That intense summer heat has really been sneaking up on us during the past couple years here in the Pacific Northwest! So, chances are, you’ve spent a lot of time watching and watering your garden for heat-related stress this season. Read on for watering best practices along with heat protection and recovery advice to help your garden survive and thrive this summer.
Factors that Complicate Watering
- Type of plant
- Exposure (sun, shade, wind, etc.)
- Growing conditions (in-ground vs. container)
- Soil type and planting media
- How long it has been planted
- Weather and season
- In-ground irrigation sprinklers
- Drip irrigation
- Soaker hoses: limit to 100 feet or less; best on level ground
- Manual sprinklers
- Hand watering
- Tree hydration bags/gators: these are okay for watering new trees for a short time, but should be removed between waterings to avoid possible root rot conditions; not effective on mature trees due to size of root zone
Getting Off to a Good Start
- Amend poor soil when necessary and plant at optimal times of the year.
- Dig a hole in the ground twice as wide as the plant root ball, but no deeper.
- Fill the hole with water to ensure proper drainage and pre-moisten the planting area.
- Pre-soak dry or root-bound plants in water, root stimulator, or compost tea; loosen up compacted roots.
- Plant at proper level in the soil and water the plant again after covering roots with soil.
- Apply 1-3 inches of mulch to cover the root zone; avoid piling mulch around stem or crown of plant; provide about 4 inches of clearance from the plant before mulch layer begins.
In-ground plantings: Newly planted items should be watered deeply every 2-3 days for the first week or two, then begin to gradually taper off to every 3-4 days, 4-5 days, etc. until you are able to provide deep watering weekly without the plant showing signs of stress (see later discussion on deep watering).
Container plantings: Container plantings often need more frequent watering when newly planted, but do not need to receive as much water as in-ground plantings since their roots do not extend far into the surrounding soil yet. When more established, the same plant will have roots throughout the container to better utilize moisture near the bottom of the pot, so should be watered more thoroughly (until water comes out drainage holes) and more frequently than in-ground plantings.
Watering new plantings to establish healthy roots usually requires about 2-3 years before plants can be considered “established.” If drought tolerance is indicated, it will only be so once the plant is established. Although drip irrigation can provide deep watering, most sprinklers only wet a shallow layer of soil and are not enough for establishing new plants.
- Dig down after watering to see how deep the moisture has penetrated to determine ideal duration of watering time.
- Test coverage/volume of sprinklers by distributing shallow, open cans (tuna cans) around the lawn before running sprinklers, then measuring amount of water in the cans after a set amount of time. The average lawn requires 1-2 inches of water per week to stay green and healthy.
Establishment Watering of Perennials, Trees & Shrubs
Deep watering is best accomplished by hand or with focused use of soaker hoses or drip irrigation; slow application of water to wet the root zone and surrounding area for 30-60 minutes.
Year 1 (Infancy): Depending on the time of year of planting, begin watering regularly once it is no longer raining or anytime during periods of dry weather often, a light shower does not wet the soil deep enough to be considered hydrating to plant roots. Begin watering frequency at every 2 or 3 days for one to two weeks, then extend the break another day for another week or two until you are watering deeply once per week. Continue watering weekly until rainfall is reliable and significant.
Year 2 (Adolescence): Again, begin regular watering once dry weather has arrived and water deeply once each week, gradually skipping more days in between to get to watering about every 10 days or so.
Year 3 (College Years): A three-year-old plant should be fairly established, but will benefit from occasional deep watering every 2-3 weeks during hot, dry seasons (typically July to early September in Portland).
Mature Landscape Plants in the Ground
Without rain, trees benefit from a thorough watering once a month during the growth period and during hot, dry weather; shrubs every 3-4 weeks—water should penetrate entire root zone (within the first 2 feet for most). Fall and winter blooming shrubs (especially Camellias) should be given supplemental water from July until rains begin since they are producing flower buds at this time of year. Perennial garden watering depends on the types of plants and other growing conditions.
- Lawns: Need frequent, shallow watering to establish from seed or sod, and once rooted, can be watered at the rate of 1-2 inches per week (split in half and applied 2X week).
- Container Plantings, Raised Beds & Hanging Baskets: Once or more per day during hot or windy weather. Hanging baskets can be taken down to sit on the ground or even sat in a shallow saucer of water during extreme heat events to conserve moisture and reduce evaporation, if you plan to be gone for a day or two.
- Annual Bedding Plants: Should be kept moist, but never soggy and not dried out to wilting point.
- Perennials: Need more water when young and during growth and bloom times.
- Annual Vegetables: Need different watering depending on crop and growing conditions; only container gardens and raised beds should be watered daily (with some exceptions, of course).
