Why acorns?
In horticultural situations, oaks can be established successfully fromeither transplants or acorns planted directly at the site. Especiallyfor restoration plantings transplants have several disadvantages relativeto direct-seeded acorns:
For these and other reasons, direct planting of acorns is the best choicefor establishing California oaks in either wildland areas or in your ownyard.
To fully understand the advantages of planting acorns directly intothe 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 intothe 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 emergesthe following spring (shoot emergence typically occurs between the endof March and early June). In most non-irrigated sites, young seedlingsrely 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 fromthe soil even when the upper levels of the soil have been dried out bycompeting annual grasses.
Acorn collection
It is best to collect acorns from native oaks growing close to your plantingsite, within a mile or so if possible. It may also be helpful to matchthe seed source to the planting site, for example collecting from a riparianarea if planting will occur along a creek. Both of these guidelineshelp ensure that the trees you plant will be adapted to the environmentalconditions at your planting site. Avoid acorns from trees that havebeen planted from nursery stock in landscaped areas. Nursery growntrees may have come from acorns of trees located hundreds of miles fromyour locale, and may not be well adapted. Planting locally nativematerial also helps preserve the local oak population, which could be uniquein its genetic makeup. If you are planting a lot of oaks, it is agood idea to collect acorns from a number of different trees. Thisincreases the genetic diversity of your planting material, which makesit even more likely that some of your oaks will be especially well-adaptedto the site.
Acorns can be picked directly from trees. Acorns areready to pick from trees when the acorn cap can be easily separated fromthe acorn without tearing the seed coat. A bamboo pole or lengthof PVC pipe can be used to tap acorns out of a tree and onto a clean tarp ifthe 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 becausemost 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 placedin moist soil.
Acorns lose viability when they dry out, so if they are not plantedimmediately, they must be stored under moist conditions. Since moisturecauses oak acorns to germinate, refrigerate the acorns to slow germination.Place acorns in plastic bags (e.g., heavy duty zip-closure bags) and storethem in the refrigerator until planting. Do not allow the acornsto freeze, as this will kill them. Even when stored in plastic bagin the refrigerator, most acorns will eventually germinate, and the emergingroots 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 filbertweevils (Cucurlio spp.) and the filbertworm (Cydia latiferreana)lay their eggs (oviposit) on the cap end of developing acorns. When theeggs 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. Whenmature, weevil and filbertworm larvae bore their way out of the acorn andlook for a suitable place to pupate. Exit holes made by the larvae areabout the size of a pencil lead. Most acorns that have these smallopen exit holes have a lot of internal damage and should be discarded.Not all eggs develop into larvae, so acorns with oviposition wounds, whichoften appear as closed pimple-like marks, may be perfectly sound. Theseare keepers. Acorns that are shriveled, lightweight, or cracked haveoften dried out excessively, and these should be discarded.
The weevil larvae are about 0.5 inch long, legless, and tend to curlinto a "C" shape when disturbed. The larvae of the filbertworm havelegs, are pinkish to grayish, and are typically quite active. Onlythe filbertworm larvae spins strands of silk. Not only can theselarvae bore their way out of acorns, they can also chew their way out ofa plastic bag. If you have some wormy acorns in cold storage fora while, a few larvae may find their way out and end up on the bottom ofyour refrigerator. You can keep your acorn bags in sealed plasticcrisper or other container to keep these escapees confined.
Some external mold growth on stored acorns can occur, but is generallynot a cause for concern unless the seed coat is discolored. If acorns are oddly dicolored andfeel soft when squeezed, they may be decayed and should be discarded. You can check a fewacorns by cutting them open. The inside (the "meat") of a healthyacorn is whitish or yellowish. Decayed acorns will be dark brownor sometimes nearly black internally.
It's a good idea to sort acorns before you store them and again at plantingtime. As you are planting, do a visual inspection for exit holesand the "feel" test for light, shriveled, and soft acorns. If youhave a large number of acorns of questionable quality, you can do the floattest: place the acorns in a bucket of water, discard the floaters,and keep the sinkers for planting. Insect damaged and dehydratedacorns typically have some air space inside the shell and tend to float.This test isn't completely accurate, but is an effective way to screena large batch of acorns.
If your acorns have germinated during storage, you can plant them ifthe 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 andstored acorns eventually deteriorate in quality. Early planted acornshave 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 spacelands or large parcels), it is usually best to wait until the first fallrains wet the soil because it is much easier to prepare planting sitesif 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 least1 foot. Let the soil dry for a few days after irrigation sothat it isn't too wet to work.
Prepare the planting site by turning over the soil with a shovel toa depth of at least 10 inches. Break up any large clods to preparea good seedbed. Select sound acorns for planting as discussedabove. At each site, plant 3 to 4 acorns spaced about 6 to 8 inchesapart. This will increase the chances of at least one successful seedlingbeing present at the site, and will keep the seedlings from being overlycrowded if more than one emerge. Plant acorns on their sides ata depth of about 2 inches. Deeper planting can decrease seedlingemergence rates (especially in heavy soils), and shallower planting canincrease the chances that the acorns may dry out or be eaten by mice orground squirrels.
If possible, mulch each planting site after planting with an organicmulch to cover the planting site and an area extending out 3 to 4 feetto a depth of about 2-3 inches. Mulch will help to suppress weedgrowth around the young seedling and helps conserve soil moisture. As the mulch decays, it also providesa slow-release source of plant nutrients for the seedling. Waste woodchips from tree pruning are an ideal mulching material as long as the chipsare not contaminated with soil or roots from diseased trees. Youcan often get a large load of waste wood chips free or for little costfrom 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.
