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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 Sept. 25

This week's warm break will revive summer crops such as peppers and tomatoes that may still be trying to produce fruit. Pumpkins and winter squash will add weight rapidly.

Be on the lookout for powdery mildew and other fungal diseases that may be enjoying this combination of warm air and moist soil.

* Late September is ideal for sowing a new lawn or re-seeding bare spots.

* Compost annuals and vegetable crops that have finished producing.

* Cultivate and add compost to the soil to replenish its nutrients for fall and winter vegetables and flowers.

* Plant for fall now. The warm soil will get cool-season veggies and flowers off to a fast start.

* Plant onions, lettuce, peas, radishes, turnips, beets, carrots, bok choy, spinach and potatoes directly into the vegetable beds.

* Transplant lettuce, cabbage, broccoli, kale, Brussels sprouts and cauliflower.

* Sow seeds of California poppies, clarkia and African daisies.

* Transplant cool-weather annuals such as pansies, violas, fairy primroses, calendulas, stocks and snapdragons.

* Divide and replant bulbs, rhizomes and perennials.

* Dig up and divide daylilies as they complete their bloom cycle.

* Divide and transplant peonies that have become overcrowded. Replant with "eyes" about an inch below the soil surface.

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