Start the Season Off Right: Fertilize with confidence when chill is inadequate and water is decent

Start the Season Off Right

Cold and wet. We need quite a bit of both when we are talking about tree nuts. The time under 45 degrees F accumulating chill hours is our way of measuring the tree’s ability to shut down for a bit and rest. That manna from heaven in the way of water when it’s raining recharges our soils, aquifers and deep moisture. As that barometer drops and clouds roll in, our hope transforms into reality when the rain arrives. If it’s cold at the same time, we get the double whammy. But what if it’s not?

I was just fortunate enough to take a break in Hawaii on Kauai. It’s one of the most beautiful places and has two main climates. The temperature is almost always perfect with minimal swings sitting in the middle of the pacific on the equator. Kawaikini is a dome volcano that has grown from the ocean floor and sticks up 5200 feet above sea level and creates quite a rain shadow. Any given year it can be the wettest point on earth. But after all that rain falls the leeward side of the island is what they call the Grand Canyon of the Pacific. It is fairly dry and more barren of the Jurassic Park vegetation you see on the north. My observation that is supposed to relate to this article is the fact that proper moisture and temperature allow vegetation to flourish. Trees, bushes, ferns and vines just don’t ever shut down. Being tropical, there is a constant supply of moisture and minerals in that environment.

Let’s relate that to chill. When decent moisture comes in winter, and sunny days that don’t get cold enough, our trees don’t exactly take a winter nap. The leaves may fall off with shorter days, but the roots continue to flush and search for nutrients in warmer soils. Energy deficits begin. Aboveground, we try to calculate our chill accumulation by degree days at specific levels. That ambient temperature is just fine with limited solar radiation. However, when we have bluebird days and the sun is bright and warm, our bud wood, stems, branches and trunks warm up a bit. This sends confusing physiological signals to the trees. Think about skiing in 25-degree (F) weather where the sun is still shining. It’s cold outside, but our faces still burn.

Our trees aren’t getting the total chill that ambient temperature models predict. Soils can heat up as well. I’m going to ask you to prove it to yourself. Go buy a thermal gun from your favorite hardware store. It should only cost you about $20 to $30 but it’s a cool tool to have. The next bright sunny day go out into your orchard at 1:00 p.m. Shoot the thermal gun at the shaded side of your trees and then at the full sun side. Do this for 20 to 30 trees and the trend will present itself. I’ve seen as much as 6- to 9-degree (F) differences. Now go back to your models. 45 degrees F gets you a one for one point in chill hours. At 45 to 55 degrees F, you get half that. If you are 9 degrees F warmer on half the tree (the sunny side), you may go from 50% less to almost no chill at all based off the model.

Now for the water. When we do get it, anaerobic conditions can hinder phosphorus conversion from the ‘poly’ form to the ‘ortho’ form. Other nutrients can get pushed through the root zone. If the roots are still flushing like they were in December this year in many areas, that root ball is depleting the available nutrients in the immediate area. We must feed the trees.

I’m sure this warrants an eye roll from a grower hearing this from a fertilizer salesman, but it seems logical to me. Our trees start out at the highest on the nutrient curves with nitrogen and phosphorus in the early spring. They take a lot of that N up in the postharvest period. With our almond budgets reduced the last two years and even pistachio prices coming down, we tend to cut nutrition. Walnuts have been suffering for a while. With a little rebound coming in two of those crops, we need to think about feeding early. If we start the year off with a nutritional deficit, we have already affected our yields. Feed them an orthophosphate early. Keep them happy and with enough energy to optimize bloom. Set yourself up for success and don’t shoot yourself in the foot early.

In years like this that may not have adequate chill but still decent water, start your season off right. Don’t skimp on the early P. When the vegetative buds start to swell, add some N. I recommend we never add more than 13 units. If trees can only absorb 10 units per week, with a 70% NUE that should be the rate. Balance your P and Ca numbers based on required ratios. Ca is almost 1:1 with N, and cells need it for division and cell wall strength. I like to add some magnesium at about three-quarters leaf -out to ramp up the chlorophyll levels. You can do this every week through April. Keep them pushing hard and give yourself the best chance at optimizing yield. With small shots more often, I find in my calculations after April I’ve actually used less and have better results. When prices dip, yield must be maximized. If both drop, next winter will seem longer, darker and colder.