Check Daily, Water As Needed

Simple answers don't begin with 'that depends.'

When people ask me how they should water their newly transplanted trees and shrubs in the summer, my answer is often not simple. It begins with 'what are you watering with?' Then we may need to discuss what type of soil they have, what kinds of plants, and so on. But gardening is full of rules of thumb, guidelines that are easy to remember. After all, gardening is a mix of science, skill, and art. So let's come up with a simple answer to the question: how should I water new plants during the summer?

Let's get one myth out of the way first: 'It's too hot to plant here during the summer.' Nonsense!

Landscape contractors don't stop working in the summer. Ideal temperatures for plants to establish and grow are 55 – 90 degrees (F), which is the majority of our summer temperature range. A plant that can grow in our climate can sustain short periods of very hot weather, even shortly after transplant.

There are advantages and disadvantages to every season here. Some advantages of summer planting?

Plants root quickly into warm soil, establishing faster than in winter or spring.

You have greater control over soil moisture, so you can dig a proper planting hole. Water thoroughly a couple of days beforehand, and the soil will be perfectly workable.

Many plants prefer warm weather: subtropicals and plants from Mediterranean climates, for example. Citrus trees planted in late winter often sulk, whereas those planted in summer begin immediate growth.

Availability and quality from wholesale growers are often better in summer. Propagation and production cycles of many plants have them retail-ready in summer: shade trees, subtropicals, and Crape myrtles are good examples.

The key to success, regardless of season, is proper planting and aftercare.

Most important is how you water. Our margin of error about watering is narrower in the summer. Plants should not be stressed to the point of wilt, while watering too often leads to crown- and root-attacking fungus. Therein lies the reputation that summer planting is difficult.

Aftercare includes the period of transplant stress and early root establishment.

When you take the plant out of the pot, you will find roots circling or bound up. Sometimes you have a solid cylinder of packed roots. Pull, tease, or even cut those roots to get them un-bound. Obviously that stresses the plant a bit. Exposing the fine root hairs to the air kills them, and those are the water-uptake part of the root. So the plant droops, even with adequate soil moisture. But that period of shock only lasts a few days: the fine root hairs re-grow immediately if the soil is loose, warm, and moist.

After the first couple of days, the roots quickly begin to grow out into the surrounding soil – especially if you have loosened it at the time of planting. This is why we recommend digging a hole at least twice as wide as the container, turning and crumbling and mixing the soil thoroughly. Full root penetration into the surrounding soil occurs within about six weeks, so that is how long it needs special watering attention.

So here is the first rule of thumb: check the plants daily, water as needed.

Simple enough. But what does 'as needed' mean?

You could wait until the plant starts to wilt slightly, poke your finger into the soil an inch or so, wait until it feels nearly dry to the touch.

Or you can calculate it.

Ok, so I actually have a degree in plant science. I should be able to calculate how much water a plant needs. The evapotranspiraton rate (ET rate) is a measure of how much water a plant uses, measured in inches (gallons per square foot). All you need to know is the plant's canopy (leaf area), look online at the weather data for Davis at ipm.ucdavis.edu, and you can calculate how much water a plant used. Let's see: 1 inch of water = 0.62 gallons per square foot. The ET rate on June 21 was .27 inches at the Davis weather station, so a 2-foot diameter plant used about one-half gallon of water (you still with me?) that day.

Now, I haven't accounted for the variation in ET rates by microclimate, because I have no way to measure ET on my own property. And plant species vary in their actual usage: the ET rate measurements are based on turf. So I'd need the landscape coefficients for my particular species to multiply (stop yawning!) the ET figures, but that would, if anything, make the actual use rate a bit lower. I'd have to do separate calculations for each plant based on size and type, or figure out the average for the whole planting area.

I'm guessing most people won't find this approach very practical.

All right, let's try a more hands-on technique. At the nursery we water nearly every plant nearly every day during the summer. How much water does it take to get the nursery soil thoroughly wet? I tell my staff to count to five slowly for a #5 can, and to ten for a #15 can. The new plant in the ground is still mostly in the nursery soil for the first couple of weeks.

How much water does your hose put out? At full throttle about 7.5 gallons per minute in our area (contact your local water district to get information for your area, or calculate it from charts based on your water pressure and meter size). So here's how much we're watering each day:

4" pot: one quart

#1 can: 0.5 gallon

#5 can: 0.75 gallon

#15 can: 1 gallon

But there are a couple of issues with this approach once the plants are in the ground.

1. Daily watering in heavy soil encourages fungus that may kill the plant, especially in hot weather. It's better to allow the soil surface to dry slightly between waterings.

So here's our second rule of thumb: Water every other day, the following amounts:

* 4" pot: 0.5 gallon

* #1 can: 1 gallon

* #5 can: 1.5 gallons

* #15 can: 2 gallons.

You can do this with a hose or with a drip system. Most drip emitters put out 1 – 2 gallons per hour, so you'll need to run the drip line for 1 – 2 hours every other day.

My friend and associate, Deborah Flower, is a horticulture instructor at Sierra College in Rocklin. When she was a graduate student at UC Davis, she field-tested (literally) this approach with #1 and #5 size shrubs planted in a field near Old Davis Rd. She made a simple berm of soil around the nursery soil and watered the measured amounts by hand. She notes, 'when I used this system I did not lose a single plant!'

2. The soil around the nursery soil also needs to get wet, or the roots won't grow outward. So use a sprinkler or your existing system to water the surrounding soil thoroughly once a week. Set out some tuna cans to make sure you're applying 1.5 – 2 inches of water a week.

In her experiment, Debbie accomplished this extra watering by thoroughly soaking the soil in a larger area around each plant.

Why not just use your sprinkler system for the new plants? Because the other plants in your yard don't need water that often. It is better to water the new plants individually. A simple drip watering system, or a soaker hose, is best for the new plants if you can't hand-water. Be sure that the drip emitter is positioned just over the nursery soil.

3. Debbie continues, 'they must be cautioned to not continue this frequency of watering once the plant is established!' So here's our third rule of thumb: keep it up for six weeks: that is, continue the individual every-other-day watering, by hand or with a drip line, for six weeks. A simple battery-operated timer can automate the process if you want to go on vacation. It's ok to give new plants an extra drink on a windy day.

A healthy plant, properly planted and watered, will begin new growth immediately and you will gain a whole summer's growth!

Note: Deborah Flower's experiment and recommendations are described in Pests of Landscape Trees and Shrubs, 2^nd Edition, Publication 3359 of the UC Integrated Pest Management Program and available online at ipm.ucdavis.edu.

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