What's Up In The Garden by Jackie Dohlman

Tips on Fertilizing Roses, Oedema on Ivy Geranium Leaves, and Good Sites for Vegetable Gardens

When should I fertilize hybrid tea roses?

Fertilize hybrid tea and other modern roses three times a year. The first application should be made in early spring after pruning. Put on the second application during the first bloom period (typically late May or early June). The third application should be made in mid to late July. Use an all-purpose garden fertilizer, such as 10-10-10. Sprinkle 1/4 cup around each plant. Do not fertilize after July 31. Late summer fertilization will encourage succulent new growth. Late season growth may not harden off properly before winter and be susceptible to winter injury.

There are blister-like growths on the undersides of ivy geranium leaves. What is the problem?

Oedema is probably responsible for the lesions on the undersides of the ivy geranium leaves. Oedema is not caused by an insect or disease pathogen. It is a physiological problem. Oedema is most often encountered in greenhouses in late winter.

While oedema can occur on many different plants, it is most commonly found on ivy geraniums. Affected leaves develop small blisters on their undersides. The blisters rupture and turn tan or brown and become corky. Seriously affected leaves may turn yellow and fall from the plant.

Oedema typically occurs when plants absorb more water through their roots than they use or transpire through their leaves. This usually happens during prolonged periods of cool, cloudy weather in late winter. Oedema develops when the soil is warm and moist, while the air is cool and moist. Water pressure builds up in the cells of the leaf causing them to enlarge and form swollen, blister-like growths.

While oedema is unsightly, it does not cause serious harm to affected plants. Problems can be minimized by not over-watering during cloudy weather and reducing humidity levels in the greenhouse.

What would be a good site for a vegetable garden?

Vegetables grow best in a well-drained, fertile soil. The garden site should also receive at least 6 hours of direct sun each day. Avoid shady sites near large trees and shrubs. Most vegetable crops will not produce well if they don’t receive sufficient sunlight. The site should also be fairly level to avoid soil erosion problems and free of difficult-to-control weeds, such as Canada thistle and field bindweed.

Got gardening questions? Contact the Hortline at (515) 294-3108 (Monday - Friday; 10 a.m. -12 noonand 1 - 4:30 p.m.) or send an e-mail to hortline@iastate.edu. For more gardening information visit us at Yard and Garden Online at www.yardandgarden.extension.iastate.edu

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