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What sort of soil amendments should I make in my perennial beds this fall? Ask an expert


What sort of soil amendments should I make in my perennial beds this fall? Ask an expert


As fall continues with some sunny weather, gardening is still on our radar. You may have questions. For answers, turn to Ask an Expert, an online question-and-answer tool from Oregon State University’s Extension Service. OSU Extension faculty and Master Gardeners reply to queries within two business days, usually less. To ask a question, simply go to the OSU Extension website, type it in and include the county where you live. Here are some questions asked by other gardeners. What’s yours?

Q: What should I be applying to my perennials this fall? Compost? Phosphorus? Nothing? – Linn County

A: Autumn is the perfect time to apply soil amendments that require time to become effective – I’m thinking specifically of lime and phosphorus. Both are best applied at this time of year because they react very slowly with soil. Lime increases soil pH. Phosphorus is an essential plant nutrient — one of the big three (nitrogen, phosphorus, potassium).

I hesitate to recommend lime or phosphorus applications without a soil test that comes with an agronomic interpretation. If not needed, lime could increase soil pH to a level higher than beneficial to plants. Phosphorus can contribute to reduced water quality.

Autumn is not the time to apply anything that contains nitrogen. Nitrogen is highly soluble in water, and any nitrogen applied before spring will be leached below the rooting level of plants. Should we have a warm autumn, nitrogen applications at this time could encourage development of tender new growth that could be damaged by subsequent cold weather.

Manure, organic fertilizers such as feather, bone or blood meal are sources of nitrogen. Applying them now would be a waste of money.

Compost is not a particularly high nitrogen source. If applied as a surface mulch, it can protect soils from raindrop splash and subsequent erosion. One-half inch of compost should be fine.

– Linda Brewer, OSU Extension research assistant

Ask an expert

Is October too late to prune my hydrangea?OSU Extension Service

Q: Due to unusual fall heat, when is the best time to prune my big leaf hydrangeas? Also, when should I prune a struggling climbing rose tree that I want to cut knee high this year.

A: If you’re watering your hydrangea, prune it as you usually do. This year with late heat and dry climate, it’s taking longer for the plants to drop leaves, so the pruning you would do some years in September is an October chore this year. Here’s a video from Clackamas Master Gardeners.

The rule of thumb for roses is to fall prune part-way, to reduce the canopy size that catches the winter winds. Then in about mid-February prune down the rest of the way. – Jacki Dougan, OSU Extension Master Gardener

Q: I am trying to find out if I can plant a big leaf maple tree where a black walnut tree was removed. The walnut was cut down and the stump ground. The chips and roots of the walnut tree are still there. The only info I can find is that most maple are tolerant of juglone but no where can I find if big leaf maples are tolerant. – Clackamas County

A: Your maple is probably fine planted where a black walnut was. Water and mulch your new tree well, after planting it correctly. There aren’t lots of research studies about the reaction of different plants to walnut. See FS 325E from WSU and the reasons to not be too concerned. Under Action Items on Page 5, it says, “Use walnut wood chips for mulch if you have them. They will not harm plants and work just as well as those from any other woody species.” – Jacki Dougan, OSU Extension Master Gardener

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