Wednesday, June 26, 2013

Growing pecan seedlings in containers

    This year I decided to raise some pecan seedlings in containers (photo at right).  There are many successful methods for growing pecan trees in pots but I've had several growers ask me for my recipe, so here goes.
   The process starts with some advanced planning. To germinate pecans effectively, the seed needs to be stratified (stored under cold moist conditions). I like to stratify my seed for at least 120 days to improve uniformity of germination. Details of the stratification process were posted previously on this blog.

A germinated pecan seed
     Before talking about pots, soil mixes, and fertilizers, I think its a good idea to become familiar with how a pecan seed germinates and grows in its natural environment. At left is a photo of a germinated pecan. To germinate, the seed must first imbibe enough water to swell the kernel and crack open the shell.  As the seed starts to grow, a vigorous tap root is the first structure formed. Shortly thereafter, a smaller, wiry shoot develops and grows upwards, poking through the soil surface. In nature, a new pecan seedling will invest most of it energy in growing a massive, deep tap root. Above ground, first-year pecan trees rarely grow more than  8-12 inches in height and produce only a hand full of  leaves. This growth pattern is the tree's way of ensuring seedling survival. Between fires, floods, grazing animals and brush hogging, seedling pecan trees often lose above ground parts. By storing a massive amount of plant energy in the tap root, a pecan tree can easily replace a lost top with a new sprout.

Tall One pot (4 x 4 x 14 inches) and
 Anderson Band (2.9 x 2.9 x 5.5 in)
     When growing pecan trees in containers, the tree's natural habit of growing a tap root must be considered. If grown in a typical "flower" pot, the pecan's tap root will hit the bottom of the pot, turn sideways and start circling the bottom of the pot.  The subsequent out-planting of pecan trees with circling roots leads to inadequate tree growth and poor long-term performance. To remedy this problem, special "bottomless" pots have been developed to deal with the tap root problem (photo at right).  By bottomless, this means that the bottom of the pot is open, with only a very limited amount of plastic cross pieces present to help hold the potting media in place. When open-bottomed pots are placed on a screen-wire nursery bench, the tap root grows to the bottom of the pot and then becomes "air-pruned". In other words, the tap root hits air at the bottom of the pot and stops growing. However, air-pruning the tap root doesn't stop all root growth. Once the tap root stops growing,  a profusion of lateral roots are created.  

          When I first starting planting pecans in containers I used the "Tall One" style pot but have since switched to the smaller "Anderson Band" container (photo above).  The smaller pot seems to create a denser profusion of lateral roots and makes transplanting in the Fall easier (smaller hole to dig).  There are many other styles and sizes of containers designed for growing tree seedlings (see Stueve & Sons, Inc.). I've thought about trying some of these other pots but then I look at the supply of pots I already have stacked in the barn and  I always decide to stick with what I've got.   

Bottom view of a Anderson Band container
    The photo at left illustrates air pruning of a pecan root system growing in an Anderson Band container. Notice that the tip of the large, freshy taproot has dried out and died after emerging into the air at at the bottom of the pot.  You can also see that limiting taproot growth promotes the growth of small lateral roots inside the pot. 
     I use a totally artificial soil mix for growing pecan trees in pots. The mix I use contains processed pine bark, peat moss, vermiculite, and perlite. Two commercially available soil mixes are Fafard Growing Mix 52 and Scotts Metro-Mix 702. Since these soil mixes have little or no nutrients needed for plant growth, I mix in a slow release fertilizers such as Osmocote 19-6-12 (12-14 months) and "Micromax Micronutrients".  For each 2.8 cu. ft. bag of soil mix, I add 3 cups of the Osmocote and 1/2 cup of the micronutrients before mixing thoroughly. Once the pecan trees have germinated in the pot, I drench each pot with a 1 % solution of "Nickel Plus" to prevent the development of nickel deficiency. A lack of nickel in the potting media is the reason so many container grown pecan trees develop symptoms of 'mouse ear' disorder.
    Air pruning the taproot requires that tree pots must be placed on a raised, wire-mesh potting bench. I have used ½ inch hardware cloth tacked onto a wooden frame built with 2x6 treated lumber (photo at left). I raised the bench to a comfortable working height by placing the wooden frame up on concrete blocks.  I planted my seeds around May 1 when temperature finally started to warm up. I set a single nut (on its side) in each pot about 1 inch deep into the potting soil. To make handling the small Anderson pots easier, I use an Anderson deep flat that can hold 25 pots (photo at left). These flats have a very open bottom that holds the pots in place yet preserves the air pruning feature of the open bottom pot.
   One of the disadvantages of using a very porous potting soil is that you will need to water the trees every day. I make sure to completely soak the soil at every watering. During the heat of the summer (temperatures above 94) water the trees in the morning and in mid-day to keep tree roots from over-heating. Heat damage to the roots can be recognized by the appearance of a marginal leaf burn on the foliage.
    I've found that placing my potting bench under the partial shade of two large oak trees has also helped me avoid problems with summer heat. When placed in direct sunlight, the black pots used for growing trees will absorb a lot of the sun's heat causing soil temperatures to rise well above the ambient air temperature. With the combination of partial shade and ample water, I am able to keep tree roots healthy in throughout our Kansas summers.
 I never try to over-winter the seedlings I grow in containers. Pecan roots will die if exposed to temperatures of less than 19 degrees F. Although preventing cold injury to potted trees is possible, I find it more hassle than its worth. I will transplant all my container-grown trees into the field starting October 1st or as soon a soil conditions allow.