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Yo-yo weather brings out odd plant behavior

Roses and others react to sudden shifts in temperature

Green shoot with no bud on a rose bush
This is a blind shoot on Gentle Giant, a hybrid tea rose. Weather fluctuations confuse plants, causing blind shoots in roses, among other examples of odd behavior. (Photo: Debbie Arrington)

Have you noticed your plants behaving oddly? It could be the weather.

So far this month, Sacramento high temperatures have jumped around wildly. According to the National Weather Service, we saw more than a 30-degree swing in just eight days. Memorial Day, the mercury hit 105 degrees. On June 8, the high reached only 74.

Wednesday was even cooler with a high of just 73 degrees. That's 15 degrees below average for this week.

Dramatic temperature fluctuations can confuse plants, especially in spring. This is when warm-season vegetables are growing rapidly, blooming and setting their first fruit.

Sudden drops or increases in temperature can cause plants to drop flowers or fruit. Tomatoes especially can see their pollination affected, refusing to set when temperatures top 95 – but growing much more slowly when temperatures fall.

Roller-coaster highs affect roses, too. These hot-cold flip-flops tend to produce “blind shoots,” growth that never produces a flower.

The stems look healthy with lots of foliage and fast growth. But no matter how long those stems grow, they won’t sprout a bloom.

Blind shoots are the result of extreme fluctuations in temperature and growing conditions. Our yo-yo June weather confused some bushes, especially when temperatures plunged back below normal.

Another oddity: Blind shoots can appear on the same bush with normal blooming stems.

Some rose varieties are more sensitive to temperature fluctuations than others. But right now in my own rose garden, at least 20 bushes have blind shoots, including usually reliable Olympiad, a red hybrid tea, and Frida Kahlo, a multicolor floribunda.

The cure for blind shoots: Prune them off. Restart the growth by cutting the cane or shoot back about 5 or 6 inches, snipping about 1/2 inch above a leaf with five leaflets.

Lower temperatures with highs in the 70s or low 80s will bring back another scourge: Powdery mildew. Triple-digit heat can stop outbreaks of that fungal disease, which attacks a wide range of plants (especially roses, bougainvillea and lilacs). But it can survive on fallen foliage or mulch. When the temperature’s right (like it is right now), those spores seem to jump back to life.

Keep an eye on plants and remove any foliage showing signs of powdery mildew, which can appear like a dusting of powdered sugar. If possible, quarantine infected plants in containers by distancing them from other plants. (Think of it as social distancing for your potted plants.)

Rust, black spot and other fungal disease can grow rapidly now, too. If working in the garden, sanitize pruning tools often by wiping the blades with a Clorox wipe or similar product. It will cut down on the spread of infection.

Breezy conditions can dry out newly planted vegetables or top soil. Check your soil moisture to make sure new seedlings are getting enough water.

Watch out for weeds and invasive insects! They love this cooler weather, too.

This cooldown is only temporary. Things soon will be back to feeling like summer – just in time for the real thing. According to the weather service, Sacramento will be back into the 90s by Tuesday with a high of 103 in the forecast for June 17.


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Dig In: Garden Checklist for week of April 7

The warm wave coming this week will shift weeds into overdrive. Get to work!

* Weed, weed, weed! Whack them before they flower.

* Mulch around plants to conserve moisture and control weeds.

* Smell orange blossoms? Feed citrus trees with a low dose of balanced fertilizer (such as 10-10-10) during bloom to help set fruit. Keep an eye out for ants.

* Apply slow-release fertilizer to the lawn.

* Thoroughly clean debris from the bottom of outdoor ponds or fountains.

* Spring brings a flush of rapid growth, and that means your garden is really hungry. Feed shrubs and trees with a slow-release fertilizer. Or mulch with a 1-inch layer of compost.

* Azaleas and camellias looking a little yellow? If leaves are turning yellow between the veins, give them a boost with chelated iron.

* Trim dead flowers but not leaves from spring-flowering bulbs such as daffodils and tulips. Those leaves gather energy to create next year's flowers. Also, give the bulbs a fertilizer boost after bloom.

* Pinch chrysanthemums back to 12 inches for fall flowers. Cut old stems to the ground.

* From seed, plant beans, beets, cantaloupes, carrots, corn, cucumbers, melons, radishes and squash. Plant onion sets.

* In the flower garden, plant seeds for asters, cosmos, celosia, marigolds, salvia, sunflowers and zinnias. Transplant petunias, zinnias, geraniums and other summer bloomers.

* Plant perennials and dahlia tubers for summer bloom. April is about the last chance to plant summer bulbs, such as gladiolus and tuberous begonias.

* Transplant lettuce and cabbage seedlings.

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