best mix for ornamentals--clay soil, poor drainage? And questions regarding directly sown seed

Asked March 18, 2020, 5:50 PM EDT

Hi: Tried to get a master gardener to no avail :'(, but hoping for a quick response so I can get this stuff ordered ASAP. Traci DiSabato-Aust's "The Well-Tended Perennial Garden" suggests amending beds with basically 2" of compost and 2" of peat moss. I'm looking to do the same for a bed that will be planted partly with perennials (carex, astilbe, ferns, meadow rue, ligularia) and partially with a "test" of directly sown annual seed (clarkia, nicotiana, nasturtium, etc.). I live in NW Portland and rent; I have poor drainage that I've tried to ameliorate by building "raised beds"--just Amerian Indian style, elevated, not boxed in and creating a teensy arroyo and rain garden. Yet the drainage problems are always a problem. So I'm buying a mix to amend the beds from MacFarlane's (see descriptions here: I'm unsure whether to go with just a yard of compost, to buy their Irish mix (40% fine compost, 40% aged barkdust, 20% sandy soil--sounds like better than just compost for aeration), whether to get 1/2 a yard of compost and 1/2 a yard of aged bark, or if there is something else I should look for (they do sell peat moss, but it's fairly pricey, and I'm on a fixed income; and I know that peat moss is a no no ecologically-speaking). My soil is of good quality, but it is clay--so I really don't need to add more topsoil, just get aeration and some humus in. Could you give a suggestion? ASAP, if possible? Also, could you solve the eternal dispute regarding whether gypsum is of any use in remediating Willamette Valley clay? Everything I've read from NW extension resources (OSU, WSU, etc.) says no--that it's some issue of salinity or the type of clay we have--but the manager at MacFarlane's was pretty insistent that was the best remedy :(. LASTLY! I'm really broke and had my heart broken when I belatedly realized what perennials would cost me. It was too late to start the appropriate seeds for my part-shade backyard inside, but as noted above, I'm going to try nicotiana, clarkia varieties, balsam, sweet alyssum, and even cornflowers, nasturtium, scarlet flax, four o'clocks, morning glories, and breadseed poppies in the sunnier areas. Wondering a few things, some of which depend on 1) starting this late and 2) considering our unique climate: 1) Any point in seeding digitalis purpurea this spring? Or should I wait for fall? What about catchfly and linaria maroccana--both apparently need cold stratification, esp the former. Violets/johnny jump ups? Just wait for the fall? (and/or a grow light set up!) 2) Is it too late to start nemophilia varieties directly outside? I know they don't like heat and they're, I think, slow to get established, but they'd be ideal in the shadier areas. Is it worth a try? I already bought the seed, so... 3) If you have any other suggestions for annuals or any other ornamentals that can be started from seed now in a backyard that gets anywhere from all-day dappled shade to 2 hours to 6 hours of direct light (northern exposure, large tree, buildings all around), would love to hear them. Sucks gardening on a budget :(. Thanks for your help. If you can't get to Part II of the extended questions, please try to get back asap on the first point re the best option for amending the soil--have a guy set to make a pick up on Saturday (or earlier). Appreciate all you all do--stay healthy!

Multnomah County Oregon

2 Responses

Hi, thanks for your question. For the time being all the MG activities have been cancelled, including the extension office and satellite offices in Portland Metro area. I am glad you found your way here. I can't answer your question this evening but I will get back to it tomorrow morning.

Gypsum is chiefly used to amend sodic soils. Sodic (saline) soils are found mainly in arid regions of the western United States and are generally higher in calcium. Addition of gypsum to soils is sometimes added to soil in our suburban gardens to increase the calcium content but a soil test should be performed to determine whether your soil is deficient in calcium. Lime or gypsum can also reduce the acidity of soil so, again, a simple soil pH test will inform you if it is needed for that. The pH you want to achieve will depend on the plants you intend to grow. Also, any effects of gypsum in clay soil will take place over a long period of time.

What is needed in our clay soil is to make sure the soil is not compacted first. Compaction will push the clay particles together, reducing or eliminating the air pockets needed for drainage. This can by done with light tilling (top 2 inches”) or by using a digging fork to penetrate the soil as deep as the fork will go in, just rocking it back and forth, then moving a few inches away repeating the process over the area you are planting. Excessive digging or tilling will more likely result in breaking down the soil structure, which can add to further compaction and poor drainage. Once you have aerated your soil do not walk on it or it will begin to compact again. If you need to access the area for maintenance place a board on the soil and walk or kneel on that.

Vegetable and flower gardens can be improved by applying and incorporating organic matter, such as compost, well-rotted manure, into the soil. Work the organic matter into the top 8 to 12 inches of soil over the entire planting area or use it as a mulch not more than 2-3” deep. The organic material will hold the clay particles apart, preserving and improving the air pockets in the soil and improving drainage.

I’ll get back to you with the plant portion of your question later today.

Iowa State University, applying gypsum to clay soil.

Oregon State University, Rebuild your soil, “Rebuild garden soil with application of lime, gypsum”