society for Horticultural Science. Root Distribution Patterns of Nine Apple Rootstock in Two Contrasting Soil Types In a loose soil of the same area, roots spread evenly through the profile.20Journal of the American Society for Horticultural Science. Root Distribution Patterns of Nine Apple Rootstock in Two Contrasting Soil Types Compacted soil also reduced shoot growth, leaf size and root weight in young trees.21HortScience. Response of Container-grown Apple Trees to Soil Compaction
Wet soil is the other problem. Waterlogged soil pushes out oxygen, and roots stop growing and stop taking up minerals.22eXtension. Apple Tree Root Depth: How Deep Do They Grow? Leaves turn yellow and stay small, and roots eventually die.23eXtension. Apple Tree Root Depth: How Deep Do They Grow? Both drought and swings between wet and dry slowed young trees’ growth.24Journal of Horticultural Science. The influence of soil moisture on apple root growth and root: shoot ratios Yellow, small leaves and weak shoots in spring can have several causes, but together with standing water around the trunk after rain they point to waterlogged roots.
Before planting, dig a hole about 60 cm deep and look at the walls for a hard layer or a pale, mottled zone, which can indicate compaction or poor drainage. Then fill the hole with water. If water is still standing after a day, the site is too wet for a standard planting. On poorly drained ground, plant on a raised mound or berm so the roots sit above the wet layer.
Watering to Match the Root Zone
Roots follow the water. Under drip irrigation, nearly half the roots were within 30 cm of the emitter.25Canadian Journal of Soil Science. Drip-fertigation of apple trees affects root distribution and development of K deficiency Under wider micro-sprinklers, roots spread to 60 cm deep and 90 cm out.26Canadian Journal of Soil Science. Drip-fertigation of apple trees affects root distribution and development of K deficiency Under a concentrated water supply, roots clustered near the wetted area.27ELSEVIER SCIENCE BV. Soil water and root distribution of apple tree (Malus pumila Mill) stands in relation to stand age and rainwater collection and infiltration system (RWCI) in a hilly region of the Loess Plateau, China So soak a wide area beneath the canopy rather than one spot at the trunk.
Water slowly and less often so the moisture reaches about 30 to 45 cm, instead of giving frequent light sprinklings that only wet the surface. To check how far it went, push a long screwdriver or probe into the soil, or dig a small test hole a day later. Widen the watered ring as the tree grows, because the roots spread outward with the canopy. Let the soil begin to dry between waterings, since soggy soil causes the oxygen problems described above.
A single emitter placed at the trunk keeps most roots within a small ball of wet soil, which dries out quickly and stresses the tree. Use several emitters spaced around the tree, or move the hose to different spots, so roots spread over a wider area and have more soil to draw on if a dry spell hits.
Grass, Mulch and Cultivation Under the Tree
Trees under mulch had more shallow roots, and trees under grass had more deep roots.28HortScience. Apple Root Growth, Turnover, and Distribution Under Different Orchard Groundcover Management Systems Tilling with compost shifted roots deeper and made them thicker.29Acta Horticulturae. Root Growth and Soil Management in Apple Orchards A dense mixed ground cover, though, held back apple root growth in the first year.30Acta Horticulturae. Root Growth and Soil Management in Apple Orchards In a dry orchard, a grass cover boosted roots in the top 1 m without changing deeper roots.31Acta Prataculturae Sinica. Effects of a cover crop on deep soil water and root characteristics in a dryland apple orchard on the Loess Plateau Deep digging near the trunk cuts surface feeder roots, so cultivate lightly and stay shallow within the root bowl.
Mulch holds moisture in the top layer, so roots grow up into it and more of the root system ends up shallow. That is fine if you keep watering, but it leaves the tree more exposed if the mulch dries out or you stop. Keep mulch a few inches back from the trunk to avoid damp bark.
For a young tree, keep a grass-free circle about 1 m across, because grass competes for water and nutrients. Once the tree is established, grass farther out from the trunk is fine.
Digging or Cutting Near the Roots
Cutting roots 60 cm from the trunk and 40 cm deep each year reduced shoot growth by about 30 percent.32Journal of the American Society for Horticultural Science. Time of Root Pruning Influences Vegetative Growth, Fruit Size, Biennial Bearing, and Yield of `Jonathan’ Apple Root pruning has also been tried on older trees, combined with beneficial soil fungi, to revive them.33Acta Scientiarum Polonorum Hortorum Cultus. REJUVENATING OLDER APPLE TREES BY ROOT PRUNING COMBINED WITH ARBUSCULAR MYCORRHIZAL FUNGI So if you must trench nearby, stay as far out as you can and cut on one side only.
Even a shallow trench can cut many roots. Dig by hand near the tree so you can see roots, and cut them cleanly with sharp tools rather than tearing them. Do the work in the dormant season, and water well afterward so the tree can recover.
If roots get cut by accident, expect shorter shoots that season. Thin the fruit to ease the stress on the tree, and skip heavy fertilizer right after the damage.
When Apple Roots Grow
Almost no new roots appear before shoot growth begins in spring.34Journal of the American Society for Horticultural Science. Root Growth Phenology, Root Longevity, and Rhizosphere Respiration of Field Grown `Mutsu’ Apple Trees on `Malling 9′ Rootstock The main burst comes in late June to early July, with a smaller one in late summer at 20 to 45 cm deep.35Journal of the American Society for Horticultural Science. Root Growth Phenology, Root Longevity, and Rhizosphere Respiration of Field Grown `Mutsu’ Apple Trees on `Malling 9′ Rootstock About half of new white roots turn brown or die within 3 to 4 weeks.36Journal of the American Society for Horticultural Science. Root Growth Phenology, Root Longevity, and Rhizosphere Respiration of Field Grown `Mutsu’ Apple Trees on `Malling 9′ Rootstock Root loss peaks at leaf fall.37Agricultural Water Management. Variability in apple tree fine-root phenology, amount and spatial distribution in response to irrigation and nitrogen application levels under drip fertigation In dry conditions, trees put more growth into deep roots.38Agricultural Water Management. Variability in apple tree fine-root phenology, amount and spatial distribution in response to irrigation and nitrogen application levels under drip fertigation
Shoots begin growing before roots do, so the tree draws on stored reserves first.
Keep soil moist from late spring through early summer, when roots grow fastest. Spread fertilizer as growth starts instead of in late fall, when roots are dying back and can’t use it. Plant bare-root trees while they’re dormant so they’re settled before spring growth begins.