Container plantings will need to be regularly watered for the duration of their lives (even during winter if the area does not receive regular rainfall).
Signs of Trouble
Wilting foliage is not always a sign that the plant is dry! Some plants partially wilt during mid-day in extreme heat and recover once the sun has become less intense. As long as plants have been properly hydrated earlier in the day, do not water in reaction to the mid-day wilt. If this is a constant seasonal pattern, it may be best to transplant to a more suitable location as it can be stressful to the plant over the long-term.
- When roots cannot take up water, the youngest foliage and new plant tissue are the first to suffer. The cause could be dry soil, curling or coiled roots, very poor drainage causing root-rot, or other issues.
- Smaller than average leaves and flowers are signs of long-term drought conditions or drought stress.
- Poor fruit yield, small fruit, or lacking flavor can be caused by drought stress.
- Early red foliage color, extreme fall leaf color, or shriveled leaves with brown edges are all signs of drought stress.
- Light green leaves, continued wilt even in moist soil, or yellowing of inner or bottom leaves are signs of overwatering and potential root-rot.
- Blossom-end rot on tomatoes (dark patch on bottom of fruit) is often caused by inconsistent watering or from allowing plants to dry out too much between waterings.
- Lettuce or other tender salad greens can turn bitter when allowed to get too dry too often.
- Cucumbers and squash will often abort their flowers and will not set fruit if not watered adequately during development.
Drought-Tolerant Plant Characteristics: Many drought tolerant plants share similar physical characteristics, such as having silver, grey, or reddish foliage, fuzzy or “hairy” leaves, small leaf surface or needle-type leaves, or thick, fleshy foliage and stems.
Extreme Heat Protection
New plantings, hanging baskets, and container plantings are most vulnerable to extreme weather conditions.
- New plantings should receive deep watering before heat arrives; mornings or evenings are best.
- Hanging baskets can be taken down and/or moved temporarily.
- Container plantings may need to be watered more than once per day. Small containers should be moved to shady locations and temporary shade provided to those that cannot be moved.
- Established plants will appreciate a deep soaking to bolster them for a coming heat wave, but always be aware of your specific plant’s needs and preferences—some plants such as Ceanothus, Cistus, and Manzanita don’t need much summer water, and others like Hydrangea may wilt even when properly watered.
Products known as anti-transpirants may be applied to certain plants to reduce moisture loss from the leaf surface. Moisture-Loc and Wilt-Pruf are two products we carry for this purpose.
Extreme Heat Recovery
Slowly evaluate and devise a recovery plan. Some plants may be better off transplanted this fall to a more ideal location.
Fertilize container plantings and hanging baskets with full-strength fertilizer to replace nutrients that were flushed out by frequent watering. Never fertilize a dry plant! Water first, then fertilize. Feed vegetables with an organic, slow-release fertilizer like G&B Tomato, Vegetable & Herb Fertilizer. Be cautious about fertilizing heavily to push soft, vulnerable new growth at this time; a diluted, slow-release feeding is better for recovery. Kelp meal or liquid seaweed is a gentle nutrient that provides potassium and trace minerals to help the plant handle stress without pushing much new growth.
If a plant has significant foliage damage due to sunburn, eventually the leaves will fall off on their own, but can be removed if completely brown. Slow-growing plants such as Camellias and Rhododendrons will take several years to recover. If stems are still green, dormant buds may still be able to grow and refresh the plant over time. Be cautious about doing any severe pruning during hot summer months; removing the top layer of foliage can often expose a more vulnerable layer underneath that will be more sensitive to sun and likely to burn.
Watch for pests! Plants weakened by stress are more vulnerable to pests and diseases; several years of mild winters have allowed insect pest populations to grow.
- Spider mites are a common problem during or following periods of hot weather; watch for signs like yellowing or browning leaves and fine webbing. Bring a plant sample in a sealed bag to our experts for confirmation and solution.
- Scale is common in sheltered areas (against house, underneath eaves), especially on broad-leaf evergreens and conifers; common signs are shiny, sticky residue on foliage or black mildew substance on leaves. Pests are often on undersides of foliage or hard to see due to their size and color. Recent reports of Hydrangea scale (larger, up to ¼”, white fuzzy things found on the under side of hydrangea leaves) may be a sign of a new pest in our area as it has not been a regular pest for our region in the past.
Overwatering and root-rot are potential problems when we are focused on watering for extreme heat. New plantings do not have extensive root systems and can only take up and store so much water at one time. Watering too frequently, before the soil has dried out a little, can cause plant roots to rot and collapse; this is often fatal. Early signs of root-rot include wilting of the plant and general loss of color.