Growing an oak from an acorn is slow, cheap and deeply satisfying. It is also one of the best ways to get an oak to thrive, because oaks grown in place, or moved young, avoid the root damage that makes large oaks hard to transplant. The steps are simple. The main thing to know is that white oak and red oak acorns behave differently.
The short answer: collect acorns as soon as they fall, drop them in water and keep the ones that sink. Plant white oak and swamp white oak acorns right away in fall, since they sprout immediately. Bur, red and pin oak acorns need several weeks of cold, moist conditions first: plant them outdoors in fall or store them in moist sand in the refrigerator at 32 to 41°F for 30 to 60 days and sow in spring. Plant ½ to 1 inch deep and protect them from squirrels.
Step 1: collect acorns at the right time
Iowa State University Extension recommends collecting acorns as soon as they fall. Fresh acorns are most likely to be viable, and they dry out or get eaten quickly on the ground. Pick plump, brown acorns without holes or cracks.
Step 2: the float test
Put the acorns in a bucket of water. According to Iowa State, sound acorns sink, and most floating acorns are not viable and can be discarded. Floaters often have weevil holes or have dried out.
Step 3: know your oak
| Oak | Group | What the acorns need |
|---|---|---|
| White oak, swamp white oak | White oak group | Plant in fall; germinate immediately |
| Bur oak | White oak group | 30 to 60 days of cold, moist stratification |
| Red oak, pin oak | Red oak group | 30 to 45 days of cold, moist stratification |
These figures are from Iowa State University Extension. Bur oak is in the white oak group botanically, but its acorns need a cold period like red oaks.
Step 4: stratify if needed
Iowa State explains that bur, pin and red oak acorns will not germinate until they have had cool, moist conditions for several weeks. You have two options:
- Let winter do it. Plant the acorns outdoors in fall and let natural winter weather break dormancy.
- Stratify indoors. Mix the acorns with moist sand and peat moss in a plastic bag or container and store them in the refrigerator at 32 to 41°F for the required time. Check monthly that the mix is still damp but not soggy. Sow them in spring.
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Step 5: plant
Iowa State recommends planting acorns ½ to 1 inch deep. You can plant in deep pots, in a protected nursery bed, or directly where you want the tree.
- Deep pots (tree pots or tall containers) suit oaks, which grow a long taproot quickly. Use a well-drained potting mix.
- A nursery bed works well for several acorns. Choose a spot where seedlings can get good care for one to two years, as Iowa State suggests, before moving them.
- Direct sowing in the final spot avoids transplanting entirely. Plant several acorns and keep the strongest seedling.
Step 6: protect them
Squirrels, jays, mice and deer love acorns and young oak seedlings. Cover pots or beds with hardware cloth until the seedlings emerge, and protect young trees with a wire cage or tree tube for the first few years.
Step 7: care for seedlings
Keep the soil evenly moist, not soggy. Give seedlings full sun once they have leafed out. Do not fertilize heavily in the first year. Pull weeds and grass around them so they do not compete for water.
Step 8: transplant young
The Morton Arboretum notes white oak is difficult to transplant because of its taproot. The younger you move an oak, the better. Move seedlings to their permanent home in their first or second year, while dormant, in fall after leaf drop or early spring before bud break, the ISA’s recommended planting seasons.
When you plant:
- Dig a hole two to three times wider than the root ball and only as deep.
- Keep as much of the taproot as you can.
- Water well and mulch, keeping mulch away from the stem.
Choosing the spot
Oaks become very large trees. White oak can reach 50 to 80 feet tall and up to 100 feet wide, according to the Morton Arboretum. Plant well away from the house and other buildings, and out from under power lines. See how close to a house you can plant an oak.
Protecting future oaks from oak wilt
Oak wilt spreads through fresh wounds in spring and early summer and through root grafts between nearby oaks, as the Morton Arboretum explains. Avoid wounding young oaks during the growing season, and do any pruning in winter.
How long until it’s a tree?
Oaks grow slowly at first while they build roots. A seedling may grow only a few inches in its first year. After a few years, growth speeds up. You are planting for the next generation, and that is part of the joy.
Sources
- How do I germinate acorns?, Iowa State University Extension
- White Oak, The Morton Arboretum
- Planting a Tree, International Society of Arboriculture, TreesAreGood.org
- Oak Wilt, The Morton Arboretum
Top photo: Andreiflorea993, CC BY-SA 4.0, via Wikimedia Commons