Growing Bareroot Blue Oak Seedlings
As interest and concern about native California oaks increase throughout the state, there is greater and greater demand for oak seedlings for restoration plantings and other artificial regeneration programs. To date, almost all of the seedlings for these projects have been grown in containers, ranging in size from a few cubic inches to 15-gallon pots. An alternative production method used widely to grow conifers is to grow seedlings in bareroot nursery beds. Unfortunately, there is little information about how this approach will work in California for oaks. Since oaks tend to produce a deep prominent tap root, it is vital to somehow prune or undercut their roots while they are in the nursery beds. This promotes a more compact, fibrous root system and facilitates lifting the seedlings when it is time to transplant them. To help develop practical guidelines for producing bareroot oak seedlings, a study was conducted at the California Department of Forestry nursery in Magalia. This study evaluated alternative undercutting treatment, as well as the effects of different nursery sowing dates.
In the fall of 1987, approximately 6,000 acorns were collected from a single blue oak tree in Butte County. Acorns were sown at a density of 14 per square foot in standard 4-foot wide nursery beds on 3 dates:
Seedlings from each of these sowing dates were given one of three undercutting treatments the following spring and/or summer: 1) early (May 23) undercutting only; 2) early and late (August 17) undercutting; or 3) late undercutting only.
The spring after planting, seedlings were evaluated once a week for emergence. The following winter they were lifted and sub-samples from each of the 9 treatment combinations were either destructively harvested for various morphological measurements, or were outplanted in an unirrigated plot at the Sierra Foothill Range Field Station. In the field they were monitored throughout the spring and summer for flushing, and at year's end were assessed for survival, height and caliper.
Nursery emergence. Seedlings sown in November or January began coming up in mid-March, while those from the March sowing didn't begin to emerge until a month later. By late April, emergence for those sown on the first two dates was nearly 100%, while emergence for the March sowing was less than 50%.
Morphological measurements. Seedlings that were undercut early only were the largest while those that were undercut late only were generally the smallest, with significantly smaller root systems and less total seedling weight. The most striking difference among treatments was in the the shape of the roots. Seedlings that were only undercut late had thick single tap roots, with a few small laterals originating from the point where the tap root was cut. Those from the other two treatments had much more branched root systems. Multiple undercutting also reduced both shoot and root weight compared with single early undercutting. There were no significant differences among sowing dates for any of the morphological measurements.
Field measurements. Year-end survival was very high for seedlings from the first two undercutting treatments, but was significantly smaller for seedlings from the late only undercutting treatment. Year-end height, caliper and number of flushes exhibited a similar pattern. Seedlings from the first two treatments grew vigorously, while those from the late only treatment remained stunted. As with morphological measurements, there were no trends or significant differences among sowing dates for any of the field measurements.
These results indicate that late sowing delayed emergence and resulted in reduced seedling production in the nursery, but had little effect on subsequent outplanting performance. Undercutting, however, dramatically affected both seedling morphology and field performance. Late undercutting alone removed such a large volume of the root that total seedling weight was greatly reduced; early undercutting alone, on the other hand, resulted in the largest seedlings. Seedlings from the early only, and early and late undercutting treatments performed well after outplanting, while those undercut late only fared poorly. While survival was nearly 100% for seedlings from the first two undercutting treatments, survival for the late only undercutting was 36%. Similarly, height and caliper were significantly lower. These results suggest that healthy, vigorous blue oak seedlings, with potential for high survival and rapid growth after outplanting can be produced in one year in bareroot nurseries, but that it's important to sow acorns by mid-winter and undercut in the spring.
prepared and edited by John M. Harper, Richard B. Standiford, and John W. LeBlanc