CO-Horts Blog

Friday, June 13, 2014

Mid-June Gardening Chores

Posted by: Jim Klett, Professor and Extension Horticulture Specialist

When should I prune my overgrown forsythia in my garden?

Along the Front Range, we generally enjoyed a good early spring for forsythia bloom due to many of the newer clones such as ‘Meadowlark’, ‘New Hampshire Gold’, ‘Sunrise’, and ‘Northern Sun’, which have better flower bud hardiness and are being sold and planted more. One should prune forsythia after flowering by thinning one third of the older canes by cutting them to ground. It is important to remove the oldest branches and weak and dead wood. This will encourage new growth and more flower buds which are formed in late summer to fall for next spring.

If the shrubs are overgrown, you may want to rejuvenate your shrub in February by trimming the entire plant to 3 to 6 inches above the ground.

No matter which of these two pruning techniques you choose, your overgrown shrub will bloom heavier and will be more showy in future years. You will lose bloom color if you choose rejuvenation  pruning but will gain it back in future years.
[photo by David Staats]
Forsythia in full bloom at the CSU Arboretum

My irises are done blooming and is this the time to divide?

Clumps of bearded irises should be divided and replanted before they become overcrowded. A single rhizome will branch many times over the years, developing into heavy criss-cross clump, often choked with old leafless rhizomes. If it is not divided, the mass of leaves will exclude sun and air from roots. This will lead to poor flowering or no flowering and often weakens the plants making them more susceptible to insects and disease.
Photo by David Staats

Dividing of irises is best done after bloom (late June into July) which is the same time that plantings should be made. Lift each clump by gently prying it loose from the soil. A spading fork is better for this than a shovel because it is less likely to cut roots and rhizomes.

Use a sharp, strong bladed knife to trim younger rhizomes into sections that include healthy looking roots and one or two strong leaf fans. Carefully wash soil off roots under low pressure from a hose. Discard old rhizomes from center sections.

Trim existing leaves to a neat fan shape and then dig a hole to replant so that the rhizome will be set no deeper than one inch. Check to be sure that all leaf fans face the same way and spread roots out evenly. Firm the soil around the rhizome to eliminate major air pockets in the soil. 

Photo by David Staats

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