Tuesday, August 17, 2010

The Heat Wave Roller Coaster

Wow- this summer has sure been a roller coaster of ultra-hot and ultra-cool, without a lot of "regular summer" weather in between. Big temperature swings between 62-92 degrees can be hard on many of the plants in our gardens, especially those that were planted this spring.

The first part of the roller coaster equation is that the extra long, cool & wet weather that continued into early summer produced an extra lush amount of new growth in the trees, shrubs, and perennials that thrive in these conditions. Hydrangea, Peony, and Fern are just a few of the plants that grew like skyrockets this spring, and that now may be experiencing some stress from the heat wave roller coaster.
This current cool spell is a great time to help all drought-stressed plants recover, because going into the cold months still stressed will make them more susceptible to winter damage.

On the other hand, this year's extra cool, wet weather delayed or stunted the growth of some garden plants. Hebe, Senecio, and many of the Ornamental Grasses stayed almost dormant until the first heat wave hit, then began to grow vigorously. The subsequent heat waves we've had are a "shot-in-the-arm" for them, and have brought most of them out of their stress.

So... since we can't control the weather, how can we help the plants that are likely to suffer in the heat, and also keep the heat-lovers happy during the cool spells?

The good news is that all heat-lovers with well-established root systems can handle the huge temperature swings without much (if any) stress, as long as their water needs are met but not exceeded. The plants that prefer cooler temps are going to experience some more stress if another heat wave hits, but they too can thrive despite it as long as their water needs are met but not exceeded. See a pattern here? On the surface (pun intended), it seems so simple: plants all do need water. And when the weather is blazing hot, it's easy to think that all plants need a great big drink. But.... the longer I am in this profession, the more I see the importance of really understanding summer watering, and its effect on plant growth and long-term health. It's a deep subject (pun intended- again!) but one well worth learning about for the sake of your plants.

Here are some tips that can help you water wisely:

1. Getting to know your soil is the biggest key to successful watering practices.
As part of my first visit to a new garden, I always take a soil sample to determine its sand, silt, and clay percentages. Sandy soil allows water to penetrate fast and deep, but then dries out quickly. Clay soil accepts water very slowly, so it can pool on the surface before being absorbed, but stays moist the longest. The ideal soil is somewhere in between, but.... whether your soil is sandy, silty, or full of clay, I always recommend adding a generous amount of compost before planting. It sounds too good to be true, but compost definitely helps any type of soil retain summer water better, yet still drain well during the rainy seasons.

However, different areas of your garden may still retain water longer than others due to shade patterns, subsoil composition, and reflected heat from nearly hardscape surfaces like your driveway or sidewalks. Getting to know how each area retains water is an important key to knowing how to deliver the right amount to keep the plants in your garden healthy and resistant to pests.

The color of your surface soil doesn't necessarily indicate when it's time to water though. In general, it's best if the soil dries out an inch or two below the surface before you add water, because it is good for plant roots to occasionally receive the air circulation that is present in drier soil. To really find out when it's time to water, you'll need to do a little digging with a hand trowel or just poke a finger down into the soil. If that underneath soil is dry, its fine to water. If that soil is still moist, don't do it- you will be oversaturating the soil which is not good for your plants. Believe it or not, summer overwatering causes as many plant problems in the Seattle area as underwatering does.

2. New plantings need regular summer watering to become well-established.
Just like us, new plants will become stressed and unhealthy if their water needs are not met. Some examples:
  • Annual vegetables and flowers are least able to handle drought stress, as they have larger water needs and less water-storage capacity in their roots than perennial plants. Lettuce and other salad greens can stop growing leaves and just "bolt", which means they send up flowers and the existing leaves begin to taste bitter. Tomatoes, squash, and other veggies can have reduced fruit production or size. Petunias and Geraniums can stop flowering altogether.
  • Newly-planted trees, shrubs, grasses, and perennials- even those which eventually will need very little water- are also very vulnerable to drought stress because their roots are not yet well-established. It can alter their blooming or fruiting patterns, or cause them to be more susceptible to insect or disease problems. It can also cause deciduous trees and shrubs to drop their leaves early, which means less food stored in their roots for getting through the coming winter.
  • The roots of all plants growing in container gardens are subjected to more direct heat than in-the-ground plants, and even mature plants may need to be treated like annuals, with a daily or every-other-day watering in order to prevent stress and/or bolting.
3. Wilted leaves don't always mean that watering is needed.
On a very hot day, most garden plants are losing more water through their leaves than they can take up with their roots, even if the soil is moist. For some shrubs and perennials, the result is leaves or stems that begin to droop or wilt a bit during mid-to-late afternoon, which is the hottest part of the day. If they perk right back up during the cooler evening or early morning hours, don't water yet- this little bit of a roller coaster ride isn't a problem. However... if you see a plant wilting on a day when it is not real hot, it's definitely time to check your soil by probing down underneath the top inch with a hand trowel or your finger. If that soil is dry, water immediately. If that soil is still moist, you may have a very different problem, and looking further is the best way to solve it. Soil compaction, root problems, and overwatering can cause symptoms that look identical to drought stress. So always check the soil before assuming that watering is needed.

4. A few more tips:
  • Many of the most drought-tolerant plants have leaves that don't lose water quickly because they are tiny, thick, and/or fuzzy.
  • Healthy and large mature trees should seldom (if ever) need summer watering, because of their deep root systems.
  • Many trees, shrubs, grasses, and perennial plants can tolerate extreme heat with dry soil conditions for a period of time without suffering any ill effects, as long as they have a well-established root system.
  • Some plants even thrive more with no summer watering once they are well-established, because they are native to areas where such conditions are the norm: Smoke Tree (Cotinus), Rockrose (Cistus), as well as the Verbena venosa and Yarrow in the photo below are members of this group.
  • Our native Kinnickinnick, Shore Pine, and Madrona thrive on hot, rocky bluffs, so once they are well-established, summer watering can actually impede their health.
  • Herbs like Rosemary, Lavender, and Thyme are even said to have more pungent fragrance and flavor when not given summer water once they are well-established.
  • Sedums such as the S. spurium 'Dragon's Blood' in the photo below need very little summer water, even when planted in containers.

Using a drip irrigation system or soaker hoses to get your new plantings well- established is something I usually recommend. However, it's not as easy as "set it and forget it", and you can learn more about how to water wisely with these methods in my July posting. I also recommend checking out The Saving Water Partnership for free, well-written information on watering new plantings for good root establishment, installing soaker hoses and drip irrigation systems, and more tips on watering wisely. www.savingwater.org/outside_watering.htm