Guide to Growing California Oaks

Collecting, storing and planting acorns

Why acorns?

In horticultural situations, oaks can be established successfully from either transplants or acorns planted directly at the site.  Especially for restoration plantings transplants have several disadvantages relative to direct-seeded acorns:

  • the dominance of the taproot is usually destroyed in the pot, and root form may be poor, which may reduce drought tolerance and retard growth of the transplanted seedling;
  • transplants require space for propagation and care in the nursery, and so are more costly than acorns, which can be collected for free;
  • transplants are more difficult to store and transport than acorns;
  • transplants available commercially are often not derived from local seed sources unless special provisions are made far in advance;
  • soil-borne pathogens or insect pests may be introduced with the transplants into the planting site;
  • transplants require more effort and care in planting, and normally require some irrigation.
  • For these and other reasons, direct planting of acorns is the best choice for establishing California oaks in either wildland areas or in your own yard.

    To fully understand the advantages of planting acorns directly into the planting site, it is necessary to understand how acorns become seedlings.  Both the shoot and root emerge from the pointed end of the acorn.  Roots usually begin to emerge by December.  The root grows down into the soil during the winter.  In species such as valley and blue oak, roots may have grown three feet deep into the soil before the shoot emerges the following spring (shoot emergence typically occurs between the end of March and early June).  In most non-irrigated sites, young seedlings rely on their deep root system for survival over the dry summer months.  The long taproot is able to extract moisture from deep in the soil profile, and this helps the young seedling to continue to extract moisture from the soil even when the upper levels of the soil have been dried out by competing annual grasses.

    Acorn collection

    It is best to collect acorns from native oaks growing close to your planting site, within a mile or so if possible. It may also be helpful to match the seed source to the planting site, for example collecting from a riparian area if planting will occur along a creek.  Both of these guidelines help ensure that the trees you plant will be adapted to the environmental conditions at your planting site.  Avoid acorns from trees that have been planted from nursery stock in landscaped areas.  Nursery grown trees may have come from acorns of trees located hundreds of miles from your locale, and may not be well adapted.  Planting locally native material also helps preserve the local oak population, which could be unique in its genetic makeup.  If you are planting a lot of oaks, it is a good idea to collect acorns from a number of different trees.  This increases the genetic diversity of your planting material, which makes it even more likely that some of your oaks will be especially well-adapted to the site.

    Acorns can be picked directly from trees. Acorns are ready to pick from trees when the acorn cap can be easily separated from the acorn without tearing the seed coat.  A bamboo pole or length of PVC pipe can be used to tap acorns out of a tree and onto a clean tarp if the acorns are out of reach.  Acorns will generally be at least somewhat green when picked from trees, but they normally turn brown in storage

    Don't collect acorns off the ground. Acorns collected from the ground usually have more insect damage than those picked directly from the tree. Also, acorns collected from the ground are more likely to have been damaged by heat or drying. Acorns can be infected by soil-borne plant pathogens while lying on the ground. Be careful not to let any soil contaminate your collected acorns. Acorns will generally be at least somewhat green when picked from trees, but they normally turn brown in storage.

    Storing acorns

    It is often necessary to store acorns for a while before planting because most acorns will ripen and drop before the soil is wet enough to work easily.  With the exception of California black oak which requires chilling to mature, most California oak acorns are ready to germinate as soon as they are placed in moist soil.

    Acorns lose viability when they dry out, so if they are not planted immediately, they must be stored under moist conditions.  Since moisture causes oak acorns to germinate, refrigerate the acorns to slow germination.  Place acorns in plastic bags (e.g., heavy duty zip-closure bags) and store them in the refrigerator until planting.  Do not allow the acorns to freeze, as this will kill them.  Even when stored in plastic bag in the refrigerator, most acorns will eventually germinate, and the emerging roots are easily damaged or may decay during storage.  Therefore, it's a good idea to plant your acorns as soon as you can.

    Sorting the good from the bad

    Not every acorn is sound.  Several different insects, including filbert weevils (Cucurlio spp.) and the filbertworm (Cydia latiferreana) lay their eggs (oviposit) on the cap end of developing acorns. When the eggs hatch, the larvae begin to feed on the inside of the developing acorn.  Larvae reach maturity at about the same time as the acorns do.  When mature, weevil and filbertworm larvae bore their way out of the acorn and look for a suitable place to pupate. Exit holes made by the larvae are about the size of a pencil lead.  Most acorns that have these small open exit holes have a lot of internal damage and should be discarded. Not all eggs develop into larvae, so acorns with oviposition wounds, which often appear as closed pimple-like marks, may be perfectly sound. These are keepers.  Acorns that are shriveled, lightweight, or cracked have often dried out excessively, and these should be discarded.

    The weevil larvae are about 0.5 inch long, legless, and tend to curl into a "C" shape when disturbed.  The larvae of the filbertworm have legs, are pinkish to grayish, and are typically quite active.  Only the filbertworm larvae spins strands of silk.  Not only can these larvae bore their way out of acorns, they can also chew their way out of a plastic bag.  If you have some wormy acorns in cold storage for a while, a few larvae may find their way out and end up on the bottom of your refrigerator.  You can keep your acorn bags in sealed plastic crisper or other container to keep these escapees confined.

    Some external mold growth on stored acorns can occur, but is generally not a cause for concern unless the seed coat is discolored.  If acorns are oddly dicolored and feel soft when squeezed, they may be decayed and should be discarded.  You can check a few acorns by cutting them open.  The inside (the "meat") of a healthy acorn is whitish or yellowish.  Decayed acorns will be dark brown or sometimes nearly black internally.

    It's a good idea to sort acorns before you store them and again at planting time.  As you are planting, do a visual inspection for exit holes and the "feel" test for light, shriveled, and soft acorns.  If you have a large number of acorns of questionable quality, you can do the float test:  place the acorns in a bucket of water, discard the floaters, and keep the sinkers for planting.  Insect damaged and dehydrated acorns typically have some air space inside the shell and tend to float.  This test isn't completely accurate, but is an effective way to screen a large batch of acorns.

    If your acorns have germinated during storage, you can plant them if the roots are still firm and light-colored.  Discard acorns with discolored, soft, or mushy roots.

    Planting your acorns

    As noted above, acorns can be planted as soon as they are collected and stored acorns eventually deteriorate in quality.  Early planted acorns have been shown to have better growth and survival than those planted later.  If you are planting in a site without access to water (e.g., open space lands or large parcels), it is usually best to wait until the first fall rains wet the soil because it is much easier to prepare planting sites if the soil is moist.

    You can plant earlier if you can pre-irrigate the planting site:  water the planting site so that the soil is wetted to a depth of at least 1 foot.   Let the soil dry for a few days after irrigation so that it isn't too wet to work.

    Prepare the planting site by turning over the soil with a shovel to a depth of at least 10 inches.  Break up any large clods to prepare a good seedbed.   Select sound acorns for planting as discussed above.  At each site, plant 3 to 4 acorns spaced about 6 to 8 inches apart. This will increase the chances of at least one successful seedling being present at the site, and will keep the seedlings from being overly crowded if more than one emerge.  Plant acorns on their sides at a depth of  about 2 inches.  Deeper planting can decrease seedling emergence rates (especially in heavy soils), and shallower planting can increase the chances that the acorns may dry out or be eaten by mice or ground squirrels.

    If possible, mulch each planting site after planting with an organic mulch to cover the planting site and an area extending out 3 to 4 feet to a depth of about 2-3 inches.  Mulch will help to suppress weed growth around the young seedling and helps conserve soil moisture. As the mulch decays, it also provides a slow-release source of plant nutrients for the seedling.  Waste wood chips from tree pruning are an ideal mulching material as long as the chips are not contaminated with soil or roots from diseased trees.  You can often get a large load of waste wood chips free or for little cost from tree pruning services.

    Plant your oak in the right place

    If you are planting the acorn for your yard, be sure that you have selected a site that will be large enough to accomodate the tree's mature size without crowding structures or being excessively crowded or shaded by other trees.  Avoid planting under power lines or over sewer and water lines that may need to be dug up at some point.  If you want to gain the maximum cooling effect from shading, the tree should be placed to the west or southwest of the structure you want to shade and at an appropriate distance to permit midsummer shading while minimizing winter shading.  Also remember that in general, our native oaks should not be planted where the area near the trunk of the tree will be irrigated in the summer.

    See also Choosing a planting site for other factors to consider.