Rooting Out Oak Radicles
Blue oak acorns readily germinate once they fall from the tree. Unlike some acorns, such as the black oaks which require a cool, moist resting period to trigger germination, blue oak acorns can start sprouting the initial root or radicle very soon after they are put away for cold storage. These acorns can develop sizeable radicles when stored for some months. While this is a good sign of the acorn's viability, there are problems with these brittle new roots. In selective sprouted acorns for outplantings, should you discard those acorns whose radicles have been discolored by presumed fungal infection or damaged by rough handling? Should you snap off the damaged radicle and proceed with planting?
A pilot study at the Sierra Foothill Range Field Station suggests some guidelines, and has produced some interesting preliminary results. In a small, fenced test plot on the typical soils of our region, 100 acorns were planted in shallow augered holes in two randomized blocks. Holes were augered to a depth of 6-8 inches, and acorns planted in the upper 1-2 inches of loosened soil. Acorns were oriented on their sides with their radicles in "normal" downward, position. The acorns had all sprouted as a result of long storage. They were sorted into four treatment (TMT) groups: TMT 1, acorns with normal whitish radicles; TMT 2, those with brown-tipped radicles, severely infected at tip only; TMT 3, those whose severely infected radicles were partially cut off, leaving 1-1.5 cm. of healthy radicle; and TMT 4, those whose infected radicles were completely cut off, flush with the acorn.
Shoot emergence was closely monitored. In late season, after the normal emergence period had passed, one-half of the non-emergers were carefully excavated and removed. The remaining non-emergers, as well as those already emerged, were left undisturbed for subsequent observation. Table 1 above shows preliminary results.
Acorns with untreated radicles had the highest percent emergence, while none of those whose radicles were trimmed off completely emerged. Although there are clear differences for the number of days it took each treatment group to emerge, there was too much variation to assign statistical significance to these findings. While TMT 1 acorns produced normal tap root development, some infected and all trimmed radicles produced multi-branched root systems of greater total mass than the single tap root systems. Those acorns with radicles trimmed all the way back to the acorns showed no evidence whatsoever of of shoot initiation, while all other acorns showed different degrees of shoot initiation and failure to penetrate to emergency. All root systems, regardless of configuration, were well developed, viable looking, and without apparent fungal problems, despite failure of shoot emergence.
At this time we conclude that slight browning of newly-sprouted radicles should be ignored for planting acorns. Intentionally trimming the radicles may alter root morphology, while severe trimming will lead to sure failure of emergence. It appears that complete radicle trimming severs the developing shoot apex. We don't know if the tap root or bushier root system is more advantageous for establishing blue oak seedlings. A single tap root would seem to allow deep, fast penetration to moist soil levels. There are, however, some indications that a more diffuse root system is not so unusual in early growth stages in nature, and diffuse root systems have proven beneficial in establishing eastern oak species. Such root systems can easily be produced for further study by radicle trimming.
prepared and edited by John M. Harper, Richard B. Standiford, and John W. LeBlanc