My primrose are done for the year and look awful burnt. I can cut them back, but would like to plant something on top of them for fall color. What do you suggest?
There are many plants suitable for fall color. Here is an article or two that you may find helpful: https://www.gardeningknowhow.com/ornamental/flowers/fgen/plants-fall-garden.htm
https://www.gardeningknowhow.com/ornamental/flowers/fgen/fall-flowering-gardens-creating-color-and-interest-during-the-off-season.htm
How large can I expect the primrose plant to require?
Depending on the variety, anywhere from 6-12 inches tall and 12-18 inches wide. Here is an article that you may find helpful: https://www.gardeningknowhow.com/ornamental/flowers/primrose/growing-primrose-primrose-plants-in-your-garden.htm
When is it the best time to split these?
Fall is generally the most suitable time for dividing primrose plants.
I have several pots of Streptocarpus (cape primrose) in bloom in the house and the stems are covered in tiny green aphids. Is there a homemade recipe for insecticidal soap before they spread to my other plants?
These articles will help you:
https://www.gardeningknowhow.com/plant-problems/pests/homemade-aphid-control-a-natural-way-to-kill-aphids.htm
https://www.gardeningknowhow.com/special/organic/natural-home-pesticides-organic-garden-pest-control.htm
I would say yes, as long as you can ensure that it will receive plenty of water and place it in an area where it won't be getting full sun in the afternoon--perhaps a partially shaded location or even morning sun only. You could also try planting it in a container and move as needed. Keep in mind that container grown plants will require more watering, especially in summer heat (twice a day in temps over 85 F.).
I have seven beautiful primrose plants. I have transplanted them to larger pots. When can I plant them in the ground outdoors? I live in Pittsburgh, Pennsylvania. It is now March 15th. I know zone planting for most flowers here is late May, early June. I just wondered if I can keep them living until then. I am anxious to see them in my garden. My crocuses are blooming and the daffodils and tulips are coming up.
Actually, you could go ahead and begin hardening them off so they will have time to acclimate before planting out permanently. Primrose plants are pretty hardy once established. Here is more information: https://www.gardeningknowhow.com/garden-how-to/seeds/how-to-harden-off-your-seedlings.htm
https://www.gardeningknowhow.com/ornamental/flowers/primrose/growing-primrose-primrose-plants-in-your-garden.htm
I planted three pretty primrose perennials two years ago and now it is spreading all over that flower bed. What do I do besides pulling it all up? It only comes back the next spring so that hasn't worked. I want to kill it but not the other flowers in that bed. Help!
Try digging up as much of the root system as you can and be sure to cut flowers off to prevent reseeding